10 Points to Secure Your ASP.NET MVC Applications

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Introduction

Lots of ASP.NET MVC developers are great in delivery, writing high performance code and so on. But when it comes to security, there is no planning done. So in this article, we will run through 10 points which will help us to make our MVC code secure.

In case you are new to MVC, I would suggest you start from this YouTube tutorial:

Contents

1. Security Misconfiguration (Error Handling Must Setup Custom Error Page)
2. Cross-Site Request Forgery (CSRF)
3. Cross-Site Scripting (XSS) attacks
4. Malicious File Upload.
5. Version Discloser
6. SQL Injection Attack
7. Sensitive Data Exposure
8. Audit Trail
9. Broken Authentication and Session Management
10. Unvalidated Redirects and Forwards

1) Security Misconfiguration (Error Handling Must Setup Custom Error Page)

In this kind of attack, the attacker intercepts form data which is submitted by end User and changes values and sends the modified data to the server.

So for such kind of scenarios, developers do put proper validations in place but when these validations display error, lot of information of the server is revealed.

So let us demonstrate the same practically.

Example

For showing demo, I have created an Employee form which takes basic Employee details.

EmployeeDetailModel View

So is data annotation validations more than enough to secure the page. No, that’s not enough for securing page. I will show you a small demo of how these validations get bypassed.

In case you are new to validation using Data annotation, please do watch this Youtube video which explains how to use data annotations for validation purposes.

The below snapshot shows that address field is validating. It asks for only 50 characters.

Intercepting Add Employee View

Now let’s intercept this form and then submit to the server from the intercept. I am using a tool called as burp suit which catches your request which is going to the server and coming from the server.

In the below snapshot, I have caught a request which is going to the server.

In the below snapshot, the request which I have caught is going to the server. You can see that I have changed Address which is only taking 50. I have added more than 50 characters and then submitted to the server.

Intercepted Address Field of Add Employee Form

Below is the snapshot which shows the request that is submitted to the server which has more than 50 characters.

Debugging Mode of Employee Form

After submitting more than 50 characters to the Address fields, the server throws an exception. Because in the database, the data type of Address field isvarchar(50) and the data is more than 50 so an exception is but obvious.

Problem with Displaying Error Occurred to Users

Now exception which occurs is directly displayed to attacker which leaks a lot of valuable information about the server and our program behavior. Using this error information, he can try various permutations and combinations to exploit our system.

Solution

So the solution here is we need to set some kind of error page which does not show the internal technical error but rather shows a custom error message.

We have two approaches for it:

1. Create a custom Error handling Attribute.
2. Set Custom Error page from Web.config file.

Solution 1

Create a custom Error handling attribute using HandleErrorAttribute or using IExceptionFilterFilter:

Showing example using HandleErrorAttribute.

using System;
usingSystem.Collections.Generic;
usingSystem.Linq;
usingSystem.Text;
usingSystem.Web.Mvc;

namespaceMvcSecurity.Filters
{
publicclassCustomErrorHandler : HandleErrorAttribute
    {
publicoverridevoidOnException(ExceptionContextfilterContext)
        {
Exception e = filterContext.Exception;
filterContext.ExceptionHandled = true;
var result = newViewResult()
            {
ViewName = "Error"
            }; ;
result.ViewBag.Error = 
    "Error Occur While Processing Your Request Please Check After Some Time";
filterContext.Result = result;     
        }
    }

After creating custom Error attribute, we need to apply this globally for the entire application. For doing this, we need to call this attribute in FilterConfig class which is in App_Start folder as shown below:

usingMvcSecurity.Filters;
usingSystem.Web;
usingSystem.Web.Mvc;

namespaceMvcSecurity
{
publicclassFilterConfig
    {
publicstaticvoidRegisterGlobalFilters(GlobalFilterCollection filters)
        {
filters.Add(newCustomErrorHandler());
        }
    }
}

Whenever error occurs, the CustomErrorHandler attribute will get called and it will redirect to Error.cshtml page. And any message you want to pass then, you can pass through @ViewBag.Errorfrom CustomErrorHandler attribute.

HTML Error Page Code Snippet

@{
    Layout = null;
}

<!DOCTYPEhtml>
<html>
<head>
<metaname="viewport"content="width=device-width"/>
<title>Error</title>
</head>
<body>

<hgroup>Error.
	<h3>@ViewBag.Error</h3>

	<h3> </h3>
</hgroup>

</body> </html>

Error Page View

Solution 2

1. Setting Custom Error page from Web.config file.
2. If you do not want to write attribute, then you can set Custom Error page in Web.config file. Before doing that, just create a simple Error page in HTML for displaying if any error occurs.

In Web.config file, there is system.web tag. Inside that, add Custom error tag as shown below:

HTML Error Page Code Snippet

<!DOCTYPEhtml>
<html>
<head>
<metaname="viewport"content="width=device-width"/>
<title>Error</title>
</head>
<body>

<hgroup>Error.
	<h2>An Error Occurred While Processing Your request...........</h2>

	<h2> </h2>
</hgroup>

</body> 
</html>

Html Error Page View

Wow, we have just started securing our web application. Let’s see the second point.

2) Cross-Site Request Forgery (CSRF)

A CSRF vulnerability allows an attacker to force a validated and logged in user to perform actions without their consent or unknowingly.

Take this simple example:

• User logs in to the bank server.
• Bank authorizes and a secure session is established between user and the bank server.
• The attacker sends an email with a malicious link saying “Earn 100000$ now” to the user.
• User clicks on the malicious link and the site tries transfer money from your account to the attackers account. Because the secure session is established, the malicious code can execute successfully.

Microsoft has recognized this threat and for preventing the same, we have something called as AntiForgeryToken.

Solution

We need to add @Html.AntiForgeryToken() helper in your form inside form tag. And on the Action method which handles your post ([HttpPost])Request we need to put [ValidateAntiForgeryToken] attribute which will check if the token is valid.

Adding [AntiForgeryToken] Helper to View

Adding [ValidateAntiForgeryToken] attribute to HttpPost[HttpPost] method.

WorkingofAntiForgeryToken

When we add AntiForgeryToken helper on View, it creates a hidden field and assigns a unique token value to it and meanwhile a session Cookie is added to the browser.

When we post form HTML, it checks for __RequestVerificationToken Hidden field and whether __RequestVerificationToken Cookie are present or not. If either the cookie or the form __RequestVerificationToken Hidden field values are missing, or the values don't match, ASP.NET MVC does not process the action. This is how we can prevent cross-site request forgery attack in ASP.NET MVC.

RequestVerificationTokenon View Snapshot

RequestVerificationToken Cookie Snapshot

3) Cross-Site Scripting (XSS) Attacks

Cross-site Scripting (XSS) is an attack in which malicious scripts are injected via input fields. This attack is most common and allows an attacker to steal credentials and valuable data that can lead to a big security breach.

In this attack, the attacker visits a website and tries to execute a malicious script in form comment box. Now if website has not checked for malicious code, then the code can get executed on the server causing damage.

Let's try to understand the same using an example. Below is simple Employee form which we are trying to save data. Now in the text box, I am trying to execute some malicious code using JavaScript using the SCRIPT tag. But if we try to submit the same, MVC throws an error that something bad is happening.

In short, by default, ASP.NET prevents Cross Site Script attack.

Understanding the Error Displayed

A potentially dangerous Request.Form value was detected from the client (worktype="<script>alert('hi');").

This error occurs because MVC is validating data which is entered by user and if user tried to execute such a script, it does not allow and that’s a good news.

But now what if we want to put SCRIPT tag. For example, programming sites like CodeProject have a genuine need that end user should submit code and script snippets. In those scenarios, we would like the end user to post code through the UI.

So let us understand how to do the same but at the same time not compromise on security.

So we have four things via which we can allow scripts to be posted.

Solution

1. [ValidateInput(false)]
2. [AllowHtml]
3. [RegularExpressionAttribute]
4. AntiXSS Library

Solution 1

ValidateInput

[ValidateInput] is an attribute which can be applied on Controller or Action Method on which we want the script to go through.

If we want Markup to be allowed, then we need to set enableValidation properties to False([ValidateInput(false)]) which will not validate input if we set to true then [ValidateInput(true)]). It will validate input. In the same way, if you apply it on Controller, then it applies to entire action methods inside the controller and if you apply it on Action Method then it will only be specific to that action method.

But ValidateInput attribute will apply to all properties of Model (EmployeeDetails).

Snapshot of applying ValidateInputAttribute on HttpPostMethod.

Snapshot after applying ValidateInputAttribute.

Solution 2

AllowHtml

[AllowHtml] attributes that is applied to Model properties such that it will not validate that particular Model property on which AllowHtml attribute is added. This allows submitting HTML for avoiding Cross Site Scripting attack.

In the below snapshot, I have applied AllowHtml attribute on Address property of EmployeeDetails model.

After applying AllowHtml attribute on that Address property, now Address property will not validate and allow HTML to be submitted in this field.

Solution 3

Regular Expression

The third solution to XSS attack is validating all your Fields with Regular Expression such that only valid data can move in.

Use Regular Expression to validate input to protect from XSS attack below is snapshot.

List of Regular Expression to Use.

• Alphabets and Space - [a-zA-Z ]+$
• Alphabets - ^[A-z]+$
• Numbers - ^[0-9]+$
• Alphanumeric - ^[a-zA-Z0-9]*$
• Email - [a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?
• Mobile no. - ^([7-9]{1})([0-9]{9})$
• Date Format( mm/dd/yyyy | mm-dd-yyyy | mm.dd.yyyy) - /^(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\\d\\d+$/
• Website URL - ^http(s)?://([\\w-]+.)+[\\w-]+(/[\\w- ./?%&=])?$
• Credit Card Numbers
  • Visa - ^4[0-9]{12}(?:[0-9]{3})?$
  • MasterCard - ^5[1-5][0-9]{14}$
  • American Express - ^3[47][0-9]{13}$
• Decimal number - ((\\d+)((\\.\\d{1,2})?))$

Solution 4

AntiXSS Library

The fourth solution to XSS attack is by using MicrosoftAntiXSSLibrary which will help to protect your application.

Now let’s start with install MicrosoftAntiXSSLibrary from NuGet. Just right click on Project, then select ManageNuGetPackages.

After selecting, a new dialog will pop up with name ManageNuGetPackages in this dialog search AntiXSSLibrary in the search box, then choose the first option which is AntiXSS click on Install button.

Reference added after installing

After installing, we are going to have a look at how to use AntiXSSLibrary.

Sanitizer Class

Below snapshot shows how to use Sanitizer Class Method

Sanitizer is a static class we can access anywhere. We just need to provide input which field we need to validate to Sanitizer class method (GetSafeHtmlFragment). It will check and return you Sanitize string.

We can use this method to filter malicious script while saving in the database and displaying in the browser.

Tip: Use [ValidateInput(false)] or [AllowHtml] before using this AntiXSS library, else it will throw error “A potentially dangerous Request.Form”.

4) Malicious File Upload

Till now, we have learned how to protect all your input fields from attack but still, we are missing one main field, it is File upload control. We need to protect from taking invalid input most attackers try to upload a malicious file which may cause a security issue. The attacker can change file extension [tuto.exe to tuto.jpeg] and the malicious script can be uploaded as an image file. Most developers just look at the file extension of the file and save in folder or database but file extension is valid not file, it may have a malicious script.

Solution

1. First thing we need to do is validate file uploads.
2. Allow access only to files extension which are required.
3. Check the file header.

First, the thing I am going to add a file upload control on View.

Adding File Upload Control

We have added File upload control on View. Next, we are going to validate file on Submit.

Validating file uploads on Index HttpPost Method

In this method first, we are going to validate Content-Length of the file if it is zero [upload.ContentLength == 0], then user has not uploaded file.

If Content-Length is greater than zero, then it has file [upload.ContentLength> 0] and we are going to read Filename of File along with Content Type and File Bytes.

[HttpPost]
   [ValidateAntiForgeryToken]
publicActionResult Index(EmployeeDetailEmployeeDetail)
   {
if (ModelState.IsValid)
       {
HttpPostedFileBase upload = Request.Files["upload"];
if (upload.ContentLength == 0)
           {
ModelState.AddModelError("File", "Please Upload your file");
           }
elseif (upload.ContentLength> 0)
           {
stringfileName = upload.FileName;                               // getting File Name

stringfileContentType = upload.ContentType;                     // getting ContentType

byte[]tempFileBytes= newbyte[upload.ContentLength];             // getting filebytes

var data = upload.InputStream.Read(tempFileBytes, 0, Convert.ToInt32(upload.ContentLength));

var types = MvcSecurity.Filters.FileUploadCheck.FileType.Image; // Setting Image type

var result = FileUploadCheck.isValidFile
             (tempFileBytes, types, fileContentType);           // Validate Header

if (result == true)
               {
intFileLength = 1024 * 1024 * 2;                                //FileLength 2 MB
if (upload.ContentLength>FileLength)
                   {
ModelState.AddModelError("File", "Maximum allowed size is: " + FileLength + " MB");
                   }
else
                   {
stringdemoAddress = Sanitizer.GetSafeHtmlFragment(EmployeeDetail.Address);
dbcon.EmployeeDetails.Add(EmployeeDetail);
dbcon.SaveChanges();
return View();
                   }
               }
           }
       }
return View(EmployeeDetail);
   }

Till now, it was basic validation we have done. Let’s Validate file which is uploaded. For doing that, I have written a static class with name FileUploadCheckin this class. There are various methods for validating different file type. For now, I am going to show you how to validate Images files and only allow image files only.

FileUploadCheck Class

In the above snapshot, there is ImageFileExtension enum which contains Image formats and File types.

privateenumImageFileExtension
 {
none = 0,
jpg = 1,
jpeg = 2,
bmp = 3,
gif = 4,
png = 5
 }
publicenumFileType
 {
     Image = 1,
     Video = 2,
     PDF = 3,
     Text = 4,
     DOC = 5,
     DOCX = 6,
     PPT = 7,
 }

If it has passed basic validation, then we are going to call isValidFile method which take bytes, File type, FileContentType as input.

publicstaticboolisValidFile(byte[] bytFile, FileTypeflType, StringFileContentType)
 {
boolisvalid = false;
if (flType == FileType.Image)
     {
isvalid = isValidImageFile(bytFile, FileContentType);//we are going call this method
     }
elseif (flType == FileType.Video)
     {
isvalid = isValidVideoFile(bytFile, FileContentType);
     }
elseif (flType == FileType.PDF)
     {
isvalid = isValidPDFFile(bytFile, FileContentType);
     }
returnisvalid;
 }

After calling isValidFilemethod, it will call another static method based on File type.

If File type is image, then it will call first method [isValidImageFile] else File type is Video, then second method [isValidVideoFile] and in similar way, if File type is PDF, then the last method [isValidPDFFile] will get called.

After completing with understanding isValidFileMethod, let’s have a look at [isValidImageFile] method which will get called.

Below is the complete code snippet of [isValidImageFile] method.

In this method, we are allowing limited Image file extensions [jpg, jpeg, png, bmp, gif]

Working of this isValidImageFile method

When we pass bytes and FileContentType to this method, then it will first check for FileContentType on the basis of that, it will set:

ImageFileExtension

After that, it is going to check header bytes which we have against uploaded file bytes. If it matches that, then File is valid [true] else file is invalid [false].

publicstaticboolisValidImageFile(byte[] bytFile, StringFileContentType)
   {
boolisvalid = false;

byte[] chkBytejpg = { 255, 216, 255, 224 };
byte[] chkBytebmp = { 66, 77 };
byte[] chkBytegif = { 71, 73, 70, 56 };
byte[] chkBytepng = { 137, 80, 78, 71 };

ImageFileExtensionimgfileExtn = ImageFileExtension.none;

if (FileContentType.Contains("jpg") | FileContentType.Contains("jpeg"))
       {
imgfileExtn = ImageFileExtension.jpg;
       }
elseif (FileContentType.Contains("png"))
       {
imgfileExtn = ImageFileExtension.png;
       }
elseif (FileContentType.Contains("bmp"))
       {
imgfileExtn = ImageFileExtension.bmp;
       }
elseif (FileContentType.Contains("gif"))
       {
imgfileExtn = ImageFileExtension.gif;
       }

if (imgfileExtn == ImageFileExtension.jpg || imgfileExtn == ImageFileExtension.jpeg)
       {
if (bytFile.Length>= 4)
           {
int j = 0;
for (Int32 i = 0; i <= 3; i++)
               {
if (bytFile[i] == chkBytejpg[i])
                   {
                       j = j + 1;
if (j == 3)
                       {
isvalid = true;
                       }
                   }
               }
           }
       }

if (imgfileExtn == ImageFileExtension.png)
       {
if (bytFile.Length>= 4)
           {
int j = 0;
for (Int32 i = 0; i <= 3; i++)
               {
if (bytFile[i] == chkBytepng[i])
                   {
                       j = j + 1;
if (j == 3)
                       {
isvalid = true;
                       }
                   }
               }
           }
       }

if (imgfileExtn == ImageFileExtension.bmp)
       {
if (bytFile.Length>= 4)
           {
int j = 0;
for (Int32 i = 0; i <= 1; i++)
               {
if (bytFile[i] == chkBytebmp[i])
                   {
                       j = j + 1;
if (j == 2)
                       {
isvalid = true;
                       }
                   }
               }
           }
       }

if (imgfileExtn == ImageFileExtension.gif)
       {
if (bytFile.Length>= 4)
           {
int j = 0;
for (Int32 i = 0; i <= 1; i++)
               {
if (bytFile[i] == chkBytegif[i])
                   {
                       j = j + 1;
if (j == 3)
                       {
isvalid = true;
                       }
                   }
               }
           }
       }

returnisvalid;
   }

Calling isValidFile method from Action Method

We are going to call (FileUploadCheck.isValidFile) method and then we are going to pass Parameter File Bytes, Types, FileContentType.

This method will return Boolean value if it is a valid file then true, else it will return false.

stringfileName = upload.FileName;                               // getting File Name
stringfileContentType = upload.ContentType;                     // getting ContentType
byte[] tempFileBytes = newbyte[upload.ContentLength];           // getting filebytes
var data = upload.InputStream.Read(tempFileBytes, 0, Convert.ToInt32(upload.ContentLength));
var types = MvcSecurity.Filters.FileUploadCheck.FileType.Image; // Setting Image type
var result = FileUploadCheck.isValidFile
             (tempFileBytes, types, fileContentType);           //Calling isValidFile method

After understanding code snippet, now let’s see a demo with how it works.

Below snapshot shows an Employee form with file upload control.

We are going to fill the below form and choose a valid file.

Choosing a valid .jpg file and check how it will validate in details

Choosing a .jpg image from disk.

Snapshot of Employee Form after Choosing a File.

In this part, we have chosen a file.

Snapshot of debugging Index Post Action Method.

In this part, we have posted an Employee form with a file. Here, we can see how it is validating basic validations.

Snapshot of debugging Index Post Action Method.

In this part, you can see the real-time value of file which we have uploaded. It has passed basic validation.

Snapshot of FileUploadCheck Class while isVaildFile Method gets called.

In this part, after calling isValidFile method, it is going to call another method according to its FileContentType.

Snapshot of isVaildImageFile Method while checking Header Bytes.

In this method, it will check header bytes of the image which is uploaded against the bytes which we have if it matches, then it is a valid file, else it is not a valid file.

5) Version Discloser

Version information can be used by an attacker to target specific attack on that Version which is disclosed.

Whenever browser sends HTTP to request to the server in response, we get response header which contains information of [Server, X-AspNet-Version, X-AspNetMvc-Version, X-Powered-By].

The server shows information of which web server is begin used.

• X-AspNet-Version shows information of which specific ASP.NET version used.
• X-AspNetMvc-Version shows information of which ASP.NET MVC version used.
• X-Powered-By shows information of which framework your website is running on.

Solution

1. For removing X-AspNetMvc-Version header

   To remove response X-AspNetMvc-Version which shows information of which ASP.NET MVC version used, we have built in property in MVC.

   Just set [MvcHandler.DisableMvcResponseHeader = true;] in Global.asax. Application start event [Application_Start()] this will remove header. It won’t be displayed any more.

   

   Fig 40. Setting property in Global.asax to removeX-AspNetMvc-Version from header.

   

   Fig 41. Response after removing X-AspNetMvc-Version from header.
2. For removingX-AspNet-Version and Server header

   To remove response of Server header which shows information of which web server is begin used and along with that, X-AspNet-Version header shows information of which specific ASP.NET Version Used.

   Just add an event in [Application_PreSendRequestHeaders()] in global.asax and then to remove header, we need to set the property as below:

   C#

   protectedvoidApplication_PreSendRequestHeaders()
   {
   Response.Headers.Remove("Server");           //Remove Server Header   
   Response.Headers.Remove("X-AspNet-Version"); //Remove X-AspNet-Version Header
   }

   

   Fig 42. Adding Application_PreSendRequestHeaders event in global.asax and then removing Response headers.

   

   Fig 43. Response after removing X-AspNet-Version and Server from header.
3. For removingX-Powered-Byheader

   To remove response of X-Powered-Byheader which shows information of which framework your website is running on.

   Just add this tag under System.webServer in Web.config file will remove [X-Powered-By] header.

   HTML

   <httpprotocol>
   <customheaders>
   
   </customheaders>
   </httpprotocol>

   

   Fig 44. Adding custom Header tag in Web.configfor removing Response headers.

   

   Fig 45. Response after removing X-Powered-By from header.

6) SQL Injection Attack

SQL Injection attack is one of the most dangerous attacks. It is ranked 1 in top 10 vulnerabilities by OWASP2013 [Open Web Application Security Project]. SQL injection attack can give valuable data to the attacker that can lead to a big security breach and can also take full access to the database server.

In SQL Injection, the attacker always tries to enter malicious SQL statement which will get executed in the database and return unwanted data to the attacker.

Simple View which shows User data

View shows Single Employee data based on EmployeeID as displayed in the below snapshot.

Simple View which Shows All User Data after SQL Injection Attack

In this browser view, as attacker saw Application URL which contains some valuable data which is ID [http://localhost:3837/EmployeeList/index?Id=2], the attacker tries SQL Injection attack as shown below:

After trying permutation & combination of SQL injection attack, the attacker gets access to all User data.

Displaying SQL injection in Debug Mode

Here, we can see in details of how attacker passed malicious SQL statement which gets executed in the database.

SQL Profiler View of SQL Statement

Solution

1. Validate inputs
2. Use of low-privileged database logins
3. Use Parameterized queries
4. Use ORM (e.g., Dapper, Entity framework)
5. Use Stored Procedures

1. Validate Inputs

Always validate input on both side Client side and Server side such that no special characters are entered in inputs which allows an attacker to get into the system.

In MVC, we use Data Annotations for Validation.

Data Annotations attributes are a simple rule that are applied on Model to validate Model data.

Client side validating inputs

Server side validating inputs

Model state is false - this indicates that Model is not valid.

2. Give Least-Privileged Database Logins

Db_owner is default role for the database which can grant and revoke access, create tables, stored procedures, views, run backups, schedule jobs can even drop the database. If this kind of Role access is given to database, then User who is using can have full access and he can perform various activity on it. We must create a new user account with least-privileged and give only that privilege that user must access.

Example, if the user has to work related to selecting, inserting and updating Employee details, then only select, Insert and update permission should be provided.

Step to Add New User and Provide Privilege

In this part, I have shown a small sample of how to create User provided specific permission to table.

1. Creating New User
2. View after creating User
3. Selecting object that user can access (table)
4. Choosing specific table to provide permission

After choosing table, we are providing permission to table as shown below, you can see we have only given permission to ‘Insert’, ‘Select’, ‘Update’ for this user.

With least-privileged, it will help us to safeguard database from attackers.

3. Use Stored Procedures

Stored procedures are a form of parameterized query. Using Stored Procedures is also one way to protect from SQL injection attack.

In the below snapshot, we have removed inline Query which we were using before now we have written code for getting data from database using stored procedure which helps us to protect against SQL injection attack, then to stored procedure, we need to pass parameter [@EmpID] and according to parameter, it is going to get records from database after passing parameter, we have used CommandType [CommandType.StoredProcedure] which indicates we are using stored procedure.

Note: Always use parameter with stored procedure if you do not use it, still you are vulnerable to SQL injection attack.

After Using Stored Procedure, let's Request GetEmployee View

Employee View which displays record.

Profiler View after using Stored Procedure

Displaying Trace of stored procedure which we have used.

After Using Stored Procedure Let’s try SQL injection Attack Again

Displaying SQL injection attack execution in Debug Mode after using Stored Procedure

If you have a look at parameter id [?Id=2 or 1=1] which contains MaliciousSQL script after passing it to stored procedure, it shows an error which indicates that it not get executed because parameter which we are passing is an integer which only take numeric values as input if we pass MaliciousSQL script [?Id=2 or 1=1] it throws an error .

Profiler View after using Stored Procedure

Output

Meanwhile, you might think that if it was varchar datatype, then we had successfully attacked. Let's see a demo of it too.

Displaying SQL injection attack execution in Debug Mode after using Stored Procedure with different parameter @name

This is the second demo in which we have passed a different parameter to stored procedure to see is it real protecting against SQL injection attack.

Output

Use Parameterized Queries

Using parameterized queries is another solution to prevent against SQL injection attack. It is similar to stored procedure. Instead of concatenating string, you need to add parameters to the SQLQuery and pass parameter value using the SqlCommand.

Profiler View of parameterized Query

Output

Use ORM (Entity framework)

ORM stands for Object relational Mapper which maps SQL object to your domain object [C#].

If you are using entity framework properly, you are not prone to SQL injection attack because entity framework internal uses parameterized query.

Normal Execution of Entity Framework

Here, we have passed the name as parameter [?name=Saineshwar].

Snapshot of the Controller During Execution

In debug mode, we can see that we have passed name parameter from Querystring which we then passed to Linq query to getting records.

Profiler View of Linq Query

Entity Framework internally uses Parameterized query it's true here is a snapshot.

Let’s try SQL injection Attack on Entity Framework

In this part, we are trying SQL injection attack on entity framework. For that, we have passed the parameter as [?name=Saineshwar or 1=1].

Snapshot of the Controller During Execution

In debug mode, we can see that we have passed name parameter from Querystring which we then passed to Linq query to get records but this time, it has malicious script along with normal parameter.

Profiler View of Linq query

If you see trace closely, it is considering the name and malicious script as a single parameter which prevents it from SQL injection attack.

7) Sensitive Data Exposure

All websites and applications always have database in which entire data is being stored. This means while we store Users personal information (which may contains Password, PAN number, Passport details, Credit Card Numbers) in this, we mostly encrypt only password right other data are stored in clear text which can lead to Sensitive Data Exposure whenever an attacker attacks, he gets access to database if he finds the table where all this personal and financial details stored steal that information.

A simple demo of how Sensitive data is been sniffed.

Snapshot of Login Page

Simple code snippet of Login page which comes default if you are choosing Internet Template while creating a project.

After having a look at how Login page Mark and View looks now, next we are going to enter credentials and login.

Entering Credentials for Login

In Login page, we are going to enter Username and Password to login to the application.

Intercepting Login Page by Attacker to Steal Credentials

When User enter Credentials in Login page and submits to server, the data [username and password] is transferred to the server in the clear text. This data [username and password] can be intercepted by the attacker and steal your Credentials.

Below snapshot shows how your values can be intercepted by an attacker.

Solution

1. Always send sensitive data to server in encrypted format using strong hashing with seed (Random Hash).
2. Always apply SSL to the Web application.
3. Do not store sensitive data if want to store using Strong Hashing techniques

Always Send Sensitive Data to Server in Encrypted format using Strong Hashing with seed

In this demo, we are going use an MD5 algorithm with seed to encrypted data on client side and send to a server such that it cannot be stolen by the attacker.

In the below snapshot, user enters credentials.

After entering User credentials when the user clicks on login button, this time, the user entered password that gets encrypted along with seed on the client side using MD5 and then it is sent to the server. In this process, if an attacker tries to sniff network, then encrypted hash is only seen and cannot be decrypted.

In details, you can see the below snapshot where the attacker has sniff network.

Blue line in snapshot indicates Username.

The red line in snapshot indicates Encrypted Password along with the seed.

The green line in snapshot indicates seed value which is generated from the server.

Till now, we have seen a snapshot of how to do this. Let's take a look at a code snippet.

Login Model

Below is a code snippet of [HttpGet] Login ActionMethod.

In this method, we are generating random hash [Seed] and then we are going to assign it to LoginModel and then pass it to Login View.

After assigning value [hdrandomSeed] to model in Login ActionMethod, now we are going to use it on Login view as a hidden field.

Now after adding a hidden field on the page, now let's see how to Encrypt data using MD5 JavaScript along with the seed.

For doing this, we are going to use jquery library 1.8 and md5.js library when user enters Credentials and clicks on Log In button, then we take password entered by User [var password1 = $('#Password');] and generate Hash [calcMD5(password).toUpperCase()] of it first then along with seed, we again generate Hash [var hash = calcMD5(seed + calcMD5(password).toUpperCase());] which is unique and then it is sent to server.

See below snapshot for details

Debugging View of Login Page

In the variable session, you can see real time values which are generated when user clicks on login button.

After client side encryption, now let's see attacker is able to intercept and see the password in clear text.

Intercepting Login page

In the below snapshot, you can see password which is entered by the user is in encrypted form and it cannot be understood by an attacker because it is a combination hash of Seed + Password which is entered by User.

After interception, next we are going to see how values are posted to Login Action method.

Login Action Method after posting Values from Login View

Here, we can clearly see password is in encrypted form.

In the next step, we are going to compare password which is stored in a database. For doing that, first we are going to get the password from username which user has entered and then we are going to compare it against which is stored in the database. The line which is marked green is Seed value. Along with that, you can see database stored password is marked as red which we get by passing Username and then yellow line which indicates that this password in posted from Login View and finally, blue line which is a combination of database password and seed. We compare it with password which is posted.

Finally, sensitive data entered by User is Secure.

2. Use SSL for Securing Web application

SSL (Secure Sockets Layer) is Layer which Secure (Encrypted) communication between client and server such that any data [Banking details, Password, and another financial transaction] passed from client and server is Secure (Encrypted).

SSL is mainly applied on the Login page and payment gateways if you want to apply on the entire application, then also you can apply.

If you want to know in detail how to enable SSL on IIS Server, there is a good article from Scott Guthrie Sir's blog.

3. Do not store Sensitive data in Database in a clear form

Always try not to store Credit Card, Debit Card and financial details and other Sensitive details in the database in Clear form. Always Use Strong Hashing techniques to encrypt data and then store in the database. If an attacker gets direct access to the database, then all data in clear form can be breached.

Below is a list of algorithms which can be used according to need.

Hash Algorithm

If someone wants just Hash, then they can use Hash Algorithm, we mostly use Hash function for Encrypting Password.

SymmetricAlgorithm

If someone wants just one key for encryption and decryption, then they can use SymmetricAlgorithm.

AsymmetricAlgorithm

If someone wants just one key for encryption (Public key) and another keydecryption (Private key), then they can use AsymmetricAlgorithm. E.g., we can use this when we are sharing Web Services and WebAPI with clients as the user.

HashAlgorithm

1. MD5
2. SHA256
3. SHA384
4. SHA512

Example:

Method to generate MD5 Hash

privatestring Generate_MD5_Hash(stringdata_To_Encrypted)
        {
using (MD5encryptor = MD5.Create())
            {
MD5md5 = System.Security.Cryptography.MD5.Create();
byte[] inputBytes = System.Text.Encoding.ASCII.GetBytes(data_To_Encrypted);
byte[] hash = md5.ComputeHash(inputBytes);

StringBuildersb = newStringBuilder();

for (int i = 0; i < hash.length; i="">

Pass text to generate hash

string Hash = Generate_MD5_Hash("Hello");

Output

SymmetricAlgorithm

1. Aes
2. DES
3. RC2
4. Rijndael
5. TripleDES

Example

Method of AES for Encryption

privatestringEncrypt_AES(stringclearText)
        {
stringEncryptionKey = "##SAI##1990##"; //Encryption Key
byte[] clearBytes = Encoding.Unicode.GetBytes(clearText);
byte[] array = Encoding.ASCII.GetBytes("##100SAINESHWAR99##"); //salt

using (Aesencryptor = Aes.Create())
            {
Rfc2898DeriveBytespdb = newRfc2898DeriveBytes(EncryptionKey, array);
encryptor.Key = pdb.GetBytes(32);
encryptor.IV = pdb.GetBytes(16);
using (MemoryStreamms = newMemoryStream())
                {
using (CryptoStreamcs = 
       newCryptoStream(ms, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                    {
cs.Write(clearBytes, 0, clearBytes.Length);
cs.Close();
                    }
clearText = Convert.ToBase64String(ms.ToArray());
                }
            }
returnclearText;
        }

Method of AES for Decryption

privatestringDecrypt_AES(stringcipherText)
        {
stringEncryptionKey = "##SAI##1990##"; //Encryption Key
byte[] cipherBytes = Convert.FromBase64String(cipherText);
byte[] array = Encoding.ASCII.GetBytes("##100SAINESHWAR99##"); //salt

using (Aesencryptor = Aes.Create())
            {
Rfc2898DeriveBytespdb = newRfc2898DeriveBytes(EncryptionKey, array);
encryptor.Key = pdb.GetBytes(32);
encryptor.IV = pdb.GetBytes(16);
using (MemoryStreamms = newMemoryStream())
                {
using (CryptoStreamcs = 
       newCryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                    {
cs.Write(cipherBytes, 0, cipherBytes.Length);
cs.Close();
                    }
cipherText = Encoding.Unicode.GetString(ms.ToArray());
                }
            }
returncipherText;
        }

Pass Text to Encrypt Value

stringDataEncrypt = Encrypt_AES("Hello");   // Encrypting Data (Pass text to Encrypt)

Pass Text to Decrypt Value

stringDataDecrypt = Decrypt_AES(DataEncrypt);  // Decrypt data (Pass Encrypt text to Decrypt)

Output

AsymmetricAlgorithm

1. DSA
2. ECDiffieHellman
3. ECDsa
4. RSA

Example

Method of RSA for Encryption

publicbyte[] Encrypt(stringpublicKeyXML, stringdataToDycript)
        {
RSACryptoServiceProviderrsa = newRSACryptoServiceProvider();
rsa.FromXmlString(publicKeyXML);
returnrsa.Encrypt(ASCIIEncoding.ASCII.GetBytes(dataToDycript), true);
        }

Method of RSA for Decryption

publicstring Decrypt(stringpublicPrivateKeyXML, byte[] encryptedData)
        {
RSACryptoServiceProviderrsa = newRSACryptoServiceProvider();
rsa.FromXmlString(publicPrivateKeyXML);
returnASCIIEncoding.ASCII.GetString(rsa.Decrypt(encryptedData, true));
        }

Output

8) Audit Trail

Audit Trail in the IT World is used to keep track of User activity on a Webapplication which he using. It is where it is important in detecting security problems, performance problems, and ApplicationsLevel Error problems. It also helps us to easily track where the problem actually is and resolve it.

Solution

Keep Audit Trail of all User activity on Web Application and always Monitor it.

For maintaining Audit Trail, first we are going to create a table in the database for storing Audit data with table name [AuditTB]. After that, we are going to create an ActionFilterAttribute with Name [UserAuditFilter] inside this on Action executing, we are going to write code for inserting data in the database of the users who are accessing our application.

AuditTB Table View

In this table, we have taken common fields which we required to recognize User and his Activity.

After displaying table view, now let’s have a look at Model which is created according to Table.

AuditTB Model

publicpartialclassAuditTB
    {
publicintUsersAuditID { get; set; }
publicintUserID { get; set; }
publicstringSessionID { get; set; }
publicstringIPAddress { get; set; }
publicstringPageAccessed { get; set; }
publicNullable<system>LoggedInAt { get; set; }
publicNullable<system.datetime>LoggedOutAt { get; set; }
publicstringLoginStatus { get; set; }
publicstringControllerName { get; set; }
publicstringActionName { get; set; }
    }

After having a look at Model which is generated now, let’s create an ActionFilter with UserAuditFilter.

UserAuditFilterActionfilter Code Snippet

UserAuditFilter is a custom ActionFilter which we have created. In this filter, we insert data of User Activity into AuditTB table. Along with this, we also check whether or not user is Logged into application. In the same way, we also insert IP address of user who is accessing the application, and time stamp of user logged in and out. For inserting this data, we use ORM entity Framework.

publicclassUserAuditFilter : ActionFilterAttribute
    {
publicoverridevoidOnActionExecuting(ActionExecutingContextfilterContext)
        {
AllSampleCodeEntitiesappcontext = newAllSampleCodeEntities();
AuditTBobjaudit = newAuditTB();
//Getting Action Name
stringactionName = filterContext.ActionDescriptor.ActionName;
//Getting Controller Name
stringcontrollerName = filterContext.ActionDescriptor.ControllerDescriptor.ControllerName;
var request = filterContext.HttpContext.Request;
if (HttpContext.Current.Session["UserID"] == null) // For Checking User is Logged in or Not 
            {
objaudit.UserID = 0;
            }
else
            {
objaudit.UserID = Convert.ToInt32(HttpContext.Current.Session["UserID"]);
            }
objaudit.UsersAuditID = 0;
objaudit.SessionID = HttpContext.Current.Session.SessionID; // Application SessionID
// User IPAddress
objaudit.IPAddress =
request.ServerVariables["HTTP_X_FORWARDED_FOR"] ?? request.UserHostAddress;
objaudit.PageAccessed = request.RawUrl;   // URL User Requested
objaudit.LoggedInAt = DateTime.Now;       // Time User Logged In || 
                                          // And time User Request Method
if (actionName == "LogOff")
            {
objaudit.LoggedOutAt = DateTime.Now;      // Time User Logged OUT
            }        
objaudit.LoginStatus = "A";
objaudit.ControllerName = controllerName; // ControllerName
objaudit.ActionName = actionName;         // ActionName
appcontext.AuditTBs.Add(objaudit);
appcontext.SaveChanges();                 // Saving in database using Entity Framework
base.OnActionExecuting(filterContext);
        }
    }

Registering UserAuditFilter in global action filter

Global action filters are mainly used for error handling and logging.

If you have a condition in which you want to apply Action Filter on all action method in the project, then you can use global action filters. Here, we need global action filters because we want to track every request of the user for Audit trail, hence we have used.

Output

The data which got inserted while user requested pages and did some activity in Audit table.

9) Broken Authentication and Session Management

If Authentication and Session management are not properly implemented in a web application which may allow an attacker to steal a password, session tokens, cookies and such issue may also allow an attacker to get access to the entire application and breach all user credentials.

Ways in which an attacker can steal data:

1. Not Secure connection (Not Using SSL)
2. Predictable login credentials
3. Not storing credentials Encrypted form
4. Improper Application logout

Attacks Possible

1. Session Fixation

Before finding a solution how to prevent this attack, let's have a look at a small demo of how Session Fixation attack occurs.

Whenever a user sends first request to server, the login page is loaded, then User enters valid Login credits to login web application. After successful login, we assign some values in session to recognize Unique User, meanwhile a [“ASP.NET_SessionId”] cookie is added to the browser for recognizing specific user who has sent a request and [“ASP.NET_SessionId”] cookie value will be always sent to the server on every request till you are not logged out from the application and on logout, we basically write code for removing session values which are created but we do not remove [“ASP.NET_SessionId”] cookie which is created on Login. This value helps an attacker to perform Session Fixation attack.

Demo of Session Fixation

When we access login page, we do not have any [“ASP.NET_SessionId”] cookie in the browser as we can see in cookie manager.

After User Enter Valid Credentials

After entering Valid Login Credentials [“ASP.NET_SessionId”], Cookie is added to the browser.

Note: When any data is saved into the Session [“ASP.NET_SessionId”] cookie is created and added to the user browser.

After Logging out from Application Cookie still exists in browser

After Logout from application, still [“ASP.NET_SessionId”] Cookie exists.

Note: Pre cookie and post cookie are the same which can lead to the session fixation.

Let’s do Some Session Fixation

The [“ASP.NET_SessionId”] Cookie which is not removed after logout that helps attacker for doing session fixation will open a browser (Chrome). In that, I will enter URL [http://localhost:3837/] of application in which we are going to do session fixation.

After entering URL in the browser, now let's check if we have any [“ASP.NET_SessionId”] Cookie created here. Oh, we don’t have any Cookie.

Cookie which is already Created in Firefox browser

In this view, I have shown [“ASP.NET_SessionId”] cookie which is created in Firefox browser when the user logged in.

Note: For managing Cookie, I have installed Cookie Manager+ addon in Chrome browser.

After having a view on [“ASP.NET_SessionId”] cookie in Firefox now, let's create [“ASP.NET_SessionId”] cookie in Chrome browser similar to Cookie which is in Firefox browser with same [“ASP.NET_SessionId”] Cookie Name and Values.

Created New [“ASP.NET_SessionId”] Cookie in Chrome browser similar to Cookie created in Firefox browser.

This is a step where we have Fixed Session. This session is live on another browser[Firefox]. We have copied values similar to that and created a [“ASP.NET_SessionId”] Cookie and assign similar values of SessionID to this Cookie.

Note: For adding Cookie, I have installed Edit this Cookie addon in Chrome browser.

After fixing cookie, now we no longer require any login to the application if we just enter Inner URL of application, we get direct access because this session is created after authentication.

Solution

1. Remove [“ASP.NET_SessionId”] after logout
2. Securing Cookies
3. Use SSL for Securing Cookies and Session

Remove [“ASP.NET_SessionId”] after logout

On logout, we are removing Session values. Along with that, we are removing [“ASP.NET_SessionId”] Cookie from browser.

//
// POST: /Account/LogOff
publicActionResultLogOff()
{
//Removing Session
Session.Abandon();
Session.Clear();
Session.RemoveAll();

//Removing ASP.NET_SessionId Cookie
if (Request.Cookies["ASP.NET_SessionId"] != null)
    {
Response.Cookies["ASP.NET_SessionId"].Value = string.Empty;
Response.Cookies["ASP.NET_SessionId"].Expires = DateTime.Now.AddMonths(-10);
    }

if (Request.Cookies["AuthenticationToken"] != null)
    {
Response.Cookies["AuthenticationToken"].Value = string.Empty;
Response.Cookies["AuthenticationToken"].Expires = DateTime.Now.AddMonths(-10);
    }

returnRedirectToAction("Login", "Account");
}

Securing Cookie

For securing cookies On Login [HttpPost] Action method, we are going to create a New Session in that Session [Session["AuthenticationToken"]] we are going save NewGuid along with that, we are going to add a cookie with Name ["AuthenticationToken"] and it will also have same Value [Guid] which we have stored in Session.

Code Snippet

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
publicActionResult Login(LoginModel model, stringreturnUrl)
{
if (ModelState.IsValid)
    {   //Getting Pasword from Database
varstoredpassword = ReturnPassword(model.UserName);
// Comparing Password With Seed
if (ReturnHash(storedpassword, model.hdrandomSeed) == model.Password)
        {
Session["Username"] = model.UserName;
Session["UserID"] = 1;

// Getting New Guid
stringguid = Convert.ToString(Guid.NewGuid());
//Storing new Guid in Session
Session["AuthenticationToken"] = guid;
//Adding Cookie in Browser
Response.Cookies.Add(newHttpCookie("AuthenticationToken", guid));

returnRedirectToAction("Index", "Dashboard");
        }
else
        {
ModelState.AddModelError("", "The user name or password provided is incorrect.");
        }
    }
return View(model);
}

Code Snippet Description

Creating a new Guid.

// Getting New Guid
stringguid = Convert.ToString(Guid.NewGuid());

Saving a new Guid in Session.

//Storing new Guid in Session
Session["AuthenticationToken"] = guid;

Saving a new Guid in Cookie and adding.

//Adding Cookie in Browser
Response.Cookies.Add(newHttpCookie("AuthenticationToken", guid));

After storing data in session and adding a cookie in the browser, now let’s match these values on every request and check whether these values are similar. If not, then we are going to redirect to Login page.

For doing this part, I am going to add an AuthorizationFilter in the project and inside that, we are going to write logic for checking whether Session and cookie values are similar or not.

AuthenticateUser ActionFilter

If you check the below code snippet, I have created an AuthorizationFilter with name AuthenticateUser. In this filter we are inheriting IAuthorizationFilter Interface and FilterAttribute class. With this, we are implementing method inside interface [OnAuthorization] and write the whole logic in this method.

using System;
usingSystem.Web.Mvc;

namespaceMvcSecurity.Filters
{
publicclassAuthenticateUser : FilterAttribute, IAuthorizationFilter
    {
publicvoidOnAuthorization(AuthorizationContextfilterContext)
        {
stringTempSession =
Convert.ToString(filterContext.HttpContext.Session["AuthenticationToken"]);
stringTempAuthCookie =
Convert.ToString(filterContext.HttpContext.Request.Cookies["AuthenticationToken"].Value);

if (TempSession != null&&TempAuthCookie != null)
            {
if (!TempSession.Equals(TempAuthCookie))
                {
ViewResult result = newViewResult();
result.ViewName = "Login";
filterContext.Result = result;
                } 
            }
else
            {
ViewResult result = newViewResult();
result.ViewName = "Login";
filterContext.Result = result;
            }
        }
    }
}

Code Snippet Description

In this method, first we are going to get session and cookie values.

stringTempSession = Convert.ToString(filterContext.HttpContext.Session["AuthenticationToken"]);
stringTempAuthCookie = Convert.ToString(filterContext.HttpContext.Request.Cookies
                       ["AuthenticationToken"].Value);

After getting values from session and cookie, now we are going to check that both session and cookie values are not null. After that we are going to see values are equal of both session and a cookie, if they are not, then we are going to redirect to login page.

if (TempSession != null&&TempAuthCookie != null)
{
if (!TempSession.Equals(TempAuthCookie))
    {
ViewResult result = newViewResult();
result.ViewName = "Login";
filterContext.Result = result;
    } 
}
else
{
ViewResult result = newViewResult();
result.ViewName = "Login";
filterContext.Result = result;
}

After understanding code snippet, now we are going apply this filter on every controller which user accesses when he is logged into application.

Applying AuthenticateUser Filter

Apply this filter on every controller which user accesses when he is logged into the application.

After applying Action filter on all the controllers which users access after getting logged into the application.

Now if the attacker knows [“ASP.NET_SessionId”] cookie value and a new cookie [Cookies["AuthenticationToken"]] value, he will still not able to Session Fixation attack here because the new [Cookies["AuthenticationToken"]] contains GUID which is unique and same values is stored in Session [Session["AuthenticationToken"]] on Web Server but attacker can’t know the Session value that is stored on the web server and these values keep changing every time user is logging to application and the old session values which attacker used to do attack will not work in this scenario.

Finally, if we will allow those users access to the application who has valid Session["AuthenticationToken"] value and Cookies["AuthenticationToken"] value.

Realtime Values of both Cookies.

Use SSL for Securing Cookies and Session Values

SSL (Secure Sockets Layer) is a Layer which has Secure (Encrypted) communication between client and server such that any data [Banking details, Password, Session, Cookie and another financial transaction] passed from client and server is Secure (Encrypted).

10) Unvalidated Redirects and Forwards

In all web applications, we do redirect from one page to another page and sometimes, we redirect to another application too but while redirecting, we won't validate URL which we are going to redirect which caused Unvalidated Redirects and Forwards Attack.

This attack is mostly used to phish user to get valuable details (User Credentials) or to install malicious malware to the User computer.

Examples

In the below snapshot, you will see simple MVC application URL along with that, a created malicious URL by an attacker that redirects users to a malicious site that performs phishing and installs malware into user computers.

Original URL: http://localhost:7426/Account/Login

Crafter URL by Attacker: ?returnUrl=https://www.google.co.in

Attack Scenario

In this attack, user gets email from attacker which contains offer related to Ecommerce shopping. When user clicks on link given below, he is redirected to shopping site [http://demotop.com] but if you take a look at the URL closely, you will see that this URL contains redirect [http://demotop.com/Login/Login?url=http://mailicious.com]. Now, after entering valid Username and Password, the User will be redirected to malicious site [http://mailicious.com] which is similar to [http://demotop.com] shopping site. On the malicious site, it will show Message “Invalid Username and Password”, then again user will enter Username and Password and he will be redirected back to the original shopping site but meanwhile User Credentials are stolen in this attack.

Solution

1. Simply avoid using redirects and forwards.
2. If you still want to use redirects and forwards, then validate URL first.
3. Use Url.IsLocalUrl to prevent redirects and forwards in MVC.

Using Url.IsLocalUrlin MVC

Below is a snapshot of the login page in MVC which contains redirect URL to www.google.com when user enters username and password, then he will be redirected to www.google.com which is invalid. To prevent this in MVC4, we have built in method called as Url.IsLocalUrl which checks that redirect URL which is crafted is Local or not, if not, then it will not redirect it.

After understanding how redirect is passed, now let’s check how this URL gets checked and executed.

The below [HttpPost] Login method gets called when user enter Credentials and submits the form. Along with that, redirect URL is also gets posted which may contain malicious URL. For showing demo, I have just checked that username and password is not null. After that, we are calling RedirectToLocalAction method and to this method, we are going to pass redirect URL (returnUrl).

After passing URL (returnUrl) to RedirectToLocalAction method, then it is going pass URL to IsLocalUrl method [Url.IsLocalUrl(returnUrl)] which is going to check whether URL is local or not and return a boolean value. If it is not, then it will redirect to Home page, else will redirect to returnUrl which is passed.

[True if the URL is local]

[False if URL is not local]

History

• 3^rd August, 2016: Initial version