To get a trial key
fill out the form below
Team License (a basic version)
Enterprise License (an extended version)
* By clicking this button you agree to our Privacy Policy statement

Request our prices
New License
License Renewal
--Select currency--
* By clicking this button you agree to our Privacy Policy statement

Free PVS-Studio license for Microsoft MVP specialists
* By clicking this button you agree to our Privacy Policy statement

To get the licence for your open-source project, please fill out this form
* By clicking this button you agree to our Privacy Policy statement

I am interested to try it on the platforms:
* By clicking this button you agree to our Privacy Policy statement

Message submitted.

Your message has been sent. We will email you at

If you haven't received our response, please do the following:
check your Spam/Junk folder and click the "Not Spam" button for our message.
This way, you won't miss messages from our team in the future.

Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS)

Jul 09 2021

XSS (cross-site scripting) is a type of attack on web applications. Malicious code is injected into a web page. This code interacts with the attacker's web server. Malicious code can get to a page as a vulnerability in the web server, or on a user's computer. When a user opens an affected web page in a browser, the introduced code will run. For example, it can steal sensitive user data stored in the browser or on the page.

XSS is subdivided into several types by attack vector and way of exposure.

XSS Classification by Attack Vector

  • Reflected XSS (non-persistent). In this type of attack, a malicious script hits the page and executes when the page opens due to lack of proper processing. Most often the code appears in the page through HTTP query parameters or through HTML forms.
  • Stored XSS (persistent). This type of XSS is more dangerous than reflected. With this type of XSS, an attacker introduces malicious code to the server. Most often, data with malicious code is saved to a database and used on a page. As a result, every time a page is displayed in a browser, you run the introduced code.
  • XSS in the DOM model. In this type, malicious code appears when a JavaScript script executes in the user's browser and changes the DOM of the site being attacked. This is why this code executes in terms of the site.

XSS classification by way of exposure

  • Active XSS. As a user, you don't need any action other than to open the page to execute the malicious script.
  • Passive XSS. As a user, you need additional actions other than to open a page in a browser, such as hover or click on an HTML page item. This will execute the malicious code.

Example of XSS vulnerability

protected void Page_Load(object sender, EventArgs e)
  Response.Cookies.Add(new HttpCookie("User_Cookie_Key", 

  const string CenterAlignFormat = "<p style='text-align: center'>{0}</p>";

  var userName = Request.Params["userName"];                          //<=
  string message;
  if (string.IsNullOrWhiteSpace(userName))
    message = string.Format(CenterAlignFormat, 
                            "Empty 'userName' parameter");
    message = string.Format(CenterAlignFormat, 
                            $"'{userName}' data has been processed.");
  Response.Write(message);                                            //<=

PVS-Studio warning: V5610 Possible XSS vulnerability. Potentially tainted data in the 'message' variable might be used to execute a malicious script. Default.aspx.cs 61

Data from the UserName query parameter is used directly without additional processing to write to Response:


This allows an attacker to sneak a user a link with malicious code that, for example, will steal a cookie from the user's browser:


In the example above, cookies are simply displayed as a demonstration using the alert(document.cookie) expression. There is nothing stopping an attacker from sending them to their server, though. By taking advantage of stolen cookies, an attacker can access a user's account. This allows them to, for example, steal sensitive data or commit malicious acts on behalf of a user.

To fix such a XSS vulnerability, you only need to encode HTML entities in a message before writing to Response using a special method:

protected void Page_Load(object sender, EventArgs e)
    var encodedUserName = 
    message = string.Format(CenterAlignFormat,
                            $"'{encodedUserName}' data has been processed.");

This way, when you open a link with a malicious script, the latter will simply appear on the page but will not be executed:


Additional Resources

Popular related articles
The Ultimate Question of Programming, Refactoring, and Everything

Date: Apr 14 2016

Author: Andrey Karpov

Yes, you've guessed correctly - the answer is "42". In this article you will find 42 recommendations about coding in C++ that can help a programmer avoid a lot of errors, save time and effort. The au…
PVS-Studio ROI

Date: Jan 30 2019

Author: Andrey Karpov

Occasionally, we're asked a question, what monetary value the company will receive from using PVS-Studio. We decided to draw up a response in the form of an article and provide tables, which will sho…
How PVS-Studio Proved to Be More Attentive Than Three and a Half Programmers

Date: Oct 22 2018

Author: Andrey Karpov

Just like other static analyzers, PVS-Studio often produces false positives. What you are about to read is a short story where I'll tell you how PVS-Studio proved, just one more time, to be more atte…
Static analysis as part of the development process in Unreal Engine

Date: Jun 27 2017

Author: Andrey Karpov

Unreal Engine continues to develop as new code is added and previously written code is changed. What is the inevitable consequence of ongoing development in a project? The emergence of new bugs in th…
The Evil within the Comparison Functions

Date: May 19 2017

Author: Andrey Karpov

Perhaps, readers remember my article titled "Last line effect". It describes a pattern I've once noticed: in most cases programmers make an error in the last line of similar text blocks. Now I want t…
The Last Line Effect

Date: May 31 2014

Author: Andrey Karpov

I have studied many errors caused by the use of the Copy-Paste method, and can assure you that programmers most often tend to make mistakes in the last fragment of a homogeneous code block. I have ne…
Technologies used in the PVS-Studio code analyzer for finding bugs and potential vulnerabilities

Date: Nov 21 2018

Author: Andrey Karpov

A brief description of technologies used in the PVS-Studio tool, which let us effectively detect a large number of error patterns and potential vulnerabilities. The article describes the implementati…
The way static analyzers fight against false positives, and why they do it

Date: Mar 20 2017

Author: Andrey Karpov

In my previous article I wrote that I don't like the approach of evaluating the efficiency of static analyzers with the help of synthetic tests. In that article, I give the example of a code fragment…
Free PVS-Studio for those who develops open source projects

Date: Dec 22 2018

Author: Andrey Karpov

On the New 2019 year's eve, a PVS-Studio team decided to make a nice gift for all contributors of open-source projects hosted on GitHub, GitLab or Bitbucket. They are given free usage of PVS-Studio s…
Appreciate Static Code Analysis!

Date: Oct 16 2017

Author: Andrey Karpov

I am really astonished by the capabilities of static code analysis even though I am one of the developers of PVS-Studio analyzer myself. The tool surprised me the other day as it turned out to be sma…

Comments (0)

Next comments
This website uses cookies and other technology to provide you a more personalized experience. By continuing the view of our web-pages you accept the terms of using these files. If you don't want your personal data to be processed, please, leave this site.
Learn More →