CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Description:
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Extended Description:
There are many variants of cross-site scripting, characterized by a variety of terms or involving different attack topologies. However, they all indicate the same fundamental weakness: improper neutralization of dangerous input between the adversary and a victim.

Affected languages: Not Language-Specific
Affected technologies: AI/ML, Web Based, Web Server
Parent weaknesses: CWE-74, CWE-74

Consequences:
  - Access Control: Bypass Protection Mechanism
  - Integrity: Execute Unauthorized Code or Commands
  - Confidentiality: Execute Unauthorized Code or Commands

Detection methods:
  - Automated Static Analysis: Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 10
  - Black Box: Use the XSS Cheat Sheet [REF-714] or automated test-generation tools to help launch a wide variety of attacks against your web application. The Cheat Sheet contains many subtle XSS variations that are

Mitigations:
  - [Architecture and Design] Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482]. 
                   Examples of libraries and frameworks that make it easier to generate properly encoded output include Microsoft's Anti-XSS librar
  - [Implementation] Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messa
  - [Architecture and Design] Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external syste
  - [Architecture and Design] For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-s

Demonstrative examples:
The following code displays a welcome message on a web page based on the HTTP GET username parameter (covers a Reflected XSS (Type 1) scenario).
Because the parameter can be arbitrary, the url of the page could be modified so $username contains scripting syntax, such as
$username = $_GET['username']; echo '<div class="header"> Welcome, ' . $username . '</div>';
http://trustedSite.example.com/welcome.php?username=<Script Language="Javascript">alert("You've been attacked!");</Script>

The following code displays a Reflected XSS (Type 1) scenario.
The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.
<% String eid = request.getParameter("eid"); %> ... Employee ID: <%= eid %>
<% protected System.Web.UI.WebControls.TextBox Login; protected System.Web.UI.WebControls.Label EmployeeID; ... EmployeeID.Text = Login.Text; %> 
                      <p><asp:label id="EmployeeID" runat="server" /></p>

Real-world examples:
  - CVE-2024-49038: XSS in AI assistant
  - CVE-2024-54142: Plugin that enables AI features allows input with html entities, leading to XSS
  - CVE-2021-25926: Python Library Manager did not sufficiently neutralize a user-supplied search term, allowing reflected XSS.
  - CVE-2021-25963: Python-based e-commerce platform did not escape returned content on error pages, allowing for reflected Cross-Site Scripting attacks.
  - CVE-2021-1879: Universal XSS in mobile operating system, as exploited in the wild per CISA KEV.