Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

Description

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user input containing HTML and JavaScript tags. When untrusted data containing characters like <, >, and & is rendered directly into a webpage, a browser may execute it as active code, not just display it as text.

Extended Description

At its core, this flaw allows attackers to inject malicious scripts into web pages viewed by other users. These scripts execute within the victim's browser context, enabling actions like stealing session cookies, logging keystrokes, defacing websites, or redirecting users to malicious sites. The risk is highest in any part of an application that displays user-provided data without proper validation, such as comment sections, user profiles, or search result pages. Preventing Basic XSS requires a consistent strategy of output encoding or contextual escaping. Instead of trying to filter out 'bad' input, you should proactively encode all user-controlled data before inserting it into HTML. This means converting special characters into their safe HTML entity equivalents (e.g., < for <). Rely on well-tested libraries for your framework to perform this encoding, and never use insecure methods like `innerHTML` or `document.write()` with raw user input.

Common Consequences 1

Scope: ConfidentialityIntegrityAvailability

Impact: Read Application DataExecute Unauthorized Code or Commands

An attacker could insert special characters that are processed client-side in the context of the user's session.

Detection Methods 1

Automated Static AnalysisHigh

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Potential Mitigations 4

Phase: Implementation

Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.

Phase: Implementation

Strategy: Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.

Phase: Implementation

With Struts, write all data from form beans with the bean's filter attribute set to true.

Phase: Implementation

Strategy: Attack Surface Reduction

To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.

Effectiveness: Defense in Depth

Demonstrative Examples 1

In the following example, a guestbook comment isn't properly encoded, filtered, or otherwise neutralized for script-related tags before being displayed in a client browser.

Code Example:Bad
JSP
jsp

Observed Examples 4

CVE-2002-0938XSS in parameter in a link.

CVE-2002-1495XSS in web-based email product via attachment filenames.

CVE-2003-1136HTML injection in posted message.

CVE-2004-2171XSS not quoted in error page.

Likelihood of Exploit

High

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Attack Patterns

Related Weaknesses

ChildOf:

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

Taxonomy Mapping

PLOVER
Software Fault Patterns

Notes

OtherAlthough this can be classified as an injection problem, the more pertinent issue is the improper conversion of such special characters to respective context-appropriate entities before displaying them to the user.