logo

Improper Neutralization of Script in an Error Message Web Page

Incomplete Variant
Structure: Simple
Description

This vulnerability occurs when an application fails to properly sanitize user-supplied input before displaying it within an error message web page. If special characters used in web scripts are not neutralized, an attacker can inject malicious code that executes in a user's browser.

Extended Description

Many web applications use custom error pages, such as 403 Forbidden or 404 Not Found pages, which can unintentionally include unsanitized input from a user's request. When an attacker can trigger an error and their input—containing JavaScript or HTML—is reflected directly into the error page's response, it creates a classic Cross-Site Scripting (XSS) opportunity. To prevent this, developers must treat all data displayed in error pages as untrusted. This means rigorously validating and encoding or escaping any user-controlled input before it is rendered, even in error-handling contexts, to ensure script code is displayed as harmless text rather than executed by the browser.
Common Consequences 1
Scope: ConfidentialityIntegrityAvailability

Impact: Read Application DataExecute Unauthorized Code or Commands
Potential Mitigations 5
Phase: Implementation
Do not write user-controlled input to error pages.
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
Observed Examples 3
CVE-2002-0840XSS in default error page from Host: header.
CVE-2002-1053XSS in error message.
CVE-2002-1700XSS in error page from targeted parameter.
References 1
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Modes of Introduction
Implementation
Operation
Related Attack Patterns
Related Weaknesses
ChildOf: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (CWE-79)
CanAlsoBe: Generation of Error Message Containing Sensitive Information (CWE-209)
CanAlsoBe: Detection of Error Condition Without Action (CWE-390)
Taxonomy Mapping
  • PLOVER
  • Software Fault Patterns