CWE-87: Improper Neutralization of Alternate XSS Syntax
Official CWE-87 CWE context with Glexia analysis, remediation guidance, related CVEs, and ATT&CK context.
Glexia's Take
CWE-87: Improper Neutralization of Alternate XSS Syntax
Improper Neutralization of Alternate XSS Syntax represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Executive Impact
- Confidentiality,Integrity,Availability: Read Application Data,Execute Unauthorized Code or Commands
Developer Pattern
CWE-87 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.
Confidence
high confidence from CWE-87, 4.20.
Official CWE Definition
CWE-87: Improper Neutralization of Alternate XSS Syntax
The product does not neutralize or incorrectly neutralizes user-controlled input for alternate script syntax.
Developer And Remediation Guidance
How teams prevent and detect this weakness
Causes
- In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent: The code only works when the "script" tag is in all lower-case, forming an incomplete denylist (CWE-184). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method, allowing an XSS attack.
Remediation
- Implementation: Resolve all input to absolute or canonical representations before processing.
- 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 tag attributes, 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.
- Implementation: [object Object]
- Implementation: With Struts, write all data from form beans with the bean's filter attribute set to true.
- Implementation: 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.
Detection
- Code review
- SAST
- DAST
- Focused regression tests
Mappings
Related CVEs, CWEs, and ATT&CK context
Related CWEs
ATT&CK Relevance
ATT&CK relevance is shown only when reviewed or responsibly inferred.