CWE-87 Variant Draft

Improper Neutralization of Alternate XSS Syntax

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.

Definition

What is CWE-87?

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.
Traditional XSS defenses often focus on blocking common vectors like `` tags or basic event handlers. However, attackers can bypass these filters by using less common syntax, encoding techniques, or obscure HTML attributes that browsers still interpret as executable code. This includes alternative event handlers, JavaScript pseudo-protocols in unexpected places, or creatively malformed tags that slip past validation routines. To prevent this, developers must adopt a positive security model that defines allowed input, rather than just blocking known bad patterns. Input validation should be combined with context-aware output encoding, ensuring user data is treated as text, not code, regardless of the syntax used. Regular security testing with tools that probe for these alternate vectors is essential for robust defense.
Real-world impact

Real-world CVEs caused by CWE-87

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable Java

In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:

Vulnerable Java 
public String preventXSS(String input, String mask) {
  	return input.replaceAll("script", mask);
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-87

  • 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 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.
  • 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 signals

How to detect CWE-87

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-87 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-87?

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.

How serious is CWE-87?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-87?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-87?

Resolve all input to absolute or canonical representations before processing. 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…

How does Plexicus detect and fix CWE-87?

Plexicus's SAST engine matches the data-flow signature for CWE-87 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-87?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/87.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-87

CWE-79 Parent

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

This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…

CWE-80 Sibling

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

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user…

CWE-81 Sibling

Improper Neutralization of Script in an Error Message Web Page

This vulnerability occurs when an application fails to properly sanitize user-supplied input before displaying it within an error message…

CWE-83 Sibling

Improper Neutralization of Script in Attributes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize or block JavaScript URIs (like 'javascript:') within HTML tag…

CWE-84 Sibling

Improper Neutralization of Encoded URI Schemes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize user-supplied input that contains malicious scripts disguised…

CWE-85 Sibling

Doubled Character XSS Manipulations

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing…

CWE-86 Sibling

Improper Neutralization of Invalid Characters in Identifiers in Web Pages

This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag…

Resources

Further reading

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