CWE-692 Compound Draft

Incomplete Denylist to Cross-Site Scripting

This vulnerability occurs when an application relies on an incomplete denylist to block cross-site scripting (XSS) attacks, leaving the door open for attackers to craft payloads that bypass the…

Definition

What is CWE-692?

This vulnerability occurs when an application relies on an incomplete denylist to block cross-site scripting (XSS) attacks, leaving the door open for attackers to craft payloads that bypass the filter.
Preventing XSS is trickier than it seems because modern web browsers and HTML parsers handle content in many different ways. An incomplete denylist, which only blocks a known set of dangerous patterns, cannot account for all the possible encoding tricks, HTML variations, and browser quirks that attackers can exploit. This creates a false sense of security. Attackers frequently use resources like the 'XSS Cheat Sheet' to find bypass techniques that evade common denylists. To effectively prevent XSS, developers should move away from denylists and instead adopt positive security models like context-aware output encoding and strict Content Security Policies (CSP).
Real-world impact

Real-world CVEs caused by CWE-692

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 pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
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-692

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-692

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-692 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.

Get a demo Try Plexicus free
Frequently asked questions

Frequently asked questions

What is CWE-692?

This vulnerability occurs when an application relies on an incomplete denylist to block cross-site scripting (XSS) attacks, leaving the door open for attackers to craft payloads that bypass the filter.

How serious is CWE-692?

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-692?

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

How can I prevent CWE-692?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-692?

Plexicus's SAST engine matches the data-flow signature for CWE-692 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-692?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/692.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-692

CWE-184 Parent

Incomplete List of Disallowed Inputs

This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but…

Resources

Further reading

Ready when you are

Don't Let Security
Weigh You Down.

Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.