CWE-804 Base Incomplete

Guessable CAPTCHA

This vulnerability occurs when a CAPTCHA challenge is too easy for automated bots to solve, either by guessing or using pattern recognition, allowing them to bypass the human verification step.

Definition

What is CWE-804?

This vulnerability occurs when a CAPTCHA challenge is too easy for automated bots to solve, either by guessing or using pattern recognition, allowing them to bypass the human verification step.
A guessable CAPTCHA defeats its core purpose by letting automated scripts mimic human users. Attackers can exploit this weakness to perform actions at an inhuman scale, such as mass account creation, comment spam, or credential stuffing attacks, overwhelming your system's defenses. Common design flaws that make CAPTCHAs vulnerable include using simple, undistorted images or audio; asking predictable questions with limited answers (like math problems or birth years); using trivia with answers available in public databases; or embedding hints in metadata, such as including the solution in an image's filename.
Real-world impact

Real-world CVEs caused by CWE-804

  • CVE-2022-4036

    Chain: appointment booking app uses a weak hash (CWE-328) for generating a CAPTCHA, making it guessable (CWE-804)

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-804

  • 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-804

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

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Frequently asked questions

Frequently asked questions

What is CWE-804?

This vulnerability occurs when a CAPTCHA challenge is too easy for automated bots to solve, either by guessing or using pattern recognition, allowing them to bypass the human verification step.

How serious is CWE-804?

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-804?

MITRE lists the following affected platforms: Web Server.

How can I prevent CWE-804?

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-804?

Plexicus's SAST engine matches the data-flow signature for CWE-804 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-804?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/804.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-804

CWE-863 Parent

Incorrect Authorization

This vulnerability occurs when an application checks if a user is allowed to perform an action or access data, but the check is flawed or…

CWE-1244 Sibling

Internal Asset Exposed to Unsafe Debug Access Level or State

This vulnerability occurs when a system's debug or test interface supports multiple access levels, but an internal asset is incorrectly…

CWE-551 Sibling

Incorrect Behavior Order: Authorization Before Parsing and Canonicalization

This vulnerability occurs when a web server checks access permissions before fully processing and normalizing a URL, potentially allowing…

CWE-639 Sibling

Authorization Bypass Through User-Controlled Key

This vulnerability occurs when an application's authorization system fails to verify that a user is allowed to access specific data before…

CWE-647 Sibling

Use of Non-Canonical URL Paths for Authorization Decisions

This vulnerability occurs when an application's authorization logic relies on specific URL paths but fails to enforce a single,…

CWE-942 Sibling

Permissive Cross-domain Security Policy with Untrusted Domains

This vulnerability occurs when a web application's cross-domain security policy, like a Content Security Policy (CSP), explicitly allows…

Resources

Further reading

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