CWE-638 Class Draft

Not Using Complete Mediation

This vulnerability occurs when software fails to verify access permissions every single time a user or process tries to use a resource. Instead, it might rely on a single, outdated check, creating a…

Definition

What is CWE-638?

This vulnerability occurs when software fails to verify access permissions every single time a user or process tries to use a resource. Instead, it might rely on a single, outdated check, creating a security gap if the user's privileges are later revoked or changed.
Complete mediation is the security principle that every access to every object must be checked for authorization. Developers often break this rule by caching an initial permission check—like when a user first opens a file or page—and then assuming that permission remains valid for the entire session. This creates a time-of-check to time-of-use (TOCTOU) window where a user's reduced privileges are not enforced, allowing continued access they should no longer have. To prevent this, implement a consistent authorization layer that validates permissions on each individual request, regardless of previous checks. Treat all access as stateless and never assume a prior grant is still valid. This is especially critical in long-running sessions, multi-step transactions, or systems with dynamic user roles, where privilege changes must be reflected immediately to close security loopholes.
Real-world impact

Real-world CVEs caused by CWE-638

  • CVE-2007-0408

    Server does not properly validate client certificates when reusing cached connections.

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-638

  • Architecture and Design Invalidate cached privileges, file handles or descriptors, or other access credentials whenever identities, processes, policies, roles, capabilities or permissions change. Perform complete authentication checks before accepting, caching and reusing data, dynamic content and code (scripts). Avoid caching access control decisions as much as possible.
  • Architecture and Design Identify all possible code paths that might access sensitive resources. If possible, create and use a single interface that performs the access checks, and develop code standards that require use of this interface.
Detection signals

How to detect CWE-638

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-638 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-638?

This vulnerability occurs when software fails to verify access permissions every single time a user or process tries to use a resource. Instead, it might rely on a single, outdated check, creating a security gap if the user's privileges are later revoked or changed.

How serious is CWE-638?

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-638?

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

How can I prevent CWE-638?

Invalidate cached privileges, file handles or descriptors, or other access credentials whenever identities, processes, policies, roles, capabilities or permissions change. Perform complete authentication checks before accepting, caching and reusing data, dynamic content and code (scripts). Avoid caching access control decisions as much as possible. Identify all possible code paths that might access sensitive resources. If possible, create and use a single interface that performs the access…

How does Plexicus detect and fix CWE-638?

Plexicus's SAST engine matches the data-flow signature for CWE-638 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-638?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/638.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-638

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Unnecessary Complexity in Protection Mechanism (Not Using 'Economy of Mechanism')

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CWE-653 Sibling

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Resources

Further reading

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