CWE-637 Class Draft

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

This weakness occurs when a security feature is implemented with excessive complexity, creating unnecessary risk. Overly intricate protection mechanisms are harder to understand, configure, and…

Definition

What is CWE-637?

This weakness occurs when a security feature is implemented with excessive complexity, creating unnecessary risk. Overly intricate protection mechanisms are harder to understand, configure, and implement correctly, often leading to security gaps and misconfigurations.
Security controls should follow the principle of 'economy of mechanism'—the simpler they are, the more reliable they become. Complex designs increase the chance of implementation errors, compatibility issues, and mismatches between the intended security model and its real-world deployment. Developers are more likely to correctly implement, test, and maintain straightforward solutions. This principle also applies to data structures and validation logic. Overly complex data specifications force you to write equally complex validation code, which is prone to bugs. Whenever possible, favor simple system architectures and data models, as they naturally require simpler and more robust security checks.
Real-world impact

Real-world CVEs caused by CWE-637

  • CVE-2007-6067

    Support for complex regular expressions leads to a resultant algorithmic complexity weakness (CWE-407).

  • CVE-2007-1552

    Either a filename extension and a Content-Type header could be used to infer the file type, but the developer only checks the Content-Type, enabling unrestricted file upload (CWE-434).

  • CVE-2007-6479

    In Apache environments, a "filename.php.gif" can be redirected to the PHP interpreter instead of being sent as an image/gif directly to the user. Not knowing this, the developer only checks the last extension of a submitted filename, enabling arbitrary code execution.

  • CVE-2005-2148

    The developer cleanses the $_REQUEST superglobal array, but PHP also populates $_GET, allowing attackers to bypass the protection mechanism and conduct SQL injection attacks against code that uses $_GET.

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-637

  • Architecture and Design Avoid complex security mechanisms when simpler ones would meet requirements. Avoid complex data models, and unnecessarily complex operations. Adopt architectures that provide guarantees, simplify understanding through elegance and abstraction, and that can be implemented similarly. Modularize, isolate and do not trust complex code, and apply other secure programming principles on these modules (e.g., least privilege) to mitigate vulnerabilities.
Detection signals

How to detect CWE-637

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-637 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-637?

This weakness occurs when a security feature is implemented with excessive complexity, creating unnecessary risk. Overly intricate protection mechanisms are harder to understand, configure, and implement correctly, often leading to security gaps and misconfigurations.

How serious is CWE-637?

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-637?

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

How can I prevent CWE-637?

Avoid complex security mechanisms when simpler ones would meet requirements. Avoid complex data models, and unnecessarily complex operations. Adopt architectures that provide guarantees, simplify understanding through elegance and abstraction, and that can be implemented similarly. Modularize, isolate and do not trust complex code, and apply other secure programming principles on these modules (e.g., least privilege) to mitigate vulnerabilities.

How does Plexicus detect and fix CWE-637?

Plexicus's SAST engine matches the data-flow signature for CWE-637 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-637?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/637.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-637

CWE-657 Parent

Violation of Secure Design Principles

This weakness occurs when a system's architecture or design fails to follow fundamental security principles, creating a flawed foundation…

CWE-1192 Sibling

Improper Identifier for IP Block used in System-On-Chip (SOC)

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CWE-1395 Sibling

Dependency on Vulnerable Third-Party Component

This vulnerability occurs when your software relies on an external library, framework, or module that contains known security flaws.

CWE-250 Sibling

Execution with Unnecessary Privileges

This vulnerability occurs when software runs with higher permissions than it actually needs to perform its tasks. This excessive privilege…

CWE-636 Sibling

Not Failing Securely ('Failing Open')

This vulnerability occurs when a system, upon encountering an error or failure, defaults to its least secure configuration instead of a…

CWE-638 Sibling

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.…

CWE-653 Sibling

Improper Isolation or Compartmentalization

This vulnerability occurs when an application fails to enforce strong boundaries between components that operate at different security…

CWE-654 Sibling

Reliance on a Single Factor in a Security Decision

This vulnerability occurs when a system's security check depends almost entirely on just one condition, object, or piece of data to decide…

CWE-655 Sibling

Insufficient Psychological Acceptability

This weakness occurs when security features are so cumbersome or confusing that well-intentioned users feel forced to turn them off or…

Resources

Further reading

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