CWE-636 Class Draft

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 safer alternative. Examples include reverting to the weakest…

Definition

What is CWE-636?

This vulnerability occurs when a system, upon encountering an error or failure, defaults to its least secure configuration instead of a safer alternative. Examples include reverting to the weakest encryption or the most permissive access rules.
Failing open creates a dangerous gap where the system operates with known, weaker security controls. This directly introduces the vulnerabilities associated with that permissive state, making it significantly easier for an attacker to exploit the system. Often, this design choice is made to prioritize uptime and reduce support overhead, mistakenly valuing continuous functionality over security. This approach fundamentally undermines security posture and provides administrators with a false sense of protection. The secure alternative is to "fail closed" or "fail safe," where the system denies access or stops operations until the issue can be safely resolved, ensuring security is never automatically compromised for convenience.
Real-world impact

Real-world CVEs caused by CWE-636

  • CVE-2007-5277

    The failure of connection attempts in a web browser resets DNS pin restrictions. An attacker can then bypass the same origin policy by rebinding a domain name to a different IP address. This was an attempt to "fail functional."

  • CVE-2006-4407

    Incorrect prioritization leads to the selection of a weaker cipher. Although it is not known whether this issue occurred in implementation or design, it is feasible that a poorly designed algorithm could be a factor.

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-636

  • Architecture and Design Subdivide and allocate resources and components so that a failure in one part does not affect the entire product.
Detection signals

How to detect CWE-636

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-636?

This vulnerability occurs when a system, upon encountering an error or failure, defaults to its least secure configuration instead of a safer alternative. Examples include reverting to the weakest encryption or the most permissive access rules.

How serious is CWE-636?

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-636?

MITRE lists the following affected platforms: Not Technology-Specific, ICS/OT.

How can I prevent CWE-636?

Subdivide and allocate resources and components so that a failure in one part does not affect the entire product.

How does Plexicus detect and fix CWE-636?

Plexicus's SAST engine matches the data-flow signature for CWE-636 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-636?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/636.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-636

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)

This weakness occurs when a System-on-Chip (SoC) lacks a secure, unique, and permanent identifier for its internal hardware components (IP…

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-637 Sibling

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…

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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