CWE-831 Variant Incomplete

Signal Handler Function Associated with Multiple Signals

This vulnerability occurs when a single function is registered to handle multiple different operating system signals, creating potential race conditions if that function isn't carefully designed.

Definition

What is CWE-831?

This vulnerability occurs when a single function is registered to handle multiple different operating system signals, creating potential race conditions if that function isn't carefully designed.
When the same function handles multiple signals, an attacker can exploit timing issues by sending a second signal while the handler is still processing the first. If the handler uses shared resources like global variables or non-reentrant functions, this interruption can corrupt program state, potentially leading to crashes, denial of service, or even code execution. Even without classic race conditions, this pattern creates risk. Attackers can trigger the handler multiple times by sending different signals sequentially, causing unintended side effects. This is especially dangerous for handlers designed to run only once, as forced re-execution can bypass security controls or destabilize the application.
Real-world impact

Real-world CVEs caused by CWE-831

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

  1. 1

    This code registers the same signal handler function with two different signals.

  2. 2

    This code registers the same signal handler function with two different signals (CWE-831). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits.

  3. 3

    The handler function uses global state (globalVar and logMessage), and it can be called by both the SIGHUP and SIGTERM signals. An attack scenario might follow these lines:

  4. 4

    - The program begins execution, initializes logMessage, and registers the signal handlers for SIGHUP and SIGTERM. - The program begins its "normal" functionality, which is simplified as sleep(), but could be any functionality that consumes some time. - The attacker sends SIGHUP, which invokes handler (call this "SIGHUP-handler"). - SIGHUP-handler begins to execute, calling syslog(). - syslog() calls malloc(), which is non-reentrant. malloc() begins to modify metadata to manage the heap. - The attacker then sends SIGTERM. - SIGHUP-handler is interrupted, but syslog's malloc call is still executing and has not finished modifying its metadata. - The SIGTERM handler is invoked. - SIGTERM-handler records the log message using syslog(), then frees the logMessage variable.

  5. 5

    At this point, the state of the heap is uncertain, because malloc is still modifying the metadata for the heap; the metadata might be in an inconsistent state. The SIGTERM-handler call to free() is assuming that the metadata is inconsistent, possibly causing it to write data to the wrong location while managing the heap. The result is memory corruption, which could lead to a crash or even code execution, depending on the circumstances under which the code is running.

Vulnerable code example

Vulnerable C

This code registers the same signal handler function with two different signals.

Vulnerable C 
void handler (int sigNum) {
  	...
  }
  int main (int argc, char* argv[]) {
  	signal(SIGUSR1, handler)
  	signal(SIGUSR2, handler)
  }
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-831

  • 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-831

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-831 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-831?

This vulnerability occurs when a single function is registered to handle multiple different operating system signals, creating potential race conditions if that function isn't carefully designed.

How serious is CWE-831?

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-831?

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

How can I prevent CWE-831?

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-831?

Plexicus's SAST engine matches the data-flow signature for CWE-831 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-831?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/831.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-831

CWE-364 Parent

Signal Handler Race Condition

A signal handler race condition occurs when a program's signal handling routine is vulnerable to timing issues, allowing its state to be…

CWE-432 Sibling

Dangerous Signal Handler not Disabled During Sensitive Operations

This vulnerability occurs when a program's signal handler, which shares resources like global variables with other handlers, can be…

CWE-828 Sibling

Signal Handler with Functionality that is not Asynchronous-Safe

This weakness occurs when a program's signal handler contains code that is not asynchronous-safe. This means the handler can be…

Resources

Further reading

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