CWE-364 Base Incomplete Medium likelihood

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 corrupted through asynchronous execution.

Definition

What is CWE-364?

A signal handler race condition occurs when a program's signal handling routine is vulnerable to timing issues, allowing its state to be corrupted through asynchronous execution.
Signal handlers are inherently risky because they can interrupt a program's normal execution at any point. If a handler modifies shared resources like global variables, uses non-reentrant functions (e.g., malloc, free, printf), or is registered for multiple signals, it can corrupt memory. This happens when the handler's actions clash with operations in the main code or other handlers, leading to use-after-free, double-free, or other memory corruption vulnerabilities that attackers can exploit for denial of service or code execution. To prevent these issues, design signal handlers to be minimal and reentrant. Avoid shared state, use only async-signal-safe functions, and consider blocking (masking) other signals within the handler to ensure atomicity. For resources that must be shared, implement proper synchronization or use a flag that the main program checks safely after the signal handler returns, moving complex logic out of the handler itself.
Real-world impact

Real-world CVEs caused by CWE-364

  • CVE-1999-0035

    Signal handler does not disable other signal handlers, allowing it to be interrupted, causing other functionality to access files/etc. with raised privileges

  • CVE-2001-0905

    Attacker can send a signal while another signal handler is already running, leading to crash or execution with root privileges

  • CVE-2001-1349

    unsafe calls to library functions from signal handler

  • CVE-2004-0794

    SIGURG can be used to remotely interrupt signal handler; other variants exist

  • CVE-2004-2259

    SIGCHLD signal to FTP server can cause crash under heavy load while executing non-reentrant functions like malloc/free.

How attackers exploit it

Step-by-step attacker path

  1. 1

    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.

  2. 2

    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:

  3. 3

    - 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.

  4. 4

    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.

  5. 5

    Note that this is an adaptation of a classic example as originally presented by Michal Zalewski [REF-360]; the original example was shown to be exploitable for code execution.

Vulnerable code example

Vulnerable C

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.

Vulnerable C 
char *logMessage;
  void handler (int sigNum) {
  		syslog(LOG_NOTICE, "%s\n", logMessage);
  		free(logMessage);
```
/* artificially increase the size of the timing window to make demonstration of this weakness easier. */* 
  		
  		sleep(10);
  		exit(0);}
  
  int main (int argc, char* argv[]) {
  ```
  		logMessage = strdup(argv[1]);
```
/* Register signal handlers. */* 
  		
  		signal(SIGHUP, handler);
  		signal(SIGTERM, handler);
  		
  		 */* artificially increase the size of the timing window to make demonstration of this weakness easier. */* 
  		
  		sleep(10);}
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-364

  • Requirements Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
  • Architecture and Design Design signal handlers to only set flags, rather than perform complex functionality. These flags can then be checked and acted upon within the main program loop.
  • Implementation Only use reentrant functions within signal handlers. Also, use validation to ensure that state is consistent while performing asynchronous actions that affect the state of execution.
Detection signals

How to detect CWE-364

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-364 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-364?

A signal handler race condition occurs when a program's signal handling routine is vulnerable to timing issues, allowing its state to be corrupted through asynchronous execution.

How serious is CWE-364?

MITRE rates the likelihood of exploit as Medium — exploitation is realistic but typically requires specific conditions.

What languages or platforms are affected by CWE-364?

MITRE lists the following affected platforms: C, C++.

How can I prevent CWE-364?

Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Design signal handlers to only set flags, rather than perform complex functionality. These flags can then be checked and acted upon within the main program loop.

How does Plexicus detect and fix CWE-364?

Plexicus's SAST engine matches the data-flow signature for CWE-364 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-364?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/364.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-364

CWE-362 Parent

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

A race condition occurs when multiple processes or threads access a shared resource simultaneously without proper coordination, creating a…

CWE-1223 Sibling

Race Condition for Write-Once Attributes

This vulnerability occurs when an untrusted software component wins a race condition and writes to a hardware register before the trusted…

CWE-1298 Sibling

Hardware Logic Contains Race Conditions

A hardware race condition occurs when security-critical logic circuits receive signals at slightly different times, creating temporary…

CWE-366 Sibling

Race Condition within a Thread

This vulnerability occurs when two or more threads within the same application access and manipulate a shared resource (like a variable,…

CWE-367 Sibling

Time-of-check Time-of-use (TOCTOU) Race Condition

This vulnerability occurs when a program verifies a resource's state (like a file's permissions or existence) but then uses it after that…

CWE-368 Sibling

Context Switching Race Condition

This vulnerability occurs when an application switches between different security contexts (like privilege levels or domains) using a…

CWE-421 Sibling

Race Condition During Access to Alternate Channel

A race condition occurs when an application opens a secondary communication channel intended for an authorized user, but fails to secure…

CWE-689 Sibling

Permission Race Condition During Resource Copy

This vulnerability occurs when a system copies a file or resource but delays setting its final permissions until the entire copy operation…

CWE-415 Can precede

Double Free

A double free vulnerability occurs when a program mistakenly calls the 'free()' function twice on the same block of memory.

Resources

Further reading

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