CWE-479 Variant Draft Low likelihood

Signal Handler Use of a Non-reentrant Function

This vulnerability occurs when a signal handler in your code calls a function that is not safe to re-enter. If that function is interrupted and called again before it finishes, it can corrupt memory…

Definition

What is CWE-479?

This vulnerability occurs when a signal handler in your code calls a function that is not safe to re-enter. If that function is interrupted and called again before it finishes, it can corrupt memory and crash your program or create security weaknesses.
Non-reentrant functions rely on global data or static memory to do their work. When a signal interrupts such a function and the handler calls the same function again, both invocations compete for and corrupt that shared state. Common examples include `malloc()`, `free()`, and `syslog()`, which use internal scratch space or metadata to track operations. This corruption can leave your application in an unpredictable and potentially exploitable state. As a developer, you must ensure that only async-signal-safe functions are called from within a signal handler. The POSIX standard defines a specific list of these safe functions. Calling anything outside this list, especially standard library functions that manage memory or perform I/O, introduces this risk of re-entrancy corruption which can lead to denial of service or, in worst cases, allow an attacker to execute arbitrary code.
Real-world impact

Real-world CVEs caused by CWE-479

  • CVE-2005-0893

    signal handler calls function that ultimately uses malloc()

  • 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

    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 C

In this example, a signal handler uses syslog() to log a message:

Vulnerable C 
char *message;
  void sh(int dummy) {
  	syslog(LOG_NOTICE,"%s\n",message);
  	sleep(10);
  	exit(0);
  }
  int main(int argc,char* argv[]) {
  	...
  	signal(SIGHUP,sh);
  	signal(SIGTERM,sh);
  	sleep(10);
  	exit(0);
  }
  	If the execution of the first call to the signal handler is suspended after invoking syslog(), and the signal handler is called a second time, the memory allocated by syslog() enters an undefined, and possibly, exploitable state.
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-479

  • Requirements Require languages or libraries that provide reentrant functionality, or otherwise make it easier to avoid this weakness.
  • Architecture and Design Design signal handlers to only set flags rather than perform complex functionality.
  • Implementation Ensure that non-reentrant functions are not found in signal handlers.
  • Implementation Use sanity checks to reduce the timing window for exploitation of race conditions. This is only a partial solution, since many attacks might fail, but other attacks still might work within the narrower window, even accidentally.
Detection signals

How to detect CWE-479

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Plexicus auto-fix

Plexicus auto-detects CWE-479 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-479?

This vulnerability occurs when a signal handler in your code calls a function that is not safe to re-enter. If that function is interrupted and called again before it finishes, it can corrupt memory and crash your program or create security weaknesses.

How serious is CWE-479?

MITRE rates the likelihood of exploit as Low — exploitation is uncommon, but the weakness should still be fixed when discovered.

What languages or platforms are affected by CWE-479?

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

How can I prevent CWE-479?

Require languages or libraries that provide reentrant functionality, or otherwise make it easier to avoid this weakness. Design signal handlers to only set flags rather than perform complex functionality.

How does Plexicus detect and fix CWE-479?

Plexicus's SAST engine matches the data-flow signature for CWE-479 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-479?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/479.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-479

CWE-828 Parent

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…

CWE-123 Can precede

Write-what-where Condition

A write-what-where condition occurs when an attacker can control both the data written and the exact memory location where it's written,…

Resources

Further reading

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