CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe

Extended Description

This can lead to an unexpected system state with a variety of potential consequences depending on context, including denial of service and code execution. Signal handlers are typically intended to interrupt normal functionality of a program, or even other signals, in order to notify the process of an event. When a signal handler uses global or static variables, or invokes functions that ultimately depend on such state or its associated metadata, then it could corrupt system state that is being used by normal functionality. This could subject the program to race conditions or other weaknesses that allow an attacker to cause the program state to be corrupted. While denial of service is frequently the consequence, in some cases this weakness could be leveraged for code execution. There are several different scenarios that introduce this issue: Invocation of non-reentrant functions from within the handler. One example is malloc(), which modifies internal global variables as it manages memory. Very few functions are actually reentrant. Code sequences (not necessarily function calls) contain non-atomic use of global variables, or associated metadata or structures, that can be accessed by other functionality of the program, including other signal handlers. Frequently, the same function is registered to handle multiple signals. The signal handler function is intended to run at most one time, but instead it can be invoked multiple times. This could happen by repeated delivery of the same signal, or by delivery of different signals that have the same handler function (CWE-831). Note that in some environments or contexts, it might be possible for the signal handler to be interrupted itself. If both a signal handler and the normal behavior of the product have to operate on the same set of state variables, and a signal is received in the middle of the normal execution's modifications of those variables, the variables may be in an incorrect or corrupt state during signal handler execution, and possibly still incorrect or corrupt upon return.

Code Examples & CVEs

Demonstrative Examples

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.

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:

BadC

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);                      }

The following code registers a signal handler with multiple signals in order to log when a specific event occurs and to free associated memory before exiting.

However, the following sequence of events may result in a double-free (CWE-415):

BadC

#include <signal.h>#include <syslog.h>#include <string.h>#include <stdlib.h>                      void *global1, *global2;char *what;void sh (int dummy) {                         syslog(LOG_NOTICE,"%s\n",what);free(global2);free(global1);                            /* Sleep statements added to expand timing window for race condition */                                                        sleep(10);exit(0);                      }                      int main (int argc,char* argv[]) {                         what=argv[1];global1=strdup(argv[2]);global2=malloc(340);signal(SIGHUP,sh);signal(SIGTERM,sh);                            /* Sleep statements added to expand timing window for race condition */                                                        sleep(10);exit(0);                      }

Observed CVE Examples (6)

CVE-2008-4109

Signal handler uses functions that ultimately call the unsafe syslog/malloc/s*printf, leading to denial of service via multiple login attempts

View Details

CVE-2006-5051

Chain: Signal handler contains too much functionality (CWE-828), introducing a race condition (CWE-362) that leads to a double free (CWE-415).

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CVE-2001-1349

unsafe calls to library functions from signal handler

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CVE-2004-0794

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

View Details

CVE-2004-2259

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

View Details

CVE-2002-1563

SIGCHLD not blocked in a daemon loop while counter is modified, causing counter to get out of sync.

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-828 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-828, Signal Handler with Functionality that is not Asynchronous-Safe, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Signal Handler with Functionality that is not Asynchronous-Safe during Implementation

MITRE associates mitigation with Implementation, and Architecture and Design; documented detection approaches include Automated Static Analysis; recorded impacts include DoS: Crash, Exit, or Restart, and Execute Unauthorized Code or Commands. Use these source-defined anchors to turn CWE-828 into implementation, review, and verification checks for the affected component.

CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe — MITRE CWE

Triage CWE-828 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-828, Signal Handler with Functionality that is not Asynchronous-Safe, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-828

Apply MITRE's full root-cause mapping guidance when using CWE-828, Signal Handler with Functionality that is not Asynchronous-Safe. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-828 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-828, Signal Handler with Functionality that is not Asynchronous-Safe. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe?+

CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product defines a signal handler that contains code sequences that are not asynchronous-safe, i.e., the functionality is not reentrant, or it can be interrupted. This can lead to an unexpected system state with a variety of potential consequences depending on context, including denial of service and code execution. Signal handlers are typically intended to interrupt normal functionality of a program, or even other signals, in order to notify the process of an event. When a signal handler uses global or static variables, or invokes functions that ultimately depend on such state or its associated metadata, then it could corrupt system state that is being used by normal functionality. This could subject the program to race conditions or other weaknesses that allow an attacker to cause the program state to be corrupted. While denial of service is frequently the consequence, in some cases this weakness could be leveraged for code execution. There are several different scenarios that introduce this issue: Invocation of non-reentrant functions from within the handler. One example is malloc(), which modifies internal global variables as it manages memory. Very few functions are actually reentrant. Code sequences (not necessarily function calls) contain non-atomic use of global variables, or associated metadata or structures, that can be accessed by other functionality of the program, including other signal handlers. Frequently, the same function is registered to handle multiple signals. The signal handler function is intended to run at most one time, but instead it can be invoked multiple times. This could happen by repeated delivery of the same signal, or by delivery of different signals that have the same handler function (CWE-831). Note that in some environments or contexts, it might be possible for the signal handler to be interrupted itself. If both a signal handler and the normal behavior of the product have to operate on the same set of state variables, and a signal is received in the middle of the normal execution's modifications of those variables, the variables may be in an incorrect or corrupt state during signal handler execution, and possibly still incorrect or corrupt upon return.

What are the security consequences of Signal Handler with Functionality that is not Asynchronous-Safe?+

If exploited, CWE-828 (Signal Handler with Functionality that is not Asynchronous-Safe) it can compromise Integrity, Confidentiality and Availability, leading to outcomes such as DoS: Crash, Exit, or Restart and Execute Unauthorized Code or Commands.

How do you prevent or mitigate Signal Handler with Functionality that is not Asynchronous-Safe?+

Recommended mitigations for CWE-828 include: Eliminate the usage of non-reentrant functionality inside of signal handlers. This includes replacing all non-reentrant library calls with reentrant calls. Note: This will not always be possible and may require large portions of the product to be rewritten or even redesigned. Sometimes reentrant-safe library alternatives will not be available. Sometimes non-reentrant interaction between the state of the system and the signal handler will be required by design. Where non-reentrant functionality must be leveraged within a signal handler, be sure to block or mask signals appropriately. This includes blocking other signals within the signal handler itself that may also leverage the functionality. It also includes blocking all signals reliant upon the functionality when it is being accessed or modified by the normal behaviors of the product.

Which programming languages are affected by Signal Handler with Functionality that is not Asynchronous-Safe?+

CWE-828 commonly affects C and Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Signal Handler with Functionality that is not Asynchronous-Safe?+

MITRE documents real CVEs mapped to CWE-828, including CVE-2008-4109, CVE-2006-5051, CVE-2001-1349, CVE-2004-0794 and CVE-2004-2259. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-828 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Phase

Description

Effectiveness

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (6)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-828 with SSDF evidence

Address Signal Handler with Functionality that is not Asynchronous-Safe during Implementation

Triage CWE-828 against known exploitation evidence

Apply precise root-cause mapping to CWE-828

Validate CWE-828 with root-cause mapping checks

Frequently Asked Questions

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