The product uses a signal handler that introduces a race condition.

Race conditions frequently occur in signal handlers, since signal handlers support asynchronous actions. These race conditions have a variety of root causes and symptoms. Attackers may be able to exploit a signal handler race condition to cause the product state to be corrupted, possibly leading to a denial of service or even code execution.
These issues occur when non-reentrant functions, or state-sensitive actions occur in the signal handler, where they may be called at any time. These behaviors can violate assumptions being made by the "regular" code that is interrupted, or by other signal handlers that may also be invoked. If these functions are called at an inopportune moment - such as while a non-reentrant function is already running - memory corruption could occur that may be exploitable for code execution. Another signal race condition commonly found occurs when free is called within a signal handler, resulting in a double free and therefore a write-what-where condition. Even if a given pointer is set to NULL after it has been freed, a race condition still exists between the time the memory was freed and the pointer was set to NULL. This is especially problematic if the same signal handler has been set for more than one signal -- since it means that the signal handler itself may be reentered.
There are several known behaviors related to signal handlers that have received the label of "signal handler race condition":

Shared state (e.g. global data or static variables) that are accessible to both a signal handler and "regular" code
Shared state between a signal handler and other signal handlers
Use of non-reentrant functionality within a signal handler - which generally implies that shared state is being used. For example, malloc() and free() are non-reentrant because they may use global or static data structures for managing memory, and they are indirectly used by innocent-seeming functions such as syslog(); these functions could be exploited for memory corruption and, possibly, code execution.
Association of the same signal handler function with multiple signals - which might imply shared state, since the same code and resources are accessed. For example, this can be a source of double-free and use-after-free weaknesses.
Use of setjmp and longjmp, or other mechanisms that prevent a signal handler from returning control back to the original functionality
While not technically a race condition, some signal handlers are designed to be called at most once, and being called more than once can introduce security problems, even when there are not any concurrent calls to the signal handler. This can be a source of double-free and use-after-free weaknesses.

Signal handler vulnerabilities are often classified based on the absence of a specific protection mechanism, although this style of classification is discouraged in CWE because programmers often have a choice of several different mechanisms for addressing the weakness. Such protection mechanisms may preserve exclusivity of access to the shared resource, and behavioral atomicity for the relevant code:

Avoiding shared state
Using synchronization in the signal handler
Using synchronization in the regular code
Disabling or masking other signals, which provides atomicity (which effectively ensures exclusivity)

Primary

Resultant

Implementation

Medium

Integrity
Confidentiality
Availability
Modify Application Data
Modify Memory
DoS: Crash, Exit, or Restart
Execute Unauthorized Code or Commands
It may be possible to cause data corruption and possibly execute arbitrary code by modifying global variables or data structures at unexpected times, violating the assumptions of code that uses this global data.

Access Control
Gain Privileges or Assume Identity
If a signal handler interrupts code that is executing with privileges, it may be possible that the signal handler will also be executed with elevated privileges, possibly making subsequent exploits more severe.

Automated Static Analysis
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.)

Requirements
Language Selection
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.

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.

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 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:

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.

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.
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.
Also note that the strdup(argv[1]) call contains a potential buffer over-read (CWE-126) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array.

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.

#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);
}

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

a SIGHUP is delivered to the process

sh() is invoked to process the SIGHUP

This first invocation of sh() reaches the point where global1 is freed

At this point, a SIGTERM is sent to the process

the second invocation of sh() might do another free of global1

this results in a double-free (CWE-415)

This is just one possible exploitation of the above code. As another example, the syslog call may use malloc calls which are not async-signal safe. This could cause corruption of the heap management structures. For more details, consult the example within "Delivering Signals for Fun and Profit" [REF-360].

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
https://www.cve.org/CVERecord?id=CVE-1999-0035

CVE-2001-0905
Attacker can send a signal while another signal handler is already running, leading to crash or execution with root privileges
https://www.cve.org/CVERecord?id=CVE-2001-0905

CVE-2001-1349
unsafe calls to library functions from signal handler
https://www.cve.org/CVERecord?id=CVE-2001-1349

CVE-2004-0794
SIGURG can be used to remotely interrupt signal handler; other variants exist
https://www.cve.org/CVERecord?id=CVE-2004-0794

CVE-2004-2259
SIGCHLD signal to FTP server can cause crash under heavy load while executing non-reentrant functions like malloc/free.
https://www.cve.org/CVERecord?id=CVE-2004-2259

Signals
Interprocess Communication

System Process

Signal handler race condition

Signal Handling Race Conditions

Race condition in signal handler

SFP19
Missing Lock

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Applicable_Platforms, Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings

CWE Content Team
MITRE
2010-09-27
updated Observed_Examples, References

CWE Content Team
MITRE
2010-12-13
updated Common_Consequences, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations, Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Demonstrative_Examples, References, Relationships

CWE Content Team
MITRE
2014-06-23
updated Demonstrative_Examples, References

CWE Content Team
MITRE
2014-07-30
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2017-11-08
updated Observed_Examples, Relationships

CWE Content Team
MITRE
2020-02-24
updated References, Relationships

CWE Content Team
MITRE
2021-03-15
updated Potential_Mitigations

CWE Content Team
MITRE
2022-04-28
updated Relationships, Research_Gaps

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated References, Relationships, Time_of_Introduction

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Detection_Factors, Weakness_Ordinalities