CWE-364: Signal Handler Race Condition

Description

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": 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:

Potential Impact

Demonstrative Examples

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);
                     }

#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);
                     }

Mitigations & Prevention

Detection Methods

• 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 sea

Real-World CVE Examples

Related Weaknesses

Taxonomy Mappings

• PLOVER: — Signal handler race condition
• 7 Pernicious Kingdoms: — Signal Handling Race Conditions
• CLASP: — Race condition in signal handler
• Software Fault Patterns: SFP19 — Missing Lock