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

Description

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: 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.

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

• CERT C Secure Coding: SIG31-C — Do not access or modify shared objects in signal handlers