Example - 1

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.

// /* artificially increase the size of the timing window to make demonstration of this weakness easier. */// 
syslog(LOG_NOTICE, "%s\n", logMessage);free(logMessage);sleep(10);exit(0);

// /* Register signal handlers. */// 
// /* artificially increase the size of the timing window to make demonstration of this weakness easier. */// 
logMessage = strdup(argv[1]);signal(SIGHUP, handler);signal(SIGTERM, handler);sleep(10);char *logMessage;void handler (int sigNum) {}int main (int argc, char* argv[]) {}

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.

Example - 2

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.

// /* Sleep statements added to expand timing window for race condition */// 
syslog(LOG_NOTICE,"%s\n",what);free(global2);free(global1);sleep(10);exit(0);

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

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

a SIGHUP is delivered to the processsh() is invoked to process the SIGHUPThis first invocation of sh() reaches the point where global1 is freedAt this point, a SIGTERM is sent to the processthe second invocation of sh() might do another free of global1this 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].