CWE-828: Signal Handler with Functionality that is not Asynchronous-Safe
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Edit Custom FilterThe 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. This table specifies different individual consequences
associated with the weakness. The Scope identifies the application security area that is
violated, while the Impact describes the negative technical impact that arises if an
adversary succeeds in exploiting this weakness. The Likelihood provides information about
how likely the specific consequence is expected to be seen relative to the other
consequences in the list. For example, there may be high likelihood that a weakness will be
exploited to achieve a certain impact, but a low likelihood that it will be exploited to
achieve a different impact.
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Relevant to the view "Research Concepts" (CWE-1000)
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. (bad code)
Example Language: C
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:
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. (bad code)
Example Language: C
#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):
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].
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