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CWE-831: Signal Handler Function Associated with Multiple Signals

Weakness ID: 831
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The software defines a function that is used as a handler for more than one signal.
+ Extended Description

While sometimes intentional and safe, when the same function is used to handle multiple signals, a race condition could occur if the function uses any state outside of its local declaration, such as global variables or non-reentrant functions, or has any side effects.

An attacker could send one signal that invokes the handler function; in many OSes, this will typically prevent the same signal from invoking the handler again, at least until the handler function has completed execution. However, the attacker could then send a different signal that is associated with the same handler function. This could interrupt the original handler function while it is still executing. If there is shared state, then the state could be corrupted. This can lead to a variety of potential consequences depending on context, including denial of service and code execution.

Another rarely-explored possibility arises when the signal handler is only designed to be executed once (if at all). By sending multiple signals, an attacker could invoke the function more than once. This may generate extra, unintended side effects. A race condition might not even be necessary; the attacker could send one signal, wait until it is handled, then send the other signal.

+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
ChildOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.364Signal Handler Race Condition
+ Relevant to the view "Software Development" (CWE-699)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.387Signal Errors
+ Common Consequences

The table below 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.

Access Control

Technical Impact: DoS: Crash, Exit, or Restart; Execute Unauthorized Code or Commands; Read Application Data; Gain Privileges or Assume Identity; Bypass Protection Mechanism; Varies by Context

The most common consequence will be a corruption of the state of the software, possibly leading to a crash or exit. However, if the signal handler is operating on state variables for security relevant libraries or protection mechanisms, the consequences can be far more severe, including protection mechanism bypass, privilege escalation, or information exposure.
+ Demonstrative Examples

Example 1

This code registers the same signal handler function with two different signals.

(bad code)
Example Language:
void handler (int sigNum) {

int main (int argc, char* argv[]) {
signal(SIGUSR1, handler)
signal(SIGUSR2, handler)

Example 2

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:
char *logMessage;

void handler (int sigNum) {
syslog(LOG_NOTICE, "%s\n", logMessage);
/* artificially increase the size of the timing window to make demonstration of this weakness easier. */


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. */


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.

+ References
[REF-360] Michal Zalewski. "Delivering Signals for Fun and Profit". <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2010-12-12CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2014-06-23CWE Content TeamMITRE
updated Demonstrative_Examples, References
2020-02-24CWE Content TeamMITRE
updated Relationships
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Page Last Updated: March 15, 2021