Common Weakness Enumeration

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CWE-364: Signal Handler Race Condition

Weakness ID: 364
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software uses a signal handler that introduces a race condition.

Extended Description

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 software 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":

  • Shared state (e.g. global data or static variables) that are accessible to both a signal handler and "regular" code

  • Shared state between a signal handler and other signal handlers

  • Use of non-reentrant functionality within a signal handler - which generally implies that shared state is being used. For example, malloc() and free() are non-reentrant because they may use global or static data structures for managing memory, and they are indirectly used by innocent-seeming functions such as syslog(); these functions could be exploited for memory corruption and, possibly, code execution.

  • Association of the same signal handler function with multiple signals - which might imply shared state, since the same code and resources are accessed. For example, this can be a source of double-free and use-after-free weaknesses.

  • Use of setjmp and longjmp, or other mechanisms that prevent a signal handler from returning control back to the original functionality

  • While not technically a race condition, some signal handlers are designed to be called at most once, and being called more than once can introduce security problems, even when there are not any concurrent calls to the signal handler. This can be a source of double-free and use-after-free weaknesses.

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:

  • Avoiding shared state

  • Using synchronization in the signal handler

  • Using synchronization in the regular code

  • Disabling or masking other signals, which provides atomicity (which effectively ensures exclusivity)

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms


C: (Sometimes)

C++: (Sometimes)

+ Common Consequences

Technical Impact: Modify application data; Modify memory; DoS: crash / exit / restart; Execute unauthorized code or commands

It may be possible to cause data corruption and possibly execute arbitrary code by modifying global variables or data structures at unexpected times, violating the assumptions of code that uses this global data.

Access Control

Technical Impact: Gain privileges / assume identity

If a signal handler interrupts code that is executing with privileges, it may be possible that the signal handler will also be executed with elevated privileges, possibly making subsequent exploits more severe.

+ Likelihood of Exploit


+ Demonstrative Examples

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)
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 [R.364.1]; 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:
#include <signal.h>
#include <syslog.h>
#include <string.h>
#include <stdlib.h>

void *global1, *global2;
char *what;
void sh (int dummy) {
/* Sleep statements added to expand timing window for race condition */

int main (int argc,char* argv[]) {
/* Sleep statements added to expand timing window for race condition */

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

  1. a SIGHUP is delivered to the process

  2. sh() is invoked to process the SIGHUP

  3. This first invocation of sh() reaches the point where global1 is freed

  4. At this point, a SIGTERM is sent to the process

  5. the second invocation of sh() might do another free of global1

  6. this 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" [R.364.1].

+ Observed Examples
Signal handler does not disable other signal handlers, allowing it to be interrupted, causing other functionality to access files/etc. with raised privileges
Attacker can send a signal while another signal handler is already running, leading to crash or execution with root privileges
unsafe calls to library functions from signal handler
SIGURG can be used to remotely interrupt signal handler; other variants exist
handler for SIGCHLD uses non-reentrant functions
+ Potential Mitigations

Phase: Requirements

Strategy: Language Selection

Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

Phase: Architecture and Design

Design signal handlers to only set flags, rather than perform complex functionality. These flags can then be checked and acted upon within the main program loop.

Phase: Implementation

Only use reentrant functions within signal handlers. Also, use sanity checks to ensure that state is consistent while performing asynchronous actions that affect the state of execution.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory361Time and State
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness ClassWeakness Class362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory387Signal Errors
Development Concepts699
ChildOfCategoryCategory634Weaknesses that Affect System Processes
Resource-specific Weaknesses (primary)631
ChildOfCategoryCategory986SFP Secondary Cluster: Missing Lock
Software Fault Pattern (SFP) Clusters (primary)888
CanPrecedeWeakness BaseWeakness Base123Write-what-where Condition
Research Concepts1000
CanPrecedeWeakness VariantWeakness Variant415Double Free
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base416Use After Free
Research Concepts1000
ParentOfWeakness BaseWeakness Base432Dangerous Signal Handler not Disabled During Sensitive Operations
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base828Signal Handler with Functionality that is not Asynchronous-Safe
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base831Signal Handler Function Associated with Multiple Signals
Development Concepts (primary)699
Research Concepts (primary)1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
PeerOfWeakness BaseWeakness Base365Race Condition in Switch
Research Concepts1000
CanAlsoBeWeakness BaseWeakness Base368Context Switching Race Condition
Research Concepts1000
+ Research Gaps

Probably under-studied.

+ Affected Resources
  • System Process
+ Functional Areas
  • Signals, interprocess communication
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERSignal handler race condition
7 Pernicious KingdomsSignal Handling Race Conditions
CLASPRace condition in signal handler
Software Fault PatternsSFP19Missing Lock
+ References
[R.364.1] [REF-53] Michal Zalewski. "Delivering Signals for Fun and Profit". <>.
[R.364.2] [REF-54] "Race Condition: Signal Handling". <>.
[R.364.3] [REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 13: Race Conditions." Page 205. McGraw-Hill. 2010.
[R364.4] [REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 13, "Signal Vulnerabilities", Page 791.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2010-09-27CWE Content TeamMITREInternal
updated Observed_Examples, References
2010-12-13CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations, Relationships
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, References, Relationships
2014-06-23CWE Content TeamMITREInternal
updated Demonstrative_Examples, References
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings

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Page Last Updated: May 05, 2017