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CWE-479: Signal Handler Use of a Non-reentrant Function

Weakness ID: 479
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The program defines a signal handler that calls a non-reentrant function.
+ Extended Description

Non-reentrant functions are functions that cannot safely be called, interrupted, and then recalled before the first call has finished without resulting in memory corruption. This can lead to an unexpected system state an unpredictable results with a variety of potential consequences depending on context, including denial of service and code execution.

Many functions are not reentrant, but some of them can result in the corruption of memory if they are used in a signal handler. The function call syslog() is an example of this. In order to perform its functionality, it allocates a small amount of memory as "scratch space." If syslog() is suspended by a signal call and the signal handler calls syslog(), the memory used by both of these functions enters an undefined, and possibly, exploitable state. Implementations of malloc() and free() manage metadata in global structures in order to track which memory is allocated versus which memory is available, but they are non-reentrant. Simultaneous calls to these functions can cause corruption of the metadata.

+ 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)
+ Relevant to the view "Development Concepts" (CWE-699)
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Architecture and Design
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

+ 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.


Technical Impact: Execute Unauthorized Code or Commands

It may be possible to execute arbitrary code through the use of a write-what-where condition.

Technical Impact: Modify Application Data

Signal race conditions often result in data corruption.
+ Likelihood Of Exploit
+ Demonstrative Examples

Example 1

In this example, a signal handler uses syslog() to log a message:

(bad code)
char *message;
void sh(int dummy) {

int main(int argc,char* argv[]) {

If the execution of the first call to the signal handler is suspended after invoking syslog(), and the signal handler is called a second time, the memory allocated by syslog() enters an undefined, and possibly, exploitable state.
+ Observed Examples
signal handler calls function that ultimately uses malloc()
SIGCHLD signal to FTP server can cause crash under heavy load while executing non-reentrant functions like malloc/free.
+ Potential Mitigations

Phase: Requirements

Require languages or libraries that provide reentrant functionality, or otherwise make it easier to avoid this weakness.

Phase: Architecture and Design

Design signal handlers to only set flags rather than perform complex functionality.

Phase: Implementation

Ensure that non-reentrant functions are not found in signal handlers.

Phase: Implementation

Use sanity checks to reduce the timing window for exploitation of race conditions. This is only a partial solution, since many attacks might fail, but other attacks still might work within the narrower window, even accidentally.

Effectiveness: Defense in Depth

+ Affected Resources
  • System Process
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPUnsafe function call from a signal handler
CERT C Secure CodingSIG30-CExactCall only asynchronous-safe functions within signal handlers
CERT C Secure CodingSIG34-CDo not call signal() from within interruptible signal handlers
CERT Java Secure CodingEXP01-JNever dereference null pointers
Software Fault PatternsSFP3Use of an improper API
+ References
[REF-62] 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 DateSubmitterOrganization
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Common_Consequences, Description, Relationships, Other_Notes, Taxonomy_Mappings
2008-11-24CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2010-09-27CWE Content TeamMITRE
updated Relationships
2010-12-13CWE Content TeamMITRE
updated Demonstrative_Examples, Description, Name, Observed_Examples, Other_Notes, Potential_Mitigations, Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2011-09-13CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated References, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITRE
updated Demonstrative_Examples
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Observed_Examples, Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2010-12-13Unsafe Function Call from a Signal Handler

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Page Last Updated: January 18, 2018