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Revision as of 18:18, 17 February 2009

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ASDR Table of Contents

Last revision (mm/dd/yy): 02/17/2009


A race condition occurs when a pair of routine programming calls in an application do not perform in the sequential manner that was intended per business rules. It is a timing event within software that can become a security vulnerability if the calls are not performed in the correct order.

Race condition in checking for certificate revocation

If the revocation status of a certificate is not checked before each privilege requiring action, the system may be subject to a race condition, in which their certificate may be used before it is checked for revocation.


  • Authentication: Trust may be assigned to an entity who is not who it claims to be.
  • Integrity: Data from an untrusted (and possibly malicious) source may be integrated.
  • Confidentiality: Date may be disclosed to an entity impersonating a trusted entity, resulting in information disclosure.

Exposure period

  • Design: Checks for certificate revocation should be included in the design of a system
  • Design: One can choose to use a language which abstracts out this part of the authentication process.


  • Languages: Languages which do not abstract out this part of the process.
  • Operating platforms: All

Required resources

Minor trust: Users must attempt to interact with the malicious system.



Likelihood of exploit


If a certificate is revoked after the initial check, all subsequent actions taken with the owner of the revoked certificate will loose all benefits guaranteed by the certificate. In fact, it is almost certain that the use of a revoked certificate indicates malicious activity.

If the certificate is checked before each access of a protected resource, the delay subject to a possible race condition becomes almost negligible and significantly reduces the risk associated with this issue.

In C/C++:

if (!(cert = SSL_get_peer(certificate(ssl)) || !host)
  if (X509_V_OK==foo)
//do stuff
 //do more stuff without the check. 

Design: Ensure that certificates are checked for revoked status before each use of a protected resource

Race condition in signal handler

Race conditions occur frequently in signal handlers, since they are asynchronous actions. These race conditions may have any number of Problem Types and symptoms.


  • Authorization: It may be possible to execute arbitrary code through the use of a write-what-where condition.
  • Integrity: Signal race conditions often result in data corruption.

Exposure period

  • Requirements specification: A language might be chosen which is not subject to this flaw.
  • Design: Signal handlers with complicated functionality may result in this issue.
  • Implementation: The use of any non-reentrant functionality or global variables in a signal handler might result in this race conditions.


  • Languages: C, C++, Assembly
  • Operating platforms: All

Required resources




Likelihood of exploit


Signal race conditions are a common issue that have only recently been seen as exploitable. These issues occur when non-reentrant functions, or state-sensitive actions occur in the signal handler, where they may be called at any time. If these functions are called at an inopportune moment - such as while a non-reentrant function is already running -, memory corruption occurs that may be exploitable.

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. This is a perfect example of a signal handler taking actions which cannot be accounted for in state. 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 prudent if the same signal handler has been set for more than one signal - since it means that the signal handler itself may be reentered.

#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[]) {  


  • Requirements specification: A language might be chosen, which is not subject to this flaw, through a guarantee of reentrant code.
  • Design: Design signal handlers to only set flags rather than perform complex functionality.
  • Implementation: Ensure that non-reentrant functions are not found in signal handlers. Also, use sanity checks to ensure that state is consistent be performing asynchronous actions which effect the state of execution.

Risk Factors

  • A common business impact of a race condition is one where a payment confirmation occurs without producing a request object for order fulfillment.
  • The technical impact of this vulnerability can be mitigated through thread management classes or other programming constructs that control the synchronization of threads



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