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Time of check, time of use race condition

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#REDIRECT Race Conditions

Last revision (mm/dd/yy): 04/7/2009


Time-of-check, time-of-use race conditions occur when between the time in which a given resource is checked, and the time that resource is used, a change occurs in the resource to invalidate the results of the check.


  • Access control: The attacker can gain access to otherwise unauthorized resources.
  • Authorization: race conditions such as this kind may be employed to gain read or write access to resources which are not normally readable or writable by the user in question.
  • Integrity: The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user.
  • Accountability: If a file or other resource is written in this method, as opposed to in a valid way, logging of the activity may not occur.
  • Non-repudiation: In some cases it may be possible to delete files a malicious user might not otherwise have access to, such as log files.

Exposure period

  • Design: Strong locking methods may be designed to protect against this flaw.
  • Implementation: Use of system APIs may prevent check, use race conditions.


  • Languages: Any
  • Platforms: All

Required resources

  • Some access to the resource in question



Likelihood of exploit

Low to Medium

Time-of-check, time-of-use race conditions occur when a resource is checked for a particular value, that value is changed, then the resource is used, based on the assumption that the value is still the same as it was at check time.

This is a broad category of race condition encompassing binding flaws, locking race conditions, and others.

Risk Factors



In C/C++:

struct stat *sb;
// it has not been updated since the last time it was read
printf("stated file\n");
if (sb->st_mtimespec==...)
  print("Now updating things\n");

Potentially the file could have been updated between the time of the check and the lstat, especially since the printf has latency.

Related Attacks

Related Vulnerabilities

Related Controls

  • Design: Ensure that some environmental locking mechanism can be used to protect resources effectively.
  • Implementation: Ensure that locking occurs before the check, as opposed to afterwards, such that the resource, as checked, is the same as it is when in use.

Related Technical Impacts