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Time-of-check Time-of-use (TOCTOU) Race Condition

ID: 367Date: (C)2012-05-14   (M)2020-03-17
Type: weaknessStatus: INCOMPLETE
Abstraction Type: Base


The software checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the software to perform invalid actions when the resource is in an unexpected state.

Extended Description

This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.

Likelihood of Exploit: Low to Medium

Applicable Platforms
Language Class: All

Time Of Introduction

  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Alter execution logic
Unexpected state
The attacker can gain access to otherwise unauthorized resources.
Modify application data
Modify files or directories
Modify memory
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.
The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user.
Hide activities
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.
In some cases it may be possible to delete files a malicious user might not otherwise have access to, such as log files.

Detection Methods

Potential Mitigations

 The most basic advice for TOCTOU vulnerabilities is to not perform a check before the use. This does not resolve the underlying issue of the execution of a function on a resource whose state and identity cannot be assured, but it does help to limit the false sense of security given by the check.
 When the file being altered is owned by the current user and group, set the effective gid and uid to that of the current user and group when executing this statement.
Architecture and Design
 Limit the interleaving of operations on files from multiple processes.
Architecture and Design
 If you cannot perform operations atomically and you must share access to the resource between multiple processes or threads, then try to limit the amount of time (CPU cycles) between the check and use of the resource. This will not fix the problem, but it could make it more difficult for an attack to succeed.
 Recheck the resource after the use call to verify that the action was taken appropriately.
Architecture and Design
 Ensure that some environmental locking mechanism can be used to protect resources effectively.
 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.

TOCTOU issues do not always involve symlinks, and not every symlink issue is a TOCTOU problem.

Related CWETypeViewChain
CWE-367 ChildOf CWE-894 Category CWE-888  

Demonstrative Examples   (Details)

  1. The following code is from a program installed setuid root. The program performs certain file operations on behalf of non-privileged users, and uses access checks to ensure that it does not use its root privileges to perform operations that should otherwise be unavailable the current user. The program uses the access() system call to check if the person running the program has permission to access the specified file before it opens the file and performs the necessary operations.
  2. This code prints the contents of a file if a user has permission. (Demonstrative Example Id DX-49)

Observed Examples

  1. CVE-2003-0813 : A multi-threaded race condition allows remote attackers to cause a denial of service (crash or reboot) by causing two threads to process the same RPC request, which causes one thread to use memory after it has been freed.
  2. CVE-2004-0594 : PHP flaw allows remote attackers to execute arbitrary code by aborting execution before the initialization of key data structures is complete.
  3. CVE-2008-2958 : chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
  4. CVE-2008-1570 : chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
A weakness where code path has:
1. start statement that validates a system resource by name rather than by reference
2. end statement that accesses the system resource by the name

Black Box Definitions

Taxynomy Mappings

PLOVER  Time-of-check Time-of-use race condition
7 Pernicious Kingdoms  File Access Race Conditions: TOCTOU
CLASP  Time of check, time of use race condition
CERT C Secure Coding FIO01-C
Be careful using functions that use file names for identification
CERT C++ Secure Coding FIO01-CPP
Be careful using functions that use file names for identification


  1. Dan Tsafrir Tomer Hertz David Wagner Dilma Da Silva .Portably Solving File TOCTTOU Races with Hardness Amplification. 2008-02-28.
  2. Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 13: Race Conditions." Page 205'. Published on 2010.
  3. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 9, "TOCTOU", Page 527.'. Published on 2006.
CVE    24

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