Time-of-check Time-of-use (TOCTOU) Race Condition
Weakness ID: 367 (Weakness Base)Status: Incomplete
+ Description

Description Summary

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 multi-threaded programs.

+ Alternate Terms

The TOCCTOU acronym expands to "Time Of Check To Time Of Use". Usage varies between TOCTOU and TOCTTOU.

+ Time of Introduction
  • Implementation
+ Applicable Platforms



+ Common Consequences
Access Control

The attacker can gain access to otherwise unauthorized resources.

Access Control

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.


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.

+ Likelihood of Exploit

Low to Medium

+ Demonstrative Examples

Example 1

(Bad Code)
Example Languages: C and C++ 
struct stat *sb;
lstat("...",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.

Example 2

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.

(Bad Code)
Example Language:
if(!access(file,W_OK)) {
f = fopen(file,"w+");
else {

fprintf(stderr,"Unable to open file %s.\n",file);

The call to access() behaves as expected, and returns 0 if the user running the program has the necessary permissions to write to the file, and -1 otherwise. However, because both access() and fopen() operate on filenames rather than on file handles, there is no guarantee that the file variable still refers to the same file on disk when it is passed to fopen() that it did when it was passed to access(). If an attacker replaces file after the call to access() with a symbolic link to a different file, the program will use its root privileges to operate on the file even if it is a file that the attacker would otherwise be unable to modify. By tricking the program into performing an operation that would otherwise be impermissible, the attacker has gained elevated privileges. This type of vulnerability is not limited to programs with root privileges. If the application is capable of performing any operation that the attacker would not otherwise be allowed perform, then it is a possible target.

+ Observed Examples
CVE-2008-2958chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
CVE-2008-1570chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
+ 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.

Phase: Implementation

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.

Do not rely on user-specified input to determine what path to format.

Phase: Architecture and Design

Limit the interleaving of operations on files from multiple processes.

Limit the spread of time (cycles) between the check and use of a resource.

Phase: Implementation

Recheck the resource after the use call to verify that the action was taken appropriately.

Phase: Architecture and Design

Ensure that some environmental locking mechanism can be used to protect resources effectively.

Phase: 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.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory361Time and State
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness ClassWeakness Class362Race Condition
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory743CERT C Secure Coding Section 09 - Input Output (FIO)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
PeerOfWeakness BaseWeakness Base373State Synchronization Error
Research Concepts1000
ParentOfWeakness BaseWeakness Base363Race Condition Enabling Link Following
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base609Double-Checked Locking
Research Concepts (primary)1000
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
PeerOfWeakness BaseWeakness Base386Symbolic Name not Mapping to Correct Object
Research Concepts1000
+ Relationship Notes

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

+ Research Gaps

Non-symlink TOCTOU issues are not reported frequently, but they are likely to occur in code that attempts to be secure.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERTime-of-check Time-of-use race condition
7 Pernicious KingdomsFile Access Race Conditions: TOCTOU
CLASPTime of check, time of use race condition
CERT C Secure CodingFIO01-CBe careful using functions that use file names for identification
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
27Leveraging Race Conditions via Symbolic Links
29Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions
+ 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

+ References
Dan Tsafrir, Tomer Hertz, David Wagner and Dilma Da Silva. "Portably Solving File TOCTTOU Races with Hardness Amplification". 2008-02-28. <http://www.usenix.org/events/fast08/tech/tsafrir.html>.
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
2008-09-08CWE Content TeamMITREInternal
updated Common Consequences, Relationships, Other Notes, Taxonomy Mappings
2008-10-14CWE Content TeamMITREInternal
updated Description, Name, Relationships
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy Mappings
2009-01-12CWE Content TeamMITREInternal
updated Alternate Terms, Observed Examples, Other Notes, References, Relationship Notes, Relationships, Research Gaps
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-07-17KDM AnalyticsExternal
Improved the White Box Definition
2009-07-27CWE Content TeamMITREInternal
updated White Box Definitions
Previous Entry Names
Change DatePrevious Entry Name
2008-10-14Time-of-check Time-of-use Race Condition