The product 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.

Primary

TOCTTOU
The TOCTTOU acronym expands to "Time Of Check To Time Of Use".

TOCCTOU
The TOCCTOU acronym is most likely a typo of TOCTTOU, but it has been used in some influential documents, so the typo is repeated fairly frequently.

Implementation
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.

Medium

Integrity
Other
Alter Execution Logic
Unexpected State
The attacker can gain access to otherwise unauthorized resources.

Integrity
Other
Modify Application Data
Modify Files or Directories
Modify Memory
Other
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
Other
Other
The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user.

Non-Repudiation
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.

Non-Repudiation
Other
Other
In some cases it may be possible to delete files a malicious user might not otherwise have access to, such as log files.

Other
Unexpected State
The product may perform invalid actions when the resource is in an unexpected state.

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
High

Implementation
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.

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.

Architecture and Design
Limit the interleaving of operations on files from multiple processes.

Implementation
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.

Implementation
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.

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.

The following code checks a file, then updates its contents.

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

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

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.

if(!access(file,W_OK)) {f = fopen(file,"w+");operate(f);...}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.

This code prints the contents of a file if a user has permission.

function readFile($filename){
$user = getCurrentUser();

//resolve file if its a symbolic link
if(is_link($filename)){$filename = readlink($filename);}
if(fileowner($filename) == $user){echo file_get_contents($realFile);return;}else{echo 'Access denied';return false;}
}

This code attempts to resolve symbolic links before checking the file and printing its contents. However, an attacker may be able to change the file from a real file to a symbolic link between the calls to is_link() and file_get_contents(), allowing the reading of arbitrary files. Note that this code fails to log the attempted access (CWE-778).

This example is adapted from [REF-18]. Assume that this code block is invoked from multiple threads. The switch statement will execute different code depending on the time when MYFILE.txt was last changed.

#include <sys/types.h>
#include <sys/stat.h>

...

struct stat sb;
stat("MYFILE.txt",&sb);
printf("file change time: %d\n",sb->st_ctime);
switch(sb->st_ctime % 2){
case 0: printf("Option 1\n"); break;
case 1: printf("Option 2\n"); break;
default: printf("this should be unreachable?\n"); break;
}

If this code block were executed within multiple threads, and MYFILE.txt changed between the operation of one thread and another, then the switch could produce different, possibly unexpected results.

CVE-2015-1743
TOCTOU in sandbox process allows installation of untrusted browser add-ons by replacing a file after it has been verified, but before it is executed
https://www.cve.org/CVERecord?id=CVE-2015-1743

CVE-2003-0813
Chain: A multi-threaded race condition (CWE-367) allows attackers to cause two threads to process the same RPC request, which causes a use-after-free (CWE-416) in one thread
https://www.cve.org/CVERecord?id=CVE-2003-0813

CVE-2004-0594
PHP flaw allows remote attackers to execute arbitrary code by aborting execution before the initialization of key data structures is complete.
https://www.cve.org/CVERecord?id=CVE-2004-0594

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.
https://www.cve.org/CVERecord?id=CVE-2008-2958

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.
https://www.cve.org/CVERecord?id=CVE-2008-1570

File or Directory

Time-of-check Time-of-use race condition

File Access Race Conditions: TOCTOU

Time of check, time of use race condition

Race condition in switch

FIO01-C
Be careful using functions that use file names for identification

SFP20
Race Condition Window

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

TOCTOU issues do not always involve symlinks, and not every symlink issue is a TOCTOU problem.
Non-symlink TOCTOU issues are not reported frequently, but they are likely to occur in code that attempts to be secure.

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

KDM Analytics
2008-08-01
added/updated white box definitions

CWE Content Team
MITRE
2008-09-08
updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings

CWE Content Team
MITRE
2008-10-14
updated Description, Name, Relationships

CWE Content Team
MITRE
2008-11-24
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2009-01-12
updated Alternate_Terms, Observed_Examples, Other_Notes, References, Relationship_Notes, Relationships, Research_Gaps

CWE Content Team
MITRE
2009-05-27
updated Demonstrative_Examples

KDM Analytics
2009-07-17
Improved the White_Box_Definition

CWE Content Team
MITRE
2009-07-27
updated White_Box_Definitions

CWE Content Team
MITRE
2010-09-27
updated Description, Relationships

CWE Content Team
MITRE
2010-12-13
updated Alternate_Terms, Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2011-06-27
updated Common_Consequences

CWE Content Team
MITRE
2011-09-13
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated Demonstrative_Examples, Observed_Examples, References, Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Demonstrative_Examples, Likelihood_of_Exploit, References, Relationships, Taxonomy_Mappings, White_Box_Definitions

CWE Content Team
MITRE
2019-06-20
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated References, Relationships

CWE Content Team
MITRE
2022-04-28
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2022-06-28
updated Observed_Examples

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Detection_Factors, References, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Affected_Resources, Observed_Examples

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated Common_Consequences, Description, Diagram, Modes_of_Introduction

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Weakness_Ordinalities

Time-of-check Time-of-use Race Condition