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Home > CWE List > CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition (4.18)
ID
CWE Glossary Definition

CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition

Weakness ID: 367
Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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Description
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. Diagram for CWE-367
Alternate Terms
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.
Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Impact Details

Alter Execution Logic; Unexpected State

Scope: Integrity, Other

The attacker can gain access to otherwise unauthorized resources.

Modify Application Data; Modify Files or Directories; Modify Memory; Other

Scope: Integrity, 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.

Other

Scope: Integrity, Other

The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user.

Hide Activities

Scope: Non-Repudiation

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.

Other

Scope: Non-Repudiation, Other

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

Unexpected State

Scope: Other

The product may perform invalid actions when the resource is in an unexpected state.
Potential Mitigations
Phase(s) Mitigation

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.
Relationships
Section Help This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (View-1000)
Nature Type ID Name
ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
ParentOf Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 363 Race Condition Enabling Link Following
PeerOf Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 386 Symbolic Name not Mapping to Correct Object
CanFollow Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 609 Double-Checked Locking
Relevant to the view "Software Development" (View-699)
Nature Type ID Name
MemberOf Category Category - a CWE entry that contains a set of other entries that share a common characteristic. 557 Concurrency Issues
Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (View-1003)
Nature Type ID Name
ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase Note
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.
Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages

Class: Not Language-Specific (Undetermined Prevalence)

Likelihood Of Exploit
Medium
Demonstrative Examples

Example 1


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

(bad code)
Example Language: 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");
updateThings();
}

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: C
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.



Example 3


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

(bad code)
Example Language: PHP
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).



Example 4


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.

(bad code)
Example Language: C
#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.



Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Reference Description
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
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
PHP flaw allows remote attackers to execute arbitrary code by aborting execution before the initialization of key data structures is complete.
chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
Detection Methods
Method Details

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.)

Effectiveness: High
Affected Resources
  • File or Directory
Memberships
Section HelpThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature Type ID Name
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 361 7PK - Time and State
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 743 CERT C Secure Coding Standard (2008) Chapter 10 - Input Output (FIO)
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 877 CERT C++ Secure Coding Section 09 - Input Output (FIO)
MemberOf ViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries). 884 CWE Cross-section
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 988 SFP Secondary Cluster: Race Condition Window
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1401 Comprehensive Categorization: Concurrency
Vulnerability Mapping Notes
Usage ALLOWED
(this CWE ID may be used to map to real-world vulnerabilities)
Reason Acceptable-Use

Rationale

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.

Comments

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.
Notes

Relationship

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

Research Gap

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 Name Node ID Fit Mapped Node Name
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
CLASP Race condition in switch
CERT C Secure Coding FIO01-C Be careful using functions that use file names for identification
Software Fault Patterns SFP20 Race Condition Window
References
[REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 5.4.6 Race condition in switch. 2005.
<https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf>. (URL validated: 2024年11月17日)
[REF-367] Dan Tsafrir, Tomer Hertz, David Wagner and Dilma Da Silva. "Portably Solving File TOCTTOU Races with Hardness Amplification". 2008年02月28日.
<https://www.usenix.org/legacy/events/fast08/tech/tsafrir.html>. (URL validated: 2023年04月07日)
[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 13: Race Conditions." Page 205. McGraw-Hill. 2010.
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "TOCTOU", Page 527. 1st Edition. Addison Wesley. 2006.
Content History
Submissions
Submission Date Submitter Organization
2006年07月19日
(CWE Draft 3, 2006年07月19日) 		PLOVER
Modifications
Modification Date Modifier Organization
2025年09月09日
(CWE 4.18, 2025年09月09日) 		CWE Content Team MITRE
updated Common_Consequences, Description, Diagram, Modes_of_Introduction
2025年04月03日
(CWE 4.17, 2025年04月03日) 		CWE Content Team MITRE
updated Affected_Resources, Observed_Examples
2023年06月29日 CWE Content Team MITRE
updated Mapping_Notes
2023年04月27日 CWE Content Team MITRE
updated Detection_Factors, References, Relationships
2023年01月31日 CWE Content Team MITRE
updated Description
2022年06月28日 CWE Content Team MITRE
updated Observed_Examples
2022年04月28日 CWE Content Team MITRE
updated Demonstrative_Examples, References, Relationships, Taxonomy_Mappings
2020年02月24日 CWE Content Team MITRE
updated References, Relationships
2019年06月20日 CWE Content Team MITRE
updated Relationships
2017年11月08日 CWE Content Team MITRE
updated Applicable_Platforms, Demonstrative_Examples, Likelihood_of_Exploit, References, Relationships, Taxonomy_Mappings, White_Box_Definitions
2014年07月30日 CWE Content Team MITRE
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings
2012年10月30日 CWE Content Team MITRE
updated Potential_Mitigations
2012年05月11日 CWE Content Team MITRE
updated Demonstrative_Examples, Observed_Examples, References, Relationships
2011年09月13日 CWE Content Team MITRE
updated Relationships, Taxonomy_Mappings
2011年06月27日 CWE Content Team MITRE
updated Common_Consequences
2011年06月01日 CWE Content Team MITRE
updated Common_Consequences
2010年12月13日 CWE Content Team MITRE
updated Alternate_Terms, Relationships
2010年09月27日 CWE Content Team MITRE
updated Description, Relationships
2009年07月27日 CWE Content Team MITRE
updated White_Box_Definitions
2009年07月17日 KDM Analytics
Improved the White_Box_Definition
2009年05月27日 CWE Content Team MITRE
updated Demonstrative_Examples
2009年01月12日 CWE Content Team MITRE
updated Alternate_Terms, Observed_Examples, Other_Notes, References, Relationship_Notes, Relationships, Research_Gaps
2008年11月24日 CWE Content Team MITRE
updated Relationships, Taxonomy_Mappings
2008年10月14日 CWE Content Team MITRE
updated Description, Name, Relationships
2008年09月08日 CWE Content Team MITRE
updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2008年08月01日 KDM Analytics
added/updated white box definitions
2008年07月01日 Eric Dalci Cigital
updated Time_of_Introduction
Previous Entry Names
Change Date Previous Entry Name
2008年10月14日 Time-of-check Time-of-use Race Condition
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Page Last Updated: September 09, 2025

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