Common Weakness Enumeration

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

Weakness ID: 367
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
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.
+ Alternate Terms
The TOCTTOU acronym expands to "Time Of Check To Time Of Use".
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.
+ Relationships

The table(s) below 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 "Development Concepts" (CWE-699)
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

+ Applicable Platforms
The listings below show 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.


Class: Language-Independent (Undetermined Prevalence)

+ Common Consequences

The table below 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.


Technical Impact: Alter Execution Logic; Unexpected State

The attacker can gain access to otherwise unauthorized resources.

Technical Impact: 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.

Technical Impact: Other

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

Technical Impact: 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.

Technical Impact: Other

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
+ Demonstrative Examples

Example 1

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

(bad code)
Example Language:
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.

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

$filename = readlink($filename);


if(fileowner($filename) == $user){
echo file_get_contents($realFile);

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).

+ Observed Examples
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.
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.
+ Potential Mitigations

Phase: 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.

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.

Phase: Architecture and Design

Limit the interleaving of operations on files from multiple processes.

Phases: 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.

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.
+ Memberships
This 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.
+ Notes


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 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
Software Fault PatternsSFP20Race Condition Window
+ References
[REF-367] Dan Tsafrir, Tomer Hertz, David Wagner and Dilma Da Silva. "Portably Solving File TOCTTOU Races with Hardness Amplification". 2008-02-28. <>.
[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
Submission DateSubmitterOrganization
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-08-01KDM Analytics
added/updated white box definitions
2008-09-08CWE Content TeamMITRE
updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2008-10-14CWE Content TeamMITRE
updated Description, Name, Relationships
2008-11-24CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2009-01-12CWE Content TeamMITRE
updated Alternate_Terms, Observed_Examples, Other_Notes, References, Relationship_Notes, Relationships, Research_Gaps
2009-05-27CWE Content TeamMITRE
updated Demonstrative_Examples
2009-07-17KDM Analytics
Improved the White_Box_Definition
2009-07-27CWE Content TeamMITRE
updated White_Box_Definitions
2010-09-27CWE Content TeamMITRE
updated Description, Relationships
2010-12-13CWE Content TeamMITRE
updated Alternate_Terms, Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2011-09-13CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, Observed_Examples, References, Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms, Demonstrative_Examples, Likelihood_of_Exploit, References, Relationships, Taxonomy_Mappings, White_Box_Definitions
Previous Entry Names
Change DatePrevious Entry Name
2008-10-14Time-of-check Time-of-use Race Condition

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Page Last Updated: January 18, 2018