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.
View on MITREThe attacker can gain access to otherwise unauthorized resources.
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.
The product may perform invalid actions when the resource is in an unexpected state.
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.
Limit the interleaving of operations on files from multiple processes.
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.
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.
No detection method information available for this CWE.
The following code checks a file, then updates its contents.
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.
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.
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.
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.
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
View DetailsChain: 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
View DetailsPHP flaw allows remote attackers to execute arbitrary code by aborting execution before the initialization of key data structures is complete.
View Detailschain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
View Detailschain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
View DetailsNo relationship information available for this CWE.
CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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.
If exploited, CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition) it can compromise Integrity, Other and Non-Repudiation, leading to outcomes such as Alter Execution Logic, Unexpected State, Modify Application Data, Modify Files or Directories, Modify Memory and Other.
Recommended mitigations for CWE-367 include: 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. Limit the interleaving of operations on files from multiple processes.
CWE-367 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.
MITRE documents real CVEs mapped to CWE-367, including CVE-2015-1743, CVE-2003-0813, CVE-2004-0594, CVE-2008-2958 and CVE-2008-1570. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.
A CWE (Common Weakness Enumeration) like CWE-367 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.