Official definition

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.

Characteristics

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.

Modes of introduction

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.

Common consequences

Impact	Scope	Explanation
Alter Execution Logic, Unexpected State	Integrity, Other	The attacker can gain access to otherwise unauthorized resources.
Modify Application Data, Modify Files or Directories, Modify Memory, Other	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	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	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	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	Other	The product may perform invalid actions when the resource is in an unexpected state.

Risk mitigations

ImplementationThe 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.
ImplementationWhen 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 DesignLimit the interleaving of operations on files from multiple processes.
Implementation, Architecture and DesignIf 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.
ImplementationRecheck the resource after the use call to verify that the action was taken appropriately.
Architecture and DesignEnsure that some environmental locking mechanism can be used to protect resources effectively.
ImplementationEnsure 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.

Detection methods

Method	Approach	Effectiveness
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

Representative vulnerabilities

Below are representative vulnerabilities related to this CWE, prioritized by severity.

Sources (4)

MITRE CWE

CWE Terms of Use The CLASP Application Security Process

usenix.org

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

What is CWE-367?

Data statistics

Vulnerabilities mapped to CWE-367

Vulnerabilities in CISA KEV for CWE-367