CWE-367

Base Abstraction Level
Pillar — Highest-level weakness category
Class — Abstract, language-independent
Base — Specific enough to detect
Variant — Tied to specific technology
Compound — Requires multiple weaknesses Incomplete MITRE CWE Status
Stable — Fully reviewed and complete
Draft — Under development, may change
Incomplete — Partially defined by MITRE
Deprecated — No longer recommended
Obsolete — Replaced by another CWE Exploit: Medium
Time-of-check Time-of-use (TOCTOU) Race Condition

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.

Top Monitored CVEs

CVE-2023-35311
HIGH KEV
8.8 15.0%
2023-07-11
CVE-2021-32708
HIGH
8.1 3.5%
2021-06-24

Consequences

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.

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.

Phase: 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.

Detection

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.)