COMMON WEAKNESS ENUMERATIONIncomplete

CWE-367

Time-of-check Time-of-use (TOCTOU) Race Condition

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.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

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.

Terminology

Alternate terms and usages

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.

Root cause

How and when does the problem arise?

Lifecycle phase

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.

Risk

What does successful exploitation lead to?

Alter Execution LogicUnexpected State

Affected security scopes: Integrity, Other

The attacker can gain access to otherwise unauthorized resources.
Modify Application DataModify Files or DirectoriesModify MemoryOther

Affected security scopes: 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

Affected security scopes: 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

Affected security scopes: 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

Affected security scopes: 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

Affected security scopes: Other

The product may perform invalid actions when the resource is in an unexpected state.

Defence

How is it prevented and fixed?

Implementation

Implementation-phase defence

Detail
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

Implementation-phase defence

Detail
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

Architecture and design approach

Detail
Limit the interleaving of operations on files from multiple processes.

Implementation · Architecture and Design

Defence to apply across several lifecycle phases

Detail
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

Implementation-phase defence

Detail
Recheck the resource after the use call to verify that the action was taken appropriately.

Architecture and Design

Architecture and design approach

Detail
Ensure that some environmental locking mechanism can be used to protect resources effectively.

Implementation

Implementation-phase defence

Detail
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.

Verification

How is it detected?

Automated Static Analysis

High
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.)

MITRE diagram

Weakness relationships and flow

CWE-367 MITRE diagramı