Weakness · Medium

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

What it is

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.

Likelihood of exploit: Medium5 exploited in the wildMITRE CWE 4.19.1

Severity (our rating): Medium
Likelihood of exploit: Medium
Introduced during: Implementation
Exploited in the wild: 5 known exploited CVEs

What an attacker can do

Alter Execution Logic (Integrity, Other)
Unexpected State (Integrity, Other)
Modify Application Data (Integrity, Other)
Modify Files or Directories (Integrity, Other)
Modify Memory (Integrity, Other)
Other (Integrity, Other, Non-Repudiation)
Hide Activities (Non-Repudiation)

How to prevent it

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.
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.
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 locking occurs before the check, as opposed to afterwards, such that the resource, as checked, is the same as it is when in use.

Architecture and Design

Limit the interleaving of operations on files from multiple processes.
Ensure that some environmental locking mechanism can be used to protect resources effectively.

How to find it

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.)

CWE-367 on MITRE's CWE site

Exploited in the wild: 5 CVEs

Vulnerabilities caused by CWE-367 that attackers have used, from CISA's Known Exploited Vulnerabilities catalog, newest first.

All known exploited vulnerabilities

H2 Security

Found it in your code?

We review where it appears, fix it with your team, and check it was not already used against you.

Talk to our team

T Academy

Stop writing it in the first place

Learn to build software and systems without weaknesses like this one, with practice on real hosts in your browser, every objective checked on the machine.

Start free

Source: MITRE Common Weakness Enumeration, CWE version 4.19.1. CWE is a trademark of The MITRE Corporation.