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.
How it's found
Time-of-check Time-of-use (TOCTOU) Race Condition is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.
Consequences
- Alter Execution Logic, Unexpected State: The attacker can gain access to otherwise unauthorized resources.
- 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.
- Other: The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user.
- 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.
- 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: The product may perform invalid actions when the resource is in an unexpected state.
Mitigations
- 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.
- 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.
- Architecture and Design: Limit the interleaving of operations on files from multiple processes.
- 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.
- Implementation: Recheck the resource after the use call to verify that the action was taken appropriately.
- Architecture and Design: Ensure that some environmental locking mechanism can be used to protect resources effectively.
- 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.
Where this fits in a TurboPentest engagement
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
Frequently asked questions
What is CWE-367?
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.
How do you find Time-of-check Time-of-use (TOCTOU) Race Condition?
Time-of-check Time-of-use (TOCTOU) Race Condition is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.
What is the impact of CWE-367?
Alter Execution Logic, Unexpected State: The attacker can gain access to otherwise unauthorized resources. 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. Other: The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user. 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. 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: The product may perform invalid actions when the resource is in an unexpected state.
Does TurboPentest test for Time-of-check Time-of-use (TOCTOU) Race Condition?
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
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Written and reviewed by
Michel Chamberland - Founder & CEO, IntegSec
CISSP, OSCP, OSCE, CEH, GIAC, CCSK · 20+ years in offensive security
Michel has spent 20+ years on offensive security teams including IBM X-Force Red and Trustwave SpiderLabs, leading penetration tests, red team engagements, and breach response for Fortune 500 customers. He is the founder of IntegSec and the architect of TurboPentest.
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