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

Opis

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.

Inne nazwy

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.

Skutki

• 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.

Zapobieganie

• 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.

Przykłady podatności

• CVE-2015-1743 TOCTOU in sandbox process allows installation of untrusted browser add-ons by replacing a file after it has been verified, but before it is executed
• CVE-2003-0813 Chain: A multi-threaded race condition (CWE-367) allows attackers to cause two threads to process the same RPC request, which causes a use-after-free (CWE-416) in one thread
• CVE-2004-0594 PHP flaw allows remote attackers to execute arbitrary code by aborting execution before the initialization of key data structures is complete.
• CVE-2008-2958 chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.
• CVE-2008-1570 chain: time-of-check time-of-use (TOCTOU) race condition in program allows bypass of protection mechanism that was designed to prevent symlink attacks.

Słabość nadrzędna

• CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

Słabości podrzędne

• CWE-363 Race Condition Enabling Link Following

Kiedy powstaje

• 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.

Dotyczy

Not Language-Specific

Uwagi

TOCTOU issues do not always involve symlinks, and not every symlink issue is a TOCTOU problem.

Non-symlink TOCTOU issues are not reported frequently, but they are likely to occur in code that attempts to be secure.

Źródła

1. The CLASP Application Security Process — Secure Software, Inc., 5.4.6 Race condition in switch, 2005
2. Portably Solving File TOCTTOU Races with Hardness Amplification — Dan Tsafrir, Tomer Hertz, David Wagner, Dilma Da Silva, 2008
3. 24 Deadly Sins of Software Security — Michael Howard, David LeBlanc, John Viega, "Sin 13: Race Conditions." Page 205, McGraw-Hill, 2010
4. The Art of Software Security Assessment — Mark Dowd, John McDonald, Justin Schuh, Chapter 9, "TOCTOU", Page 527, Addison Wesley, 2006