Time-of-check Time-of-use (TOCTOU) Race Condition
|ID: 367||Date: (C)2012-05-14 (M)2020-02-26|
|Type: weakness||Status: INCOMPLETE|
|Abstraction Type: Base|
The software 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. This can cause the software to
perform invalid actions when the resource is in an unexpected
Extended DescriptionThis 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
Likelihood of Exploit: Low to Medium
Applicable PlatformsLanguage Class: All
Time Of Introduction
Related Attack Patterns
|IntegrityOther ||Alter execution
logicUnexpected state ||The attacker can gain access to otherwise unauthorized
|IntegrityOther ||Modify application
dataModify files or
directoriesModify memoryOther ||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. |
|IntegrityOther ||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-RepudiationOther ||Other ||In some cases it may be possible to delete files a malicious user
might not otherwise have access to, such as log files. |
|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. || || |
|ImplementationArchitecture 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. || || |
RelationshipsTOCTOU issues do not always involve symlinks, and not every symlink issue
is a TOCTOU problem.
|CWE-367 ChildOf CWE-894 ||Category ||CWE-888 || |
Demonstrative Examples (Details)
- The following code is from a program installed setuid root. The
program performs certain file operations on behalf of non-privileged users,
and uses access checks to ensure that it does not use its root privileges to
perform operations that should otherwise be unavailable the current user.
The program uses the access() system call to check if the person running the
program has permission to access the specified file before it opens the file
and performs the necessary operations.
- This code prints the contents of a file if a user has permission. (Demonstrative Example Id DX-49)
- CVE-2003-0813 : A multi-threaded race condition allows remote attackers to cause a denial of service (crash or reboot) by causing two threads to process the same RPC request, which causes one thread to use memory after it has been freed.
- 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.
For more examples, refer to CVE relations in the bottom box.
White Box DefinitionsA weakness where code path has:1. start statement that validates a system resource by name rather
than by reference2. end statement that accesses the system resource by the name
Black Box Definitions None
|PLOVER || ||Time-of-check Time-of-use race condition || |
|7 Pernicious Kingdoms || ||File Access Race Conditions: TOCTOU || |
|CLASP || ||Time of check, time of use race condition || |
|CERT C Secure Coding ||FIO01-C ||Be careful using functions that use file names for
identification || |
|CERT C++ Secure Coding ||FIO01-CPP ||Be careful using functions that use file names for
identification || |
- Dan Tsafrir Tomer Hertz David Wagner Dilma Da Silva .Portably Solving File TOCTTOU Races with Hardness
- Michael Howard David LeBlanc John Viega .24 Deadly Sins of Software Security. McGraw-Hill. Section:'"Sin 13: Race Conditions." Page 205'. Published on 2010.
- Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 9, "TOCTOU", Page 527.'. Published on 2006.