Time-of-check Time-of-use (TOCTOU) Race Condition |
Weakness ID: 367 (Weakness Base) | Status: Incomplete |
Description Summary
Extended Description
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 multi-threaded programs.
TOCTTOU: | The TOCCTOU acronym expands to "Time Of Check To Time Of Use". Usage varies between TOCTOU and TOCTTOU. |
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Scope | Effect |
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Access Control | The attacker can gain access to otherwise unauthorized resources. |
Access Control Authorization | 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 | The resource in question, or other resources (through the corrupted one), may be changed in undesirable ways by a malicious user. |
Accountability | 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 | In some cases it may be possible to delete files a malicious user might not otherwise have access to, such as log files. |
Example 1
Potentially the file could have been updated between the time of the check and the lstat, especially since the printf has latency.
Example 2
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.
The call to access() behaves as expected, and returns 0 if the user running the program has the necessary permissions to write to the file, and -1 otherwise. However, because both access() and fopen() operate on filenames rather than on file handles, there is no guarantee that the file variable still refers to the same file on disk when it is passed to fopen() that it did when it was passed to access(). If an attacker replaces file after the call to access() with a symbolic link to a different file, the program will use its root privileges to operate on the file even if it is a file that the attacker would otherwise be unable to modify. By tricking the program into performing an operation that would otherwise be impermissible, the attacker has gained elevated privileges. This type of vulnerability is not limited to programs with root privileges. If the application is capable of performing any operation that the attacker would not otherwise be allowed perform, then it is a possible target.
Reference | Description |
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CVE-2003-0813 | |
CVE-2004-0594 | |
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. |
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. |
Phase: 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. |
Do not rely on user-specified input to determine what path to format. |
Phase: Architecture and Design Limit the interleaving of operations on files from multiple processes. |
Limit the spread of time (cycles) between the check and use of a resource. |
Phase: Implementation Recheck the resource after the use call to verify that the action was taken appropriately. |
Phase: Architecture and Design Ensure that some environmental locking mechanism can be used to protect resources effectively. |
Phase: 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. |
Nature | Type | ID | Name | View(s) this relationship pertains to |
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ChildOf | Category | 361 | Time and State | Seven Pernicious Kingdoms (primary)700 |
ChildOf | Weakness Class | 362 | Race Condition | Development Concepts (primary)699 Research Concepts (primary)1000 |
ChildOf | Category | 743 | CERT C Secure Coding Section 09 - Input Output (FIO) | Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734 |
PeerOf | Weakness Base | 373 | State Synchronization Error | Research Concepts1000 |
ParentOf | Weakness Base | 363 | Race Condition Enabling Link Following | Development Concepts (primary)699 Research Concepts (primary)1000 |
ParentOf | Weakness Base | 609 | Double-Checked Locking | Research Concepts (primary)1000 |
MemberOf | View | 630 | Weaknesses Examined by SAMATE | Weaknesses Examined by SAMATE (primary)630 |
PeerOf | Weakness Base | 386 | Symbolic Name not Mapping to Correct Object | Research Concepts1000 |
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. |
Mapped Taxonomy Name | Node ID | Fit | Mapped Node Name |
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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 |
A weakness where code path has: 1. start statement that validates a system resource by name rather than by reference 2. end statement that accesses the system resource by the name |
Dan Tsafrir, Tomer Hertz, David Wagner and Dilma Da Silva. "Portably Solving File TOCTTOU Races with Hardness Amplification". 2008-02-28. <http://www.usenix.org/events/fast08/tech/tsafrir.html>. |
Submissions | ||||
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Submission Date | Submitter | Organization | Source | |
PLOVER | Externally Mined | |||
Modifications | ||||
Modification Date | Modifier | Organization | Source | |
2008-07-01 | Eric Dalci | Cigital | External | |
updated Time of Introduction | ||||
2008-08-01 | KDM Analytics | External | ||
added/updated white box definitions | ||||
2008-09-08 | CWE Content Team | MITRE | Internal | |
updated Common Consequences, Relationships, Other Notes, Taxonomy Mappings | ||||
2008-10-14 | CWE Content Team | MITRE | Internal | |
updated Description, Name, Relationships | ||||
2008-11-24 | CWE Content Team | MITRE | Internal | |
updated Relationships, Taxonomy Mappings | ||||
2009-01-12 | CWE Content Team | MITRE | Internal | |
updated Alternate Terms, Observed Examples, Other Notes, References, Relationship Notes, Relationships, Research Gaps | ||||
2009-05-27 | CWE Content Team | MITRE | Internal | |
updated Demonstrative Examples | ||||
2009-07-17 | KDM Analytics | External | ||
Improved the White Box Definition | ||||
2009-07-27 | CWE Content Team | MITRE | Internal | |
updated White Box Definitions | ||||
Previous Entry Names | ||||
Change Date | Previous Entry Name | |||
2008-10-14 | Time-of-check Time-of-use Race Condition | |||