Status: Incomplete Weakness ID: 367 (Weakness Base)Summary 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 state. 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. 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: C/C++ Example: struct stat *sb; ... lstat("...",sb); // it has not been updated since the last time it
was read printf("stated file\n"); if (sb->st_mtimespec==...){ print("Now updating things\n"); updateThings(); } 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. C Example: if(!access(file,W_OK)) { f = fopen(file,"w+"); operate(f); ... } else { fprintf(stderr,"Unable to open file %s.\n",file); } 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.
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. Do not rely on user-specified input to determine what path to format. 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. 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.
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.
A weakness where code path has: 1. start statement that validates a system resource by a reference 2. end statement that accesses the system resource by the reference 3. the validated system resource is not equal to accessed system resource Dan Tsafrir, Tomer Hertz, David Wagner
and Dilma Da Silva. "Portably Solving File TOCTTOU Races with Hardness
Amplification". 2008-02-28. <http:/ Submissions PLOVER. (Externally Mined) Modifications Eric Dalci. Cigital. 2008-07-01. (External) updated Time_of_Introduction KDM Analytics. 2008-08-01. (External) added/updated white box definitions CWE Content Team. MITRE. 2008-09-08. (Internal) updated Common_Consequences, Relationships, Other_Notes,
Taxonomy_Mappings CWE Content Team. MITRE. 2008-10-14. (Internal) updated Description, Name,
Relationships CWE Content Team. MITRE. 2008-11-24. (Internal) updated Relationships,
Taxonomy_Mappings CWE Content Team. MITRE. 2009-01-12. (Internal) updated Alternate_Terms, Observed_Examples, Other_Notes,
References, Relationship_Notes, Relationships,
Research_Gaps CWE Content Team. MITRE. 2009-05-27. (Internal) updated Demonstrative_Examples Previous Entry Names Time-of-check Time-of-use
Race Condition (changed
2008-10-14) |
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Page Last Updated:
May 26, 2009
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