CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition

Description

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.

Common consequences

• 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.

Potential 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.

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