Race Condition Enabling Link Following
The software checks the status of a file or directory before accessing it, which produces a race condition in which the file can be replaced with a link before the access is performed, causing the software to access the wrong file.
While developers might expect that there is a very narrow time window between the time of check and time of use, there is still a race condition. An attacker could cause the software to slow down (e.g. with memory consumption), causing the time window to become larger. Alternately, in some situations, the attacker could win the race by performing a large number of attacks.
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
This code prints the contents of a file if a user has permission.
This code attempts to resolve symbolic links before checking the file and printing its contents. However, an attacker may be able to change the file from a real file to a symbolic link between the calls to is_link() and file_get_contents(), allowing the reading of arbitrary files. Note that this code fails to log the attempted access (CWE-778).
Weaknesses in this category are related to the rules and recommendations in the POSIX (POS) section of the SEI CERT C Coding Standard.
This category identifies Software Fault Patterns (SFPs) within the Race Condition Window cluster (SFP20).
Weaknesses in this category are related to concurrent use of shared resources.
This view (slice) covers all the elements in CWE.
This view (slice) lists weaknesses that can be introduced during implementation.
This view (slice) lists weaknesses that can be introduced during design.