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.
Extended Description
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.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Confidentiality
Integrity
Technical Impact: Read files or
directories; Modify files or
directories
Demonstrative Examples
Example 1
This code prints the contents of a file if a user has permission.
(Bad Code)
Example
Language: PHP
function readFile($filename){
$user = getCurrentUser();
//resolve file if its a symbolic link
if(is_link($filename)){
$filename = readlink($filename);
}
if(fileowner($filename) == $user){
echo file_get_contents($realFile);
return;
}
else{
echo 'Access denied';
return false;
}
}
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).
This is already covered by the "Link Following" weakness (CWE-59). It is included here because so many people associate race conditions with link problems; however, not all link following issues involve race conditions.
Taxonomy Mappings
Mapped Taxonomy Name
Node ID
Fit
Mapped Node Name
PLOVER
Race condition enabling link following
CERT C Secure Coding
POS35-C
Avoid race conditions while checking for the existence of a
symbolic link
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "Race Conditions", Page 526.. 1st Edition. Addison Wesley. 2006.
Description
