Common Attack Pattern Enumeration and Classification
A Community Resource for Identifying and Understanding Attacks
This attack leverages the use of symbolic links (Symlinks) in order to write to sensitive files. An attacker can create a Symlink link to a target file not otherwise accessible to her. When the privileged program tries to create a temporary file with the same name as the Symlink link, it will actually write to the target file pointed to by the attackers' Symlink link. If the attacker can insert malicious content in the temporary file she will be writing to the sensitive file by using the Symlink. The race occurs because the system checks if the temporary file exists, then creates the file. The attacker would typically create the Symlink during the interval between the check and the creation of the temporary file.
In this naive example, the Unix program foo is setuid. Its function is to retrieve information for the accounts specified by the user. For "efficiency," it sorts the requested accounts into a temporary file (/tmp/foo naturally) before making the queries.
The directory /tmp is world-writable. Malicious user Mallory creates a symbolic link to the file /.rhosts named /tmp/foo. Then, she invokes foo with + + as the requested account. The program creates the (temporary) file /tmp/foo (really creating /.rhosts) and puts the requested account (+ +) in it. It removes the temporary file (merely removing the symbolic link).
Now the /.rhosts contains + +, which is the incantation necessary to allow anyone to use rlogin to log into the computer as the superuser.
GNU ed before 0.3 allows local users to overwrite arbitrary files via a symlink attack on temporary files, possibly in the open_sbuf function.
OpenmosixCollector and OpenMosixView in OpenMosixView 1.5 allow local users to overwrite or delete arbitrary files via a symlink attack on (1) temporary files in the openmosixcollector directory or (2) nodes.tmp.
Setuid product allows file reading by replacing a file being edited with a symlink to the targeted file, leaking the result in error messages when parsing fails.
Skill or Knowledge Level: Medium
This attack is sophisticated because the attacker has to overcome a few challenges such as creating symlinks on the target host during a precise timing, inserting malicious data in the temporary file and have knowledge about the temporary files created (file name and function which creates them).
The attacker will certainly look for file system locations where he can write and create Symlink links.
The attacker may also observe the system and locate the temporary files created during a call to a certain function.
Use safe libraries when creating temporary files. For instance the standard library function mkstemp can be used to safely create temporary files. For shell scripts, the system utility mktemp does the same thing.
Access to the directories should be restricted as to prevent attackers from manipulating the files. Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file.
Follow the principle of least privilege when assigning access rights to files.
Ensure good compartmentalization in the system to provide protected areas that can be trusted.
The content of the temporary file which is copied to the file pointed to by the Symlink.
This attack can cause privilege escalation, modification of resources or denial of services.
[R.27.1] [REF-6] "Wikipedia". Symlink race. The Wikimedia Foundation, Inc. <http://en.wikipedia.org/wiki/Symlink_race>.
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