Compound Element ID: 61 (Compound Element Variant: Composite)
Status: Incomplete
Description
Description Summary
The software, when opening a file or directory, does not sufficiently account for when the file is a symbolic link that resolves to a target outside of the intended control sphere. This could allow an attacker to cause the software to operate on unauthorized files.
Extended Description
A software system that allows UNIX symbolic links (symlink) as part of paths whether in internal code or through user input can allow an attacker to spoof the symbolic link and traverse the file system to unintended locations or access arbitrary files. The symbolic link can permit an attacker to read/write/corrupt a file that they originally did not have permissions to access.
Alternate Terms
Symlink following
symlink vulnerability
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Confidentiality
Integrity
Technical Impact: Read files or
directories; Modify files or
directories
Some versions of Perl follows symbolic links when
running with the -e option, which allows local users to overwrite arbitrary
files via a symlink attack.
Text editor follows symbolic links when creating
a rescue copy during an abnormal exit, which allows local users to overwrite
the files of other users.
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.
Symbolic link attacks often occur when a program creates a tmp
directory that stores files/links. Access to the directory should be
restricted to the program as to prevent attackers from manipulating the
files.
Phase: Architecture and Design
Strategy: Separation of Privilege
Follow the principle of least privilege when assigning access rights
to entities in a software system.
Denying access to a file can prevent an attacker from replacing that
file with a link to a sensitive file. Ensure good compartmentalization
in the system to provide protected areas that can be trusted.
Symlink vulnerabilities are regularly found in C and shell programs, but
all programming languages can have this problem. Even shell programs are
probably under-reported.
"Second-order symlink vulnerabilities" may exist in programs that invoke
other programs that follow symlinks. They are rarely reported but are likely
to be fairly common when process invocation is used. Reference:
[Christey2005]
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "Symbolic Link Attacks", Page
518.. 1st Edition. Addison Wesley. 2006.
Description
