CWE-61: UNIX Symbolic Link (Symlink) Following

Description

The product, 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 product to operate on unauthorized files.

Extended Description

A product 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.

ThreatScore

Threat Mapped score: 0.0

Industry: Finiancial

Threat priority: Unclassified

Observed Examples (CVEs)

CVE: CVE-1999-1386

Some versions of Perl follow symbolic links when running with the -e option, which allows local users to overwrite arbitrary files via a symlink attack.
CVE: CVE-2000-1178

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.
CVE: CVE-2004-0217

Antivirus update allows local users to create or append to arbitrary files via a symlink attack on a logfile.
CVE: CVE-2003-0517

Symlink attack allows local users to overwrite files.
CVE: CVE-2004-0689

Possible interesting example
CVE: CVE-2005-1879

Second-order symlink vulnerabilities
CVE: CVE-2005-1880

Second-order symlink vulnerabilities
CVE: CVE-2005-1916

Symlink in Python program
CVE: CVE-2000-0972

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.
CVE: CVE-2005-0824

Signal causes a dump that follows symlinks.
CVE: CVE-2015-3629

A Libcontainer used in Docker Engine allows local users to escape containerization and write to an arbitrary file on the host system via a symlink attack in an image when respawning a container.
CVE: CVE-2020-26277

In a MySQL database deployment tool, users may craft a maliciously packaged tarball that contains symlinks to files external to the target and once unpacked, will execute.
CVE: CVE-2021-21272

"Zip Slip" vulnerability in Go-based Open Container Initiative (OCI) registries product allows writing arbitrary files outside intended directory via symbolic links or hard links in a gzipped tarball.

Related Attack Patterns (CAPEC)

CAPEC-27

Attack TTPs

N/A

Modes of Introduction

Phase	Note
Implementation	These are typically reported for temporary files or privileged programs.

Common Consequences

Impact: Read Files or Directories, Modify Files or Directories — Notes:

Potential Mitigations

Implementation: 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. (N/A)
Architecture and Design: 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. (N/A)

Applicable Platforms

None (Not Language-Specific, Undetermined)

Demonstrative Examples

N/A

Notes

Research Gap: 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 [REF-493].

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