Improper Link Resolution Before File Access ('Link Following')

Description

This vulnerability occurs when an application uses a filename to access a file but fails to properly check if that name points to a symbolic link, shortcut, or junction. This allows an attacker to manipulate the link's target, causing the application to read or write to an unintended, potentially sensitive location.

Extended Description

At its core, this is a path traversal issue enabled by symbolic links. When a program doesn't verify that a file is a regular file and not a link, an attacker can create a link that points outside the intended directory—like a system file or another user's data. The application then blindly follows this link, leading to unauthorized access, data corruption, or a denial of service. To prevent this, developers must explicitly resolve symbolic links before performing file operations. This involves using system calls or functions that return the canonical path of a file, checking the file's type, and implementing proper access controls. Always assume filenames provided by users or external sources are untrusted and validate the final resolved path against a safe allowlist.

Common Consequences 2

Scope: ConfidentialityIntegrityAccess Control

Impact: Read Files or DirectoriesModify Files or DirectoriesBypass Protection Mechanism

An attacker may be able to traverse the file system to unintended locations and read or overwrite the contents of unexpected files. If the files are used for a security mechanism then an attacker may be able to bypass the mechanism.

Scope: Other

Impact: Execute Unauthorized Code or Commands

Windows simple shortcuts, sometimes referred to as soft links, can be exploited remotely since a ".LNK" file can be uploaded like a normal file. This can enable remote execution.

Detection Methods 7

Automated Static Analysis - Binary or BytecodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Bytecode Weakness Analysis - including disassembler + source code weakness analysis

Manual Static Analysis - Binary or BytecodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Binary / Bytecode disassembler - then use manual analysis for vulnerabilities & anomalies

Dynamic Analysis with Automated Results InterpretationSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Web Application Scanner Web Services Scanner Database Scanners

Dynamic Analysis with Manual Results InterpretationSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Fuzz Tester Framework-based Fuzzer

Manual Static Analysis - Source CodeHigh

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Focused Manual Spotcheck - Focused manual analysis of source Manual Source Code Review (not inspections)

Automated Static Analysis - Source CodeSOAR Partial

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Source code Weakness Analyzer Context-configured Source Code Weakness Analyzer

Architecture or Design ReviewHigh

According to SOAR [REF-1479], the following detection techniques may be useful: ``` Highly cost effective: ``` Formal Methods / Correct-By-Construction ``` Cost effective for partial coverage: ``` Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)

Potential Mitigations 1

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.

Observed Examples 28

CVE-1999-1386Some 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-2000-1178Text 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-2004-0217Antivirus update allows local users to create or append to arbitrary files via a symlink attack on a logfile.

CVE-2003-0517Symlink attack allows local users to overwrite files.

CVE-2004-0689Window manager does not properly handle when certain symbolic links point to "stale" locations, which could allow local users to create or truncate arbitrary files.

CVE-2005-1879Second-order symlink vulnerabilities

CVE-2005-1880Second-order symlink vulnerabilities

CVE-2005-1916Symlink in Python program

CVE-2000-0972Setuid 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-2005-0824Signal causes a dump that follows symlinks.

CVE-2001-1494Hard link attack, file overwrite; interesting because program checks against soft links

CVE-2002-0793Hard link and possibly symbolic link following vulnerabilities in embedded operating system allow local users to overwrite arbitrary files.

CVE-2003-0578Server creates hard links and unlinks files as root, which allows local users to gain privileges by deleting and overwriting arbitrary files.

CVE-1999-0783Operating system allows local users to conduct a denial of service by creating a hard link from a device special file to a file on an NFS file system.

CVE-2004-1603Web hosting manager follows hard links, which allows local users to read or modify arbitrary files.

CVE-2004-1901Package listing system allows local users to overwrite arbitrary files via a hard link attack on the lockfiles.

CVE-2005-1111Hard link race condition

CVE-2000-0342Mail client allows remote attackers to bypass the user warning for executable attachments such as .exe, .com, and .bat by using a .lnk file that refers to the attachment, aka "Stealth Attachment."

CVE-2001-1042FTP server allows remote attackers to read arbitrary files and directories by uploading a .lnk (link) file that points to the target file.

CVE-2001-1043FTP server allows remote attackers to read arbitrary files and directories by uploading a .lnk (link) file that points to the target file.

CVE-2005-0587Browser allows remote malicious web sites to overwrite arbitrary files by tricking the user into downloading a .LNK (link) file twice, which overwrites the file that was referenced in the first .LNK file.

CVE-2001-1386".LNK." - .LNK with trailing dot

CVE-2003-1233Rootkits can bypass file access restrictions to Windows kernel directories using NtCreateSymbolicLinkObject function to create symbolic link

CVE-2002-0725File system allows local attackers to hide file usage activities via a hard link to the target file, which causes the link to be recorded in the audit trail instead of the target file.

CVE-2003-0844Web server plugin allows local users to overwrite arbitrary files via a symlink attack on predictable temporary filenames.

CVE-2015-3629A 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-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.

CVE-2020-27833"Zip Slip" vulnerability in container management product allows writing arbitrary files outside intended directory via a container image (.tar format) with filenames that are symbolic links that point to other files within the same tar file; however, the files being pointed to can also be symbolic links to destinations outside the intended directory, bypassing the initial check.

References 3

The Art of Software Security Assessment

Mark Dowd, John McDonald, and Justin Schuh

Addison Wesley

2006

ID: REF-62

Zip Slip Vulnerability

Snyk

05-06-2018

https://security.snyk.io/research/zip-slip-vulnerability

ID: REF-1282

State-of-the-Art Resources (SOAR) for Software Vulnerability Detection, Test, and Evaluation

Gregory Larsen, E. Kenneth Hong Fong, David A. Wheeler, and Rama S. Moorthy

07-2014

https://www.ida.org/-/media/feature/publications/s/st/stateoftheart-resources-soar-for-software-vulnerability-detection-test-and-evaluation/p-5061.ashx(2025-09-05)

ID: REF-1479

Likelihood of Exploit

Medium

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Attack Patterns

Alternate Terms

insecure temporary file

Some people use the phrase "insecure temporary file" when referring to a link following weakness, but other weaknesses can produce insecure temporary files without any symlink involvement at all.

Zip Slip

"Zip slip" is an attack that uses file archives (e.g., ZIP, tar, rar, etc.) that contain filenames with path traversal sequences that cause the files to be written outside of the directory under which the archive is expected to be extracted [REF-1282]. It is most commonly used for relative path traversal (Relative Path Traversal) and link following (Improper Link Resolution Before File Access ('Link Following')).

Functional Areas

File Processing

Affected Resources

File or Directory

Related Weaknesses

ChildOf:

Use of Incorrectly-Resolved Name or Reference (CWE-706)

ChildOf:

Use of Incorrectly-Resolved Name or Reference (CWE-706)

Taxonomy Mapping

PLOVER
CERT C Secure Coding
CERT C Secure Coding
SEI CERT Perl Coding Standard
Software Fault Patterns

Notes

Theoretical Link following vulnerabilities are Multi-factor Vulnerabilities (MFV). They are the combination of multiple elements: file or directory permissions, filename predictability, race conditions, and in some cases, a design limitation in which there is no mechanism for performing atomic file creation operations. Some potential factors are race conditions, permissions, and predictability.