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
CWE-59 Base Draft
Improper Link Resolution Before File Access ('Link Following')
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…
Definition
What is CWE-59?
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.
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.
Real-world impact
Real-world CVEs caused by CWE-59
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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.
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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.
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Antivirus update allows local users to create or append to arbitrary files via a symlink attack on a logfile.
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Symlink attack allows local users to overwrite files.
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Window manager does not properly handle when certain symbolic links point to "stale" locations, which could allow local users to create or truncate arbitrary files.
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Second-order symlink vulnerabilities
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Second-order symlink vulnerabilities
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Symlink in Python program
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-59
- 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.
Detection signals
How to detect CWE-59
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
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
According to SOAR [REF-1479], the following detection techniques may be useful: ``` Cost effective for partial coverage: ``` Fuzz Tester Framework-based Fuzzer
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)
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
CWE-59
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Frequently asked questions What is CWE-59?
How serious is CWE-59?
What languages or platforms are affected by CWE-59?
How can I prevent CWE-59?
How does Plexicus detect and fix CWE-59?
Where can I learn more about CWE-59?
Frequently asked questions
What is CWE-59?
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.
How serious is CWE-59?
MITRE rates the likelihood of exploit as Medium — exploitation is realistic but typically requires specific conditions.
What languages or platforms are affected by CWE-59?
MITRE lists the following affected platforms: Windows, Unix.
How can I prevent CWE-59?
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.
How does Plexicus detect and fix CWE-59?
Plexicus's SAST engine matches the data-flow signature for CWE-59 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-59?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/59.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-59
CWE-706 Parent
Use of Incorrectly-Resolved Name or Reference
This vulnerability occurs when software uses a name, path, or reference to access a resource, but that identifier points to something…
CWE-178 Sibling
Improper Handling of Case Sensitivity
This vulnerability occurs when software fails to consistently handle uppercase and lowercase letters when checking or accessing resources,…
CWE-22 Sibling
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
This vulnerability occurs when an application builds a file path using user input but fails to properly validate it, allowing an attacker…
CWE-386 Sibling
Symbolic Name not Mapping to Correct Object
This vulnerability occurs when a program uses a fixed symbolic name (like a constant or identifier) to refer to an object, but that name…
CWE-41 Sibling
Improper Resolution of Path Equivalence
This vulnerability occurs when an application fails to properly handle different text representations that refer to the same file or…
CWE-66 Sibling
Improper Handling of File Names that Identify Virtual Resources
This vulnerability occurs when software incorrectly processes a filename that points to a 'virtual' resource—like a device, pipe, or…
CWE-827 Sibling
Improper Control of Document Type Definition
This vulnerability occurs when an application fails to properly restrict which Document Type Definitions (DTDs) can be referenced during…
CWE-98 Sibling
Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
This vulnerability occurs when a PHP application uses unvalidated or insufficiently restricted user input directly within file inclusion…
CWE-1386 Child
Insecure Operation on Windows Junction / Mount Point
This vulnerability occurs when a Windows application opens a file or directory without properly validating that the path is not a symbolic…
Resources
Further reading
- MITRE — official CWE-59 https://cwe.mitre.org/data/definitions/59.html
- Zip Slip Vulnerability https://security.snyk.io/research/zip-slip-vulnerability
- State-of-the-Art Resources (SOAR) for Software Vulnerability Detection, Test, and Evaluation https://www.ida.org/-/media/feature/publications/s/st/stateoftheart-resources-soar-for-software-vulnerability-detection-test-and-evaluation/p-5061.ashx
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