CWE-59: Improper Link Resolution Before File Access ('Link Following')

Definición

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.

Impacto en el mundo real

Real-world CVEs caused by CWE-59

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-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-2004-0217

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

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

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.
CVE-2005-1879

Second-order symlink vulnerabilities
CVE-2005-1880

Second-order symlink vulnerabilities
CVE-2005-1916

Symlink in Python program

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

1
Identifica una ruta de código que maneje entrada no confiable sin validación.
2
Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.
3
Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.
4
Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.

Ejemplo de código vulnerable

Vulnerable pseudo

MITRE no ha publicado un ejemplo de código para esta CWE. El patrón siguiente es ilustrativo — consulta Recursos para referencias canónicas.

Vulnerable pseudo

// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}

Ejemplo de código seguro

Secure pseudo

Seguro 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.

Lista de prevención

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.

Señales de detección

How to detect CWE-59

Automated Static Analysis - Binary or Bytecode SOAR 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 Bytecode SOAR 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 Interpretation SOAR 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 Interpretation SOAR 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 Code High

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 Code SOAR 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

CWE-59

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Preguntas frecuentes

Frequently asked questions

¿Qué es 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.

¿Qué gravedad tiene CWE-59?

MITRE califica la probabilidad de explotación como Media — la explotación es realista pero suele requerir condiciones específicas.

¿Qué lenguajes o plataformas se ven afectados por CWE-59?

MITRE lists the following affected platforms: Windows, Unix.

¿Cómo puedo prevenir 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.

¿Cómo detecta y corrige Plexicus CWE-59?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-59 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-59?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/59.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-59

CWE-706 Padre

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Plexicus es Proof-Driven AppSec: hallazgos validados, comprensión contextual y remediación revisada — anclada en evidencia, acotada contigo.

Reservar Pentest

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

What is CWE-59?

Real-world CVEs caused by CWE-59

Ruta del atacante paso a paso

Vulnerable pseudo

Secure pseudo

How to prevent CWE-59

How to detect CWE-59

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-59

Use of Incorrectly-Resolved Name or Reference

Improper Handling of Case Sensitivity

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Symbolic Name not Mapping to Correct Object

Improper Resolution of Path Equivalence

Improper Handling of File Names that Identify Virtual Resources

Improper Control of Document Type Definition

Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')

Insecure Operation on Windows Junction / Mount Point

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`