CWE-61: UNIX Symbolic Link (Symlink) Following

Definição

What is CWE-61?

This vulnerability occurs when a software application opens files or directories without properly checking if they are symbolic links (symlinks). If an attacker can create or control a symlink, they can redirect file operations to locations outside the intended directory, potentially accessing or modifying unauthorized files.

In Unix-like systems, symbolic links are shortcuts that point to other files or directories. When an application uses paths that can contain symlinks—whether from internal logic or user-supplied input—an attacker can create a malicious symlink that points anywhere in the filesystem. This bypasses the application's intended directory restrictions, allowing the attacker to traverse to sensitive locations they shouldn't access. Without proper validation, the application will follow the symlink and perform operations like reading, writing, or deleting on the target file. This means an attacker could expose confidential data, corrupt critical system files, or escalate privileges by manipulating files originally protected by normal permissions. The core issue is that the application trusts the path structure without verifying that symlinks haven't redirected it outside its allowed scope.

Impacto no mundo real

Real-world CVEs caused by CWE-61

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

Possible interesting example
CVE-2005-1879

Second-order symlink vulnerabilities
CVE-2005-1880

Second-order symlink vulnerabilities
CVE-2005-1916

Symlink in Python program

Como os atacantes a exploram

Trajeto do atacante passo a passo

1
Identificar um caminho de código que trata input não confiável sem validação.
2
Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.
3
Entregar o payload através de um pedido normal e observar a reação da aplicação.
4
Iterar até que a resposta exponha dados, execute código do atacante ou escale privilégios.

Exemplo de código vulnerável

Vulnerable pseudo

A MITRE não publicou um exemplo de código para este CWE. O padrão abaixo é ilustrativo — consulte os Recursos para referências canónicas.

Vulnerável pseudo

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

Exemplo 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 verificação de prevenção

How to prevent CWE-61

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.
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.

Sinais de deteção

How to detect CWE-61

SAST High

Executar análise estática (SAST) na base de código à procura do padrão inseguro no fluxo de dados.

DAST Moderate

Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.

Runtime Moderate

Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.

Code review Moderate

Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.

CWE-61

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

Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.

Request AI Swarm Pentest

Perguntas frequentes

Frequently asked questions

O que é o CWE-61?

This vulnerability occurs when a software application opens files or directories without properly checking if they are symbolic links (symlinks). If an attacker can create or control a symlink, they can redirect file operations to locations outside the intended directory, potentially accessing or modifying unauthorized files.

Qual a gravidade do CWE-61?

A MITRE classifica a probabilidade de exploração como Alta — esta fraqueza é ativamente explorada em campo e deve ser priorizada para remediação.

Que linguagens ou plataformas são afetadas pelo CWE-61?

A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.

Como posso prevenir o CWE-61?

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. 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…

Como é que o Plexicus deteta e corrige o CWE-61?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-61 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.

Onde posso saber mais sobre o CWE-61?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/61.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-61

CWE-59 Pai

Pronto para validar o que importa.

O Plexicus é Proof-Driven AppSec: achados validados, compreensão contextual e remediação revisada — ancorada em evidência, escopada com você.

Reservar Pentest

UNIX Symbolic Link (Symlink) Following

What is CWE-61?

Real-world CVEs caused by CWE-61

Trajeto do atacante passo a passo

Vulnerable pseudo

Secure pseudo

How to prevent CWE-61

How to detect CWE-61

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

Frequently asked questions

Weaknesses related to CWE-61

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

Insecure Operation on Windows Junction / Mount Point

UNIX Hard Link

Windows Shortcut Following (.LNK)

Windows Hard Link

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

Generation of Predictable Numbers or Identifiers

Symbolic Name not Mapping to Correct Object

Incorrect Permission Assignment for Critical Resource

Further reading

Pronto para validar o que importa.

Verifique se o AI Swarm Pentest se adequa ao seu ambiente.

Pedido recebido. Marque o próximo passo.

Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`