CWE-61 Composto Incompleto High likelihood

UNIX Symbolic Link (Symlink) Following

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…

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. 1

    Identificar um caminho de código que trata input não confiável sem validação.

  2. 2

    Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.

  3. 3

    Entregar o payload através de um pedido normal e observar a reação da aplicação.

  4. 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.

Correção automática do Plexicus

O Plexicus deteta automaticamente o CWE-61 e abre um PR de correção em menos de 60 segundos.

O Codex Remedium analisa cada commit, identifica esta fraqueza exata e entrega um pull request pronto para revisão com o patch. Sem tickets. Sem transferências.

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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

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…

CWE-1386 Irmão

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…

CWE-62 Irmão

UNIX Hard Link

This vulnerability occurs when an application opens a file or directory without verifying if the name points to a hard link that leads…

CWE-64 Irmão

Windows Shortcut Following (.LNK)

This vulnerability occurs when an application opens a file or directory without properly validating that it is a Windows shortcut (.LNK).…

CWE-65 Irmão

Windows Hard Link

This vulnerability occurs when a Windows application opens a file or directory without properly verifying if the path points to a hard…

CWE-362 Par

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

A race condition occurs when multiple processes or threads access a shared resource simultaneously without proper coordination, creating a…

CWE-340 Par

Generation of Predictable Numbers or Identifiers

This vulnerability occurs when a system creates numbers or identifiers that are too easy to guess, undermining security mechanisms that…

CWE-386 Par

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-732 Par

Incorrect Permission Assignment for Critical Resource

This vulnerability occurs when a system grants overly permissive access to a sensitive resource, allowing unauthorized users or processes…

Recursos

Further reading

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