CWE-29 Variante Incomplet

Path Traversal: '\..\filename'

This vulnerability occurs when an application builds file paths using user input but fails to block '\..\filename' sequences. Attackers can use these leading backslash and dot-dot sequences to…

Définition

What is CWE-29?

This vulnerability occurs when an application builds file paths using user input but fails to block '\..\filename' sequences. Attackers can use these leading backslash and dot-dot sequences to escape the intended directory and access unauthorized files or folders elsewhere on the system.

This flaw is a specific variant of path traversal that leverages the backslash (\) directory separator, common in Windows environments, instead of the forward slash (/). It's particularly effective at bypassing simple defenses that only check for patterns like '..\' at the very start of an input string, because a preceding backslash (as in '\..\') can slip past those filters. Developers often focus protection on the forward slash, assuming it's the only valid separator. This oversight allows attackers to use '\..\' sequences to navigate the filesystem. Understanding this technique is crucial for implementing robust validation that accounts for all possible path separators and relative sequence positions, not just those at the beginning of a user-supplied string.

Impact réel

Real-world CVEs caused by CWE-29

CVE-2002-1987

Protection mechanism checks for "/.." but doesn't account for Windows-specific "\.." allowing read of arbitrary files.
CVE-2005-2142

Directory traversal vulnerability in FTP server allows remote authenticated attackers to list arbitrary directories via a "\.." sequence in an LS command.

Comment les attaquants l'exploitent

Parcours de l'attaquant étape par étape

1
Identifier un chemin de code qui traite des entrées non fiables sans validation.
2
Élaborer une charge utile qui exploite le comportement non sécurisé — injection, traversal, débordement ou abus de logique.
3
Délivrer la charge utile via une requête normale et observer la réaction de l'application.
4
Itérer jusqu'à ce que la réponse divulgue des données, exécute le code de l'attaquant ou élève les privilèges.

Exemple de code vulnérable

Vulnerable pseudo

MITRE n'a pas publié d'exemple de code pour ce CWE. Le motif ci-dessous est illustratif — voir Ressources pour les références canoniques.

Vulnérable pseudo

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

Exemple de code sécurisé

Secure pseudo

Sécurisé 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.

Liste de contrôle de prévention

How to prevent CWE-29

Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a list of allowable file extensions, which will help to avoid CWE-434. Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.
Implementation Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Signaux de détection

How to detect CWE-29

SAST High

Exécuter une analyse statique (SAST) sur le code source à la recherche du motif non sécurisé dans le flux de données.

DAST Moderate

Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.

Runtime Moderate

Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.

Code review Moderate

Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.

Correction automatique Plexicus

Plexicus détecte automatiquement CWE-29 et ouvre une PR de correction en moins de 60 secondes.

Codex Remedium analyse chaque commit, identifie cette faiblesse précise et livre une pull request prête à être relue avec le correctif. Pas de tickets. Pas de transferts.

Obtenir une démo Essayer Plexicus gratuitement

Questions fréquentes

Frequently asked questions

Qu'est-ce que CWE-29 ?

Quelle est la gravité de CWE-29 ?

MITRE n'a pas publié de note de probabilité d'exploitation pour cette faiblesse. Traitez-la comme un impact moyen jusqu'à ce que votre modèle de menace prouve le contraire.

Quels langages ou plateformes sont affectés par CWE-29 ?

MITRE lists the following affected platforms: Windows.

Comment puis-je prévenir CWE-29 ?

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and…

Comment Plexicus détecte et corrige CWE-29 ?

Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-29 à chaque commit. Lorsqu'une correspondance est trouvée, notre agent Codex Remedium ouvre une PR de correction avec le code corrigé, les tests et un résumé d'une ligne pour le relecteur.

Où puis-je en savoir plus sur CWE-29 ?

MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/29.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.

Faiblesses associées

Weaknesses related to CWE-29

CWE-23 Parent

Relative Path Traversal

This vulnerability occurs when an application builds file paths using user-supplied input without properly validating or sanitizing it.…

CWE-24 Frère

Path Traversal: '../filedir'

Path traversal, often called directory traversal, occurs when an application builds a file path using user input without properly blocking…

CWE-25 Frère

Path Traversal: '/../filedir'

This vulnerability, often called directory traversal, occurs when an application builds a file path using user input without properly…

CWE-26 Frère

Path Traversal: '/dir/../filename'

This vulnerability occurs when an application builds a file path using user input but fails to properly block directory traversal…

CWE-27 Frère

Path Traversal: 'dir/../../filename'

This vulnerability occurs when an application builds file paths using user input but fails to properly block sequences like…

CWE-28 Frère

Path Traversal: '..\filedir'

This vulnerability occurs when an application builds a file path using user input but fails to block or properly handle '..\' sequences.…

CWE-30 Frère

Path Traversal: '\dir\..\filename'

This vulnerability occurs when an application builds file paths using user input but fails to properly sanitize sequences like…

CWE-31 Frère

Path Traversal: 'dir\..\..\filename'