CWE-29 Variant Incomplete

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…

Definition

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.

Auswirkungen in der Praxis

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.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

1
Identifiziere einen Codepfad, der nicht vertrauenswürdige Eingaben ohne Validierung verarbeitet.
2
Erzeuge eine Payload, die das unsichere Verhalten auslöst — Injection, Traversal, Overflow oder Logik-Missbrauch.
3
Liefere die Payload über einen normalen Request aus und beobachte die Reaktion der Anwendung.
4
Iteriere, bis die Antwort Daten preisgibt, Angreifer-Code ausführt oder Berechtigungen eskaliert.

Verwundbares Codebeispiel

Vulnerable pseudo

MITRE hat kein Codebeispiel für diese CWE veröffentlicht. Das untenstehende Muster ist illustrativ — kanonische Referenzen findest du unter Ressourcen.

Verwundbar pseudo

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

Sicheres Codebeispiel

Secure pseudo

Sicher 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.

Präventions-Checkliste

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.

Erkennungssignale

How to detect CWE-29

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

Plexicus Auto-Fix

Plexicus erkennt CWE-29 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen

Häufig gestellte Fragen

Frequently asked questions

Was ist CWE-29?

Wie gravierend ist CWE-29?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-29 betroffen?

MITRE lists the following affected platforms: Windows.

Wie kann ich CWE-29 verhindern?

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…

Wie erkennt und behebt Plexicus CWE-29?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-29 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-29?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/29.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

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 Sibling

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 Sibling

Path Traversal: '/../filedir'

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

CWE-26 Sibling

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 Sibling

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 Sibling

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 Sibling

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 Sibling

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