CWE-177 Variant Draft

Improper Handling of URL Encoding (Hex Encoding)

This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or…

Definition

What is CWE-177?

This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or inconsistently handling encoded characters across different security checks.
URL encoding (like %20 for a space) is a standard way to safely transmit special characters in web requests. The security flaw arises when an application's validation logic, filtering routines, and business logic treat the same encoded input differently. For example, a security filter might check the raw encoded string, while a later processing step decodes it, allowing malicious payloads to slip through undetected. This inconsistency creates a classic security bypass. An attacker can embed malicious code (like SQL commands or script tags) within encoded sequences that evade initial validation. To prevent this, developers must ensure all security controls—input validation, output encoding, and business logic—operate on the same canonical, decoded representation of the data, and apply decoding only once, at the very beginning of the request pipeline.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-177

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

    Identifiziere einen Codepfad, der nicht vertrauenswürdige Eingaben ohne Validierung verarbeitet.

  2. 2

    Erzeuge eine Payload, die das unsichere Verhalten auslöst — Injection, Traversal, Overflow oder Logik-Missbrauch.

  3. 3

    Liefere die Payload über einen normalen Request aus und beobachte die Reaktion der Anwendung.

  4. 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-177

  • Architecture and Design Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
  • 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.
  • 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-177

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-177 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-177?

This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or inconsistently handling encoded characters across different security checks.

Wie gravierend ist CWE-177?

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-177 betroffen?

MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.

Wie kann ich CWE-177 verhindern?

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names. 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…

Wie erkennt und behebt Plexicus CWE-177?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-177 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-177?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/177.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-177

CWE-172 Parent

Encoding Error

This vulnerability occurs when software incorrectly transforms data between different formats, leading to corrupted or misinterpreted…

CWE-173 Sibling

Improper Handling of Alternate Encoding

This vulnerability occurs when software fails to correctly process input that arrives in a different character encoding than expected,…

CWE-174 Sibling

Double Decoding of the Same Data

This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or…

CWE-175 Sibling

Improper Handling of Mixed Encoding

This vulnerability occurs when software fails to correctly process input that contains multiple character encodings within the same data…

CWE-176 Sibling

Improper Handling of Unicode Encoding

This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data…

Ressourcen

Further reading

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