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CWE-180 Variant Draft
Incorrect Behavior Order: Validate Before Canonicalize
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
Definition
What is CWE-180?
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
When validation runs before canonicalization (the process of converting data into a standard, consistent form), attackers can exploit the gap between these two steps. They can submit input that appears safe during the initial check but transforms into a dangerous payload after it's standardized. For example, an attacker might use alternate character encodings, multiple slashes, or dot sequences that resolve to a forbidden path after canonicalization.
This flaw effectively neutralizes security defenses like allow-lists or injection filters, creating a false sense of security. To prevent this, always canonicalize input first—convert it to its simplest, canonical form—and then perform validation and sanitization on that standardized data. This ensures your security logic evaluates the actual data the application will use.
Auswirkungen in der Praxis
Real-world CVEs caused by CWE-180
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Product allows remote attackers to view restricted files via an HTTP request containing a "*" (wildcard or asterisk) character.
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Product modifies the first two letters of a filename extension after performing a security check, which allows remote attackers to bypass authentication via a filename with a .ats extension instead of a .hts extension.
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Database consumes an extra character when processing a character that cannot be converted, which could remove an escape character from the query and make the application subject to SQL injection attacks.
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Overlaps "fakechild/../realchild"
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Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.
Wie Angreifer es ausnutzen
Angreiferpfad Schritt für Schritt
- 1
The following code attempts to validate a given input path by checking it against an allowlist and then return the canonical path. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".
- 2
The problem with the above code is that the validation step occurs before canonicalization occurs. An attacker could provide an input path of "/safe_dir/../" that would pass the validation step. However, the canonicalization process sees the double dot as a traversal to the parent directory and hence when canonicized the path would become just "/".
- 3
To avoid this problem, validation should occur after canonicalization takes place. In this case canonicalization occurs during the initialization of the File object. The code below fixes the issue.
Verwundbares Codebeispiel
Vulnerable Java
The following code attempts to validate a given input path by checking it against an allowlist and then return the canonical path. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".
String path = getInputPath();
if (path.startsWith("/safe_dir/"))
{
File f = new File(path);
return f.getCanonicalPath();
} Sicheres Codebeispiel
Secure Java
To avoid this problem, validation should occur after canonicalization takes place. In this case canonicalization occurs during the initialization of the File object. The code below fixes the issue.
String path = getInputPath();
File f = new File(path);
if (f.getCanonicalPath().startsWith("/safe_dir/"))
{
return f.getCanonicalPath();
} 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-180
- 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-180
Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.
Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.
Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.
CWE-180
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.
Häufig gestellte Fragen Was ist CWE-180?
Wie gravierend ist CWE-180?
Welche Sprachen oder Plattformen sind von CWE-180 betroffen?
Wie kann ich CWE-180 verhindern?
Wie erkennt und behebt Plexicus CWE-180?
Wo erfahre ich mehr über CWE-180?
Frequently asked questions
Was ist CWE-180?
This vulnerability occurs when a system checks user input for malicious content before standardizing its format, allowing specially crafted data to bypass security checks.
Wie gravierend ist CWE-180?
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-180 betroffen?
MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.
Wie kann ich CWE-180 verhindern?
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.
Wie erkennt und behebt Plexicus CWE-180?
Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-180 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-180?
MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/180.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.
Verwandte Schwachstellen
Weaknesses related to CWE-180
CWE-179 Parent
Incorrect Behavior Order: Early Validation
This vulnerability occurs when an application validates user input before applying security filters or data normalization. Attackers can…
CWE-181 Sibling
Incorrect Behavior Order: Validate Before Filter
This vulnerability occurs when a system checks user input for validity before cleaning or filtering it. This flawed sequence allows…
Ressourcen
Further reading
Bereit, das Wesentliche zu validieren?
Bereit zu validieren, was zählt.
Plexicus ist Proof-Driven AppSec: validierte Funde, kontextuelles Verständnis und geprüfte Remediation — in Evidenz verankert, mit Ihnen gescoped.