CWE

Incorrect Behavior Order: Validate Before Canonicalize

ID: 180		Date: (C)2012-05-14   (M)2022-10-10
Type: weakness		Status: DRAFT
Abstraction Type: Base

Description

The software validates input before it is canonicalized, which prevents the software from detecting data that becomes invalid after the canonicalization step.

Extended Description

This can be used by an attacker to bypass the validation and launch attacks that expose weaknesses that would otherwise be prevented, such as injection.

Applicable Platforms
Language Class: All

Time Of Introduction

• Implementation

Related Attack Patterns

• CAPEC-267
• CAPEC-3
• CAPEC-4
• CAPEC-71
• CAPEC-78
• CAPEC-79
• CAPEC-80

Common Consequences

Scope	Technical Impact	Notes
Access_Control	Bypass protection mechanism

Detection Methods
None

Potential Mitigations

Phase	Strategy	Description	Effectiveness	Notes
Implementation	Input Validation	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 whitelist validation schemes by introducing dangerous inputs after they have been checked.

Relationships
This overlaps other categories.

Related CWE	Type	View	Chain
CWE-180 ChildOf CWE-896	Category	CWE-888

Demonstrative Examples   (Details)

1. The following code attempts to validate a given input path by checking it against a whitelist and then return the canonical path. In this specific case, the path is considered valid if it starts with the string "/safe_dir/". (Demonstrative Example Id DX-35)

Observed Examples

1. CVE-2002-0433 : Product allows remote attackers to view restricted files via an HTTP request containing a "*" (wildcard or asterisk) character.
2. CVE-2003-0332 : 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.
3. CVE-2002-0802 : 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.
4. CVE-2000-0191 : Overlaps "fakechild/../realchild"
5. CVE-2004-2363 : Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

Taxynomy	Id	Name	Fit
PLOVER		Validate-Before-Canonicalize
OWASP Top Ten 2004	A1	Unvalidated Input	CWE_More_Specific
CERT Java Secure Coding	IDS01-J	Normalize strings before validating them

References:
None