logo

Incorrect Behavior Order: Validate Before Canonicalize

Draft Variant
Structure: Simple
Description

The product validates input before it is canonicalized, which prevents the product 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.
Common Consequences 1
Scope: Access Control

Impact: Bypass Protection Mechanism
Potential Mitigations 1
Phase: Implementation

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated (Incorrect Behavior Order: Validate Before Canonicalize). Make sure that the application does not decode the same input twice (Double Decoding of the Same Data). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Demonstrative Examples 1

ID : DX-35

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/".

Code Example:

Bad
Java
java
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 "/".
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.

Code Example:

Good
Java
java
Observed Examples 5
CVE-2002-0433Product allows remote attackers to view restricted files via an HTTP request containing a "*" (wildcard or asterisk) character.
CVE-2003-0332Product 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.
CVE-2002-0802Database 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.
CVE-2000-0191Overlaps "fakechild/../realchild"
CVE-2004-2363Product checks URI for "<" and other literal characters, but does it before hex decoding the URI, so "%3E" and other sequences are allowed.
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Modes of Introduction
Implementation
Related Attack Patterns
Related Weaknesses
ChildOf: Incorrect Behavior Order: Early Validation (CWE-179)
Taxonomy Mapping
  • PLOVER
  • OWASP Top Ten 2004
  • The CERT Oracle Secure Coding Standard for Java (2011)
  • SEI CERT Oracle Coding Standard for Java
Notes
RelationshipThis overlaps other categories.