CWE WEAKNESSES  /  CWE-180

CWE-180

Incorrect Behavior Order: Validate Before Canonicalize

What it is

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

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.

Impact

Mitigations

• [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.

Real-world CVE examples

• CVE-2002-0433 — List files in web server using "*.ext"
• 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 f
• 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 ap
• CVE-2000-0191 — Overlaps "fakechild/../realchild"
• 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.

Related weaknesses

Source: MITRE CWE. View on cwe.mitre.org →