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.

Primary

Implementation

Access Control
Bypass Protection Mechanism

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 allowlist validation schemes by introducing dangerous inputs after they have been checked.

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();}

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.

String path = getInputPath();File f = new File(path);if (f.getCanonicalPath().startsWith("/safe_dir/")){return f.getCanonicalPath();}

CVE-2002-0433
List files in web server using "*.ext"
https://www.cve.org/CVERecord?id=CVE-2002-0433

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.
https://www.cve.org/CVERecord?id=CVE-2003-0332

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.
https://www.cve.org/CVERecord?id=CVE-2002-0802

CVE-2000-0191
Overlaps "fakechild/../realchild"
https://www.cve.org/CVERecord?id=CVE-2000-0191

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.
https://www.cve.org/CVERecord?id=CVE-2004-2363

Validate-Before-Canonicalize

A1
Unvalidated Input
CWE More Specific

IDS01-J
Normalize strings before validating them
Exact

IDS01-J
Normalize strings before validating them
Exact

Allowed
This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

This overlaps other categories.

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Potential_Mitigations, Time_of_Introduction

Veracode
2008-08-15
Suggested OWASP Top Ten 2004 mapping

CWE Content Team
MITRE
2008-09-08
updated Relationships, Other_Notes, Taxonomy_Mappings, Type

CWE Content Team
MITRE
2008-10-14
updated Description

CWE Content Team
MITRE
2009-05-27
updated Other_Notes, Relationship_Notes

CWE Content Team
MITRE
2010-02-16
updated Demonstrative_Examples

CWE Content Team
MITRE
2011-03-29
updated Potential_Mitigations

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2012-05-11
updated Demonstrative_Examples, Observed_Examples, Related_Attack_Patterns, Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-01-19
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Functional_Areas

CWE Content Team
MITRE
2019-01-03
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2019-06-20
updated Related_Attack_Patterns

CWE Content Team
MITRE
2020-02-24
updated Relationships, Type

CWE Content Team
MITRE
2020-06-25
updated Demonstrative_Examples, Potential_Mitigations

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Observed_Examples, Relationships, Weakness_Ordinalities

Validate-Before-Canonicalize