The product does not properly handle when all or part of an input has been URL encoded.

Primary

Implementation

Integrity
Unexpected State

Architecture and Design
Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Implementation
Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

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.

CVE-2000-0900
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
https://www.cve.org/CVERecord?id=CVE-2000-0900

CVE-2005-2256
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
https://www.cve.org/CVERecord?id=CVE-2005-2256

CVE-2004-2121
Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
https://www.cve.org/CVERecord?id=CVE-2004-2121

CVE-2004-0280
"%20" (encoded space)
https://www.cve.org/CVERecord?id=CVE-2004-0280

CVE-2003-0424
"%20" (encoded space)
https://www.cve.org/CVERecord?id=CVE-2003-0424

CVE-2001-0693
"%20" (encoded space)
https://www.cve.org/CVERecord?id=CVE-2001-0693

CVE-2001-0778
"%20" (encoded space)
https://www.cve.org/CVERecord?id=CVE-2001-0778

CVE-2002-1831
Crash via hex-encoded space "%20".
https://www.cve.org/CVERecord?id=CVE-2002-1831

CVE-2000-0671
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2000-0671

CVE-2004-0189
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2004-0189

CVE-2002-1291
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2002-1291

CVE-2002-1031
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2002-1031

CVE-2001-1140
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2001-1140

CVE-2004-0760
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2004-0760

CVE-2002-1025
"%00" (encoded null)
https://www.cve.org/CVERecord?id=CVE-2002-1025

CVE-2002-1213
"%2f" (encoded slash)
https://www.cve.org/CVERecord?id=CVE-2002-1213

CVE-2004-0072
"%5c" (encoded backslash) and "%2e" (encoded dot) sequences
https://www.cve.org/CVERecord?id=CVE-2004-0072

CVE-2004-0847
web framework for .NET allows remote attackers to bypass authentication for .aspx files in restricted directories via a request containing a (1) "\" (backslash) or (2) "%5C" (encoded backslash)
https://www.cve.org/CVERecord?id=CVE-2004-0847

CVE-2002-1575
"%0a" (overlaps CRLF)
https://www.cve.org/CVERecord?id=CVE-2002-1575

URL Encoding (Hex Encoding)

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.

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, Observed_Example, Taxonomy_Mappings

CWE Content Team
MITRE
2009-07-27
updated Potential_Mitigations

CWE Content Team
MITRE
2010-12-13
updated Name

CWE Content Team
MITRE
2011-03-29
updated Potential_Mitigations

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2011-06-27
updated Common_Consequences

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

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms

CWE Content Team
MITRE
2019-01-03
updated Related_Attack_Patterns

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

CWE Content Team
MITRE
2020-06-25
updated 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, Weakness_Ordinalities

URL Encoding (Hex Encoding)
Failure to Handle URL Encoding (Hex Encoding)