The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.

Improperly handled case sensitive data can lead to several possible consequences, including:

case-insensitive passwords reducing the size of the key space, making brute force attacks easier
bypassing filters or access controls using alternate names
multiple interpretation errors using alternate names.

Primary

Implementation

Access Control
Bypass Protection Mechanism

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.

In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:

public String preventXSS(String input, String mask) {return input.replaceAll("script", mask);}

The code only works when the "script" tag is in all lower-case, forming an incomplete denylist (CWE-184). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method, allowing an XSS attack.

CVE-2000-0499
Application server allows attackers to bypass execution of a jsp page and read the source code using an upper case JSP extension in the request.
https://www.cve.org/CVERecord?id=CVE-2000-0499

CVE-2000-0497
The server is case sensitive, so filetype handlers treat .jsp and .JSP as different extensions. JSP source code may be read because .JSP defaults to the filetype "text".
https://www.cve.org/CVERecord?id=CVE-2000-0497

CVE-2000-0498
The server is case sensitive, so filetype handlers treat .jsp and .JSP as different extensions. JSP source code may be read because .JSP defaults to the filetype "text".
https://www.cve.org/CVERecord?id=CVE-2000-0498

CVE-2001-0766
A URL that contains some characters whose case is not matched by the server's filters may bypass access restrictions because the case-insensitive file system will then handle the request after it bypasses the case sensitive filter.
https://www.cve.org/CVERecord?id=CVE-2001-0766

CVE-2001-0795
Server allows remote attackers to obtain source code of CGI scripts via URLs that contain MS-DOS conventions such as (1) upper case letters or (2) 8.3 file names.
https://www.cve.org/CVERecord?id=CVE-2001-0795

CVE-2001-1238
Task Manager does not allow local users to end processes with uppercase letters named (1) winlogon.exe, (2) csrss.exe, (3) smss.exe and (4) services.exe via the Process tab which could allow local users to install Trojan horses that cannot be stopped.
https://www.cve.org/CVERecord?id=CVE-2001-1238

CVE-2003-0411
chain: Code was ported from a case-sensitive Unix platform to a case-insensitive Windows platform where filetype handlers treat .jsp and .JSP as different extensions. JSP source code may be read because .JSP defaults to the filetype "text".
https://www.cve.org/CVERecord?id=CVE-2003-0411

CVE-2002-0485
Leads to interpretation error
https://www.cve.org/CVERecord?id=CVE-2002-0485

CVE-1999-0239
Directories may be listed because lower case web requests are not properly handled by the server.
https://www.cve.org/CVERecord?id=CVE-1999-0239

CVE-2005-0269
File extension check in forum software only verifies extensions that contain all lowercase letters, which allows remote attackers to upload arbitrary files via file extensions that include uppercase letters.
https://www.cve.org/CVERecord?id=CVE-2005-0269

CVE-2004-1083
Web server restricts access to files in a case sensitive manner, but the filesystem accesses files in a case insensitive manner, which allows remote attackers to read privileged files using alternate capitalization.
https://www.cve.org/CVERecord?id=CVE-2004-1083

CVE-2002-2119
Case insensitive passwords lead to search space reduction.
https://www.cve.org/CVERecord?id=CVE-2002-2119

CVE-2004-2214
HTTP server allows bypass of access restrictions using URIs with mixed case.
https://www.cve.org/CVERecord?id=CVE-2004-2214

CVE-2004-2154
Mixed upper/lowercase allows bypass of ACLs.
https://www.cve.org/CVERecord?id=CVE-2004-2154

CVE-2005-4509
Bypass malicious script detection by using tokens that aren't case sensitive.
https://www.cve.org/CVERecord?id=CVE-2005-4509

CVE-2002-1820
Mixed case problem allows "admin" to have "Admin" rights (alternate name property).
https://www.cve.org/CVERecord?id=CVE-2002-1820

CVE-2007-3365
Chain: uppercase file extensions causes web server to return script source code instead of executing the script.
https://www.cve.org/CVERecord?id=CVE-2007-3365

CVE-2021-39155
Chain: A microservice integration and management platform compares the hostname in the HTTP Host header in a case-sensitive way (CWE-178, CWE-1289), allowing bypass of the authorization policy (CWE-863) using a hostname with mixed case or other variations.
https://www.cve.org/CVERecord?id=CVE-2021-39155

File Processing

File or Directory

Case Sensitivity (lowercase, uppercase, mixed case)

Allowed
This CWE entry is at the Base 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.

These are probably under-studied in Windows and Mac environments, where file names are case-insensitive and thus are subject to equivalence manipulations involving case.

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

Sean Eidemiller
Cigital
2008-07-01
added/updated demonstrative examples

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

CWE Content Team
MITRE
2008-09-08
updated Description, Relationships, Observed_Example, Taxonomy_Mappings

CWE Content Team
MITRE
2008-10-14
updated Observed_Examples

CWE Content Team
MITRE
2008-11-24
updated Observed_Examples

CWE Content Team
MITRE
2009-03-10
updated Description

CWE Content Team
MITRE
2009-07-27
updated Potential_Mitigations

CWE Content Team
MITRE
2010-06-21
updated Demonstrative_Examples

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
2012-05-11
updated Observed_Examples, Relationships

CWE Content Team
MITRE
2012-10-30
updated Demonstrative_Examples, Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

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

CWE Content Team
MITRE
2019-06-20
updated Relationships

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

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

CWE Content Team
MITRE
2021-03-15
updated Demonstrative_Examples

CWE Content Team
MITRE
2022-10-13
updated Observed_Examples

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 Applicable_Platforms, Weakness_Ordinalities

Case Sensitivity (Lowercase, Uppercase, Mixed Case)
Failure to Resolve Case Sensitivity