Improper Handling of Case Sensitivity
Incomplete Base
Structure: Simple
Description
This vulnerability occurs when software fails to consistently handle uppercase and lowercase letters when checking or accessing resources, leading to unpredictable behavior and security gaps.
When an application treats 'Admin', 'admin', and 'ADMIN' as different entities, it creates inconsistencies that attackers can exploit. For example, a filter blocking 'admin.php' might be bypassed by requesting 'Admin.PHP', or a system might create duplicate user accounts with only case differences, corrupting data integrity. From a security perspective, mishandling case sensitivity weakens defenses. It can drastically reduce password complexity by making 'Password' equivalent to 'password', simplifying brute-force attacks. It also allows attackers to evade security checks by using alternate capitalizations to access restricted files or bypass input validation rules.
Common Consequences 1
Scope: Access Control
Impact: Bypass Protection Mechanism
Potential Mitigations 3
Phase: Architecture and Design
Strategy: Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
Phase: Implementation
Strategy: 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.
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-141
In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:Code Example:
Bad
Java
javaThe code only works when the "script" tag is in all lower-case, forming an incomplete denylist (Incomplete List of Disallowed Inputs). Equivalent tags such as "SCRIPT" or "ScRiPt" will not be neutralized by this method, allowing an XSS attack.
Observed Examples 18
CVE-2000-0499Application server allows attackers to bypass execution of a jsp page and read the source code using an upper case JSP extension in the request.
CVE-2000-0497The 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".
CVE-2000-0498The 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".
CVE-2001-0766A 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.
CVE-2001-0795Server 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.
CVE-2001-1238Task 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.
CVE-2003-0411chain: 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".
CVE-2002-0485Leads to interpretation error
CVE-1999-0239Directories may be listed because lower case web requests are not properly handled by the server.
CVE-2005-0269File 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.
CVE-2004-1083Web 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.
CVE-2002-2119Case insensitive passwords lead to search space reduction.
CVE-2004-2214HTTP server allows bypass of access restrictions using URIs with mixed case.
CVE-2004-2154Mixed upper/lowercase allows bypass of ACLs.
CVE-2005-4509Bypass malicious script detection by using tokens that aren't case sensitive.
CVE-2002-1820Mixed case problem allows "admin" to have "Admin" rights (alternate name property).
CVE-2007-3365Chain: uppercase file extensions causes web server to return script source code instead of executing the script.
CVE-2021-39155Chain: A microservice integration and management platform compares the hostname in the HTTP Host header in a case-sensitive way (Improper Handling of Case Sensitivity, Improper Validation of Unsafe Equivalence in Input), allowing bypass of the authorization policy (Incorrect Authorization) using a hostname with mixed case or other variations.
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Modes of Introduction
Implementation
Functional Areas
- File Processing
Affected Resources
- File or Directory
Related Weaknesses
Taxonomy Mapping
- PLOVER
Notes
Research GapThese are probably under-studied in Windows and Mac environments, where file names are case-insensitive and thus are subject to equivalence manipulations involving case.