Improper Validation of Unsafe Equivalence in Input

Description

This vulnerability occurs when an application accepts user input as a reference (like a file path or resource identifier) but fails to properly check if that input is functionally equivalent to a dangerous or restricted value.

Extended Description

Attackers can bypass security checks by crafting inputs that look safe to the validation layer but are processed as dangerous by a downstream system. For instance, a filter might block the exact string `<script>` using a case-sensitive match, but a browser interprets HTML tags case-insensitively, so an input like `<ScRiPt>` would bypass the filter and still execute. This happens because validation logic often makes assumptions about how data will be interpreted later in the processing chain. To prevent this, security checks must validate inputs based on the same parsing rules used by the ultimate consumer of that data, such as the operating system, database, or web browser.

Common Consequences 1

Scope: Other

Impact: Varies by Context

Potential Mitigations 1

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.

Effectiveness: High

Observed Examples 5

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.

CVE-2020-11053Chain: Go-based Oauth2 reverse proxy can send the authenticated user to another site at the end of the authentication flow. A redirect URL with HTML-encoded whitespace characters can bypass the validation (Improper Validation of Unsafe Equivalence in Input) to redirect to a malicious site (URL Redirection to Untrusted Site ('Open Redirect'))

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-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-2004-2214HTTP server allows bypass of access restrictions using URIs with mixed case.

Applicable Platforms

Languages:

Not Language-Specific : Often

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Improper Input Validation (CWE-20)

PeerOf:

Improper Resolution of Path Equivalence (CWE-41)

PeerOf:

Improper Handling of Case Sensitivity (CWE-178)

Notes

MaintenanceThis entry is still under development and will continue to see updates and content improvements.