Authentication Bypass by Alternate Name

Description

This vulnerability occurs when a system checks access based on a resource or user name, but fails to account for all the different names or aliases that could refer to the same entity, allowing attackers to bypass authentication.

Extended Description

This flaw often appears in systems that rely on simple string matching for access control. For example, a file server might block access to 'admin.txt' but overlook 'ADMIN.TXT', 'Admin.Txt', or a symlink pointing to the same file. Similarly, a user authentication system might verify 'administrator' but not check for 'admin', 'root', or other common aliases for privileged accounts. Attackers exploit this by simply using an alternate name the system doesn't recognize as restricted. To prevent this, developers must implement canonicalization, where all names are resolved to a single, standard form before the security check. This means normalizing file paths, converting usernames to a consistent case, and resolving symbolic links. Access control logic should never rely on raw, unvalidated input strings. Instead, it should compare the canonical identity of the resource or user against a policy that is enforced consistently for all access attempts.

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.

Observed Examples 2

CVE-2003-0317Protection mechanism that restricts URL access can be bypassed using URL encoding.

CVE-2004-0847Bypass of authentication for files using "\" (backslash) or "%5C" (encoded backslash).

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Architecture and Design

Implementation

Related Weaknesses

ChildOf:

Weak Authentication (CWE-1390)

Taxonomy Mapping

PLOVER
The CERT Oracle Secure Coding Standard for Java (2011)
SEI CERT Oracle Coding Standard for Java

Notes

RelationshipOverlaps equivalent encodings, canonicalization, authorization, multiple trailing slash, trailing space, mixed case, and other equivalence issues.

TheoreticalAlternate names are useful in data driven manipulation attacks, not just for authentication.