COMMON WEAKNESS ENUMERATIONIncomplete

CWE-647

Use of Non-Canonical URL Paths for Authorization Decisions

The product defines policy namespaces and makes authorization decisions based on the assumption that a URL is canonical. This can allow a non-canonical URL to bypass the authorization.

Abstraction

Variant

Structure

Simple

Likelihood of Exploit

High

Understand the problem

What is the problem?

The product defines policy namespaces and makes authorization decisions based on the assumption that a URL is canonical. This can allow a non-canonical URL to bypass the authorization.

Extended description

If an application defines policy namespaces and makes authorization decisions based on the URL, but it does not require or convert to a canonical URL before making the authorization decision, then it opens the application to attack. For example, if the application only wants to allow access to http://www.example.com/mypage, then the attacker might be able to bypass this restriction using equivalent URLs such as: - http://WWW.EXAMPLE.COM/mypage - http://www.example.com/%6Dypage (alternate encoding) - http://192.168.1.1/mypage (IP address) - http://www.example.com/mypage/ (trailing /) - http://www.example.com:80/mypage Therefore it is important to specify access control policy that is based on the path information in some canonical form with all alternate encodings rejected (which can be accomplished by a default deny rule).

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Lifecycle phase

Operation

Risk

What does successful exploitation lead to?

Bypass Protection Mechanism

Affected security scopes: Access Control

An attacker may be able to bypass the authorization mechanism to gain access to the otherwise-protected URL.
Read Files or Directories

Affected security scopes: Confidentiality

If a non-canonical URL is used, the server may choose to return the contents of the file, instead of pre-processing the file (e.g. as a program).

Defence

How is it prevented and fixed?

Architecture and Design

Architecture and design approach

Detail
Make access control policy based on path information in canonical form. Use very restrictive regular expressions to validate that the path is in the expected form.

Architecture and Design

Architecture and design approach

Detail
Reject all alternate path encodings that are not in the expected canonical form.

Verification

How is it detected?

Automated Static Analysis

High
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)