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.
How it's found
Use of Non-Canonical URL Paths for Authorization Decisions is a specific, narrowly defined instance of a broader pattern. Testers confirm it with targeted code review and a proof-of-concept input that exercises the exact code path this weakness describes.
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: 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).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
Consequences
- Bypass Protection Mechanism: An attacker may be able to bypass the authorization mechanism to gain access to the otherwise-protected URL.
- Read Files or Directories: 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).
Mitigations
- Architecture and Design: 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: Reject all alternate path encodings that are not in the expected canonical form.
Where this fits in a TurboPentest engagement
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
Frequently asked questions
What is CWE-647?
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.
How do you find Use of Non-Canonical URL Paths for Authorization Decisions?
Use of Non-Canonical URL Paths for Authorization Decisions is a specific, narrowly defined instance of a broader pattern. Testers confirm it with targeted code review and a proof-of-concept input that exercises the exact code path this weakness describes.
What is the impact of CWE-647?
Bypass Protection Mechanism: An attacker may be able to bypass the authorization mechanism to gain access to the otherwise-protected URL. Read Files or Directories: 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).
Does TurboPentest test for Use of Non-Canonical URL Paths for Authorization Decisions?
This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.
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Written and reviewed by
Michel Chamberland - Founder & CEO, IntegSec
CISSP, OSCP, OSCE, CEH, GIAC, CCSK · 20+ years in offensive security
Michel has spent 20+ years on offensive security teams including IBM X-Force Red and Trustwave SpiderLabs, leading penetration tests, red team engagements, and breach response for Fortune 500 customers. He is the founder of IntegSec and the architect of TurboPentest.
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