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check_resource_allowed treats /api/../admin as under /api #3464
Description
What happened
check_resource_allowed pads trailing slashes and then does requested_path.startswith(configured_path). It does not collapse . / ...
On main @ 08a3bc8 these return True:
- requested
https://example.com/api/../adminvs configuredhttps://example.com/api - requested
https://example.com/mcp/..vs configuredhttps://example.com/mcp - requested
https://example.com/api/%2e%2e/adminvs configuredhttps://example.com/api
tests/shared/test_auth_utils.py already rejects /api123 vs /api (path-boundary). Dot-segments are not covered.
What I expected
A requested path that walks out of the configured prefix should not match. /api/foo/../v1 vs /api can still match after normalisation, because it stays under /api.
How to reproduce
from mcp.shared.auth_utils import check_resource_allowed
check_resource_allowed("https://example.com/api/../admin", "https://example.com/api")
# True today. I expected False.I can send a PR that percent-decodes once, runs posixpath.normpath, then keeps the existing trailing-slash prefix rule. Happy to do that if you want it.
Written with AI assistance. I read auth_utils.py next to the path-boundary tests and reproduced it locally.
Activity
AbarnaaSree commentedon Sep 7, 2026
I'd like to work on this. I'll first verify the current
check_resource_allowedbehavior and existing path-boundary tests, then add regression coverage for dot-segments and percent-encoded traversal while preserving the existing prefix semantics. I'll also check how URL paths are parsed/decoded elsewhere in the SDK before settling on the normalization approach.zsxh1990 commentedon Sep 7, 2026
I'd like to work on this security fix.
Root cause: check_resource_allowed does a naive prefix check on the URI path. A request to /api/../admin passes the /api prefix check because the path is not normalized before comparison.
Approach: Normalize the request path with posixpath.normpath (or equivalent) before the prefix check, resolving .. and . segments. This ensures /api/../admin correctly resolves to /admin and gets rejected.
I will include test cases for various traversal patterns (.., encoded variants).
1747687484-collab commentedon Sep 8, 2026
Confirmed the reproduction locally on
main:check_resource_allowed("https://example.com/api/../admin", "https://example.com/api")evaluates toTruebecause trailing-slash padding produces/api/../admin/which naively.startswith("/api/")./mcp/..and percent-encoded variants (%2e%2e/%2E%2E).To address this cleanly without breaking existing prefix semantics:
urllib.parse.unquote()and normalize withposixpath.normpath()(ensuring a leading/)./fooand/foo/equivalence) so path-boundary protection against prefix collisions (like/api123/vs/api/) is preserved.requested_pathandconfigured_pathbefore checkingrequested_path.startswith(configured_path).I have this implemented and verified locally with comprehensive regression tests covering out-of-boundary traversal (
..,%2e%2e,../../admin,/mcp/..), intra-boundary traversals that legitimately stay under the configured root (/api/v1/../v2,/api/./v1), and all 16 existing auth tests passing cleanly.Happy to open the PR if maintainers would like to assign this or open it for external contributions!
Written with AI assistance; reproduced and tested locally.
Kludex commentedon Oct 10, 2026
Both reports reproduce the same
check_resource_allowedpath-prefix bypass using raw and percent-encoded dot segments. This is tracked in #3303, so I’m closing this as a duplicate. AI-assisted triage; I reviewed both reports.