fix(skills): unquote percent-encoded dot-segments during URI traversal validation

[chauvuusvn]

Summary

Fixes #3685.

In src/mcp/shared/skills.py (added in #3485 for the SEP-2640 Skills extension), _validate_resource_uri_in_skill() and validate_directory_result() validated URI traversal dot-segments (. and ..) against the raw percent-encoded URI path. As reported in #3685, percent-encoded traversal sequences (such as %2e%2e, %2E%2E, %2e) bypassed the literal checks.

Changes

• In _validate_resource_uri_in_skill(): unquote resource_parts.path before checking if any split segment equals . or ...
• In validate_directory_result(): unquote relative before validating that no / or traversal segment (., ..) exists.
• Added comprehensive unit tests in tests/shared/test_skills.py covering percent-encoded traversal dot-segments (%2e%2e, %2E%2E, %2e) and directory listing bypass attempts.

Verification

• Validated with pytest tests/shared/test_skills.py.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3685.

If a maintainer assigns you to #3685, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

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• We're a small team with very little capacity to review community PRs right now.
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Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.