refactor(authn)!: remove ClientIDs allowlist; rely on audience for token binding

[mushrafmim]

Summary

Removes the ClientIDs allowlist from authn. Token validation no longer rejects a token because its client_id claim isn't on a configured list.

The audience check already binds a token to the service it was issued for, and the IdP decides which clients may obtain a token for that audience. Deciding which apps may call a service is authorization policy, like roles and scopes, so it doesn't belong in token validation. The allowlist also forced every service to list and maintain its callers' client IDs just to start up.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Other (please describe):

Changes Made

• authn/config.go: removed Config.ClientIDs (clientIDs in yaml) and its Validate check.
• authn/token_parser.go: removed the expectedClientIDs parameter from NewTokenExtractor, NewTokenExtractorWithClient and newExtractor, and removed the allowlist check from validateConfig and from token extraction. A token must still carry a client_id claim, because ClientPrincipal needs it.
• authn/manager.go: no longer passes ClientIDs to the extractor.
• authn/README.md: removed ClientIDs from the examples. Added a note that audience binds a token to the service, and that limiting which clients can call it belongs in authz.
• Tests: dropped the missing client ids and unexpected client_id cases. Added TestTokenExtractor_AcceptsAnyClientIDForAudience, which checks that a token is accepted whatever its client_id when aud matches.

Testing

• I have tested this change locally
• I have added tests that prove my fix is effective or that my feature works
• I have tested edge cases
• All existing tests pass

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked that there are no merge conflicts

Related Issues

Closes #289

Screenshots/Demo

N/A

Additional Notes

Breaking changes:

• Remove ClientIDs / clientIDs from authn.Config values and yaml files.
• Drop the client-IDs argument from direct NewTokenExtractor / NewTokenExtractorWithClient calls.

Any client the IdP issues a token for the service's audience is now accepted. A service that relied on the allowlist to limit its callers should enforce that in authz using the principal's ClientID.

Exposing the originating client ID on user principals is a related follow-up and is out of scope here.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Behavior Changes
  • Tokens are no longer rejected solely because their client_id is absent from a configured allowlist. Valid tokens must still include a client_id and meet the existing service-audience checks.
  • Client authorization is handled separately from token validation, such as through authorization rules or application logic.
• Documentation
  • Updated authentication guidance to distinguish service audience validation from client authorization.

[coderabbitai]

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📝 Walkthrough

📝 Walkthrough

📝 Walkthrough

Walkthrough

The authn package removes ClientIDs from configuration and removes the client-ID allowlist from token extractor constructors and parsing. Audience validation remains, and parsed tokens still require a nonempty client_id.

Changes

Client-ID Allowlist Removal

Layer / File(s)	Summary
Remove client-ID validation authn/config.go, authn/config_test.go, authn/token_parser.go, authn/token_parser_test.go	Config and the token extractor no longer accept or require a client-ID allowlist. Token parsing no longer rejects a nonempty client_id because it is absent from a configured list. Tests cover acceptance of a client-credentials token with an arbitrary client ID.
Update manager wiring and examples authn/manager.go, authn/manager_test.go, authn/extra_claims_test.go, authn/fixed_schema_test.go, authn/test_helpers_test.go, authn/README.md	Manager wiring, tests, and examples use the revised configuration and constructor signatures. The README states that audience identifies the service and that client access restrictions belong in authorization policy or the application.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Suggested reviewers: sthanikan2000

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Merge Risk: 🔵 Low · up to e0bd9

Clarify how services using user tokens should preserve client restrictions. This is a bounded migration-guidance issue, not a demonstrated authentication failure.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 31.03% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 29 functions across 9 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely identifies the main change: removing the ClientIDs allowlist and relying on audience for token binding.
Description check		The description covers the summary, change type, implementation changes, testing, checklist, related issue, breaking changes, and out-of-scope items. It provides sufficient context for review.
Linked Issues check		PR #294 satisfies the coding requirements in directly linked issue #289. authn.Config no longer has ClientIDs, and Config.Validate no longer validates that allowlist. NewTokenExtractor, `NewTo…
Out of Scope Changes check		The reviewed changes stay within #289's scope. The source changes remove the configuration field, constructor parameters, and token allowlist check. The test changes update callers and verify the new …

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Full details: Docstring Coverage

Explanation

Docstring coverage is 31.03% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 29 functions across 9 files. (1 skipped: 1 unsupported.)

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[sthanikan2000]

LGTM!

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @authn/README.md:
- Line 36: Update the audience guidance in the README to specify that
ClientPrincipal.ClientID supports restricting client-credentials authorization.
Clarify that authorization_code tokens produce a UserPrincipal without ClientID,
so applications needing to restrict those user tokens by originating client must
obtain that client ID through a separate application or authorization path.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 34749d46-174a-4ddc-bd5a-9e479e902bc8

📥 Commits

Reviewing files that changed from the base of the PR and between 643bf60 and e0bd9f6.

📒 Files selected for processing (10)

• authn/README.md
• authn/config.go
• authn/config_test.go
• authn/extra_claims_test.go
• authn/fixed_schema_test.go
• authn/manager.go
• authn/manager_test.go
• authn/test_helpers_test.go
• authn/token_parser.go
• authn/token_parser_test.go

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[sthanikan2000]

LGTM!