Improve token exchange errors

[thiva-k]

Purpose

The token exchange grant returned a single generic Invalid subject_token for every subject-token failure, so a caller could not tell an expired token from an untrusted issuer or an audience mismatch (#3835, item 2). This PR attributes the reasons that are safe to disclose and reports them in the error_description, applying the same treatment to actor_token and to the jwt-bearer ID-JAG assertion.

Approach

• Add three sentinel errors in the tokenservice package (ErrTokenExpired, ErrIssuerNotTrusted, ErrAudienceNotAccepted), named for the failure rather than the token that carried it so any validator in the package can return them. The validator wraps them with %w, so errors.Is classifies the failure while the underlying context stays in the server-side debug log.
• Each grant handler owns its client-facing wording and OAuth2 error code: the token exchange handler branches on the sentinels for subject_token and actor_token (invalid_request); the jwt-bearer handler does the same for the assertion (invalid_grant). Any unattributed failure keeps the original generic description.
• Disclosure is safe because the token endpoint is client-authenticated and the client already holds the token it presented. All descriptions comply with the RFC 6749 §5.2 error_description character set.
• ValidateSubjectToken keeps its existing signature, so no interface or mock changes.

Related Issues

• Issues with Token exchange grant #3835

[coderabbitai]

Warning

Review limit reached

@thiva-k, you've reached your PR review limit, so we couldn't start this review.

Next review available in: 12 minutes

Enable usage-based reviews in Billing to review now. Otherwise, wait until the next included review is available.
You're only billed for reviews past your plan's rate limits ($0.25/file).

How can I continue?

After more reviews become available, a review can be triggered using the @coderabbitai review command as a PR comment. Alternatively, push new commits to this PR.

To avoid repeated limits, reduce automatic review volume by pausing incremental auto-reviews earlier, using label-based review opt-in, excluding WIP or generated PR titles, or requesting reviews manually when the PR is ready. If your team needs uninterrupted high-volume reviews, an organization admin can enable usage-based reviews.

How do review limits work?

CodeRabbit enforces per-developer PR review limits for each organization. Most developers receive the normal plan review availability.

For paid Pro and Pro+ PR reviews, CodeRabbit uses adaptive limits for sustained high-volume activity. When a developer's recent PR review activity reaches the 95th percentile or higher among CodeRabbit users, additional reviews become available more gradually as earlier reviews age out of the rolling window.

Please refer docs for additional details.

Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: c540e8e1-7619-4b9b-b2a5-88a43396485e

📥 Commits

Reviewing files that changed from the base of the PR and between 7b7ed80 and f5ec16d.

📒 Files selected for processing (7)

• backend/internal/oauth/oauth2/granthandlers/jwt_bearer.go
• backend/internal/oauth/oauth2/granthandlers/jwt_bearer_test.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange_test.go
• backend/internal/oauth/oauth2/tokenservice/error_constants.go
• backend/internal/oauth/oauth2/tokenservice/validator.go
• tests/integration/oauth/token/tokenexchange_test.go

📝 Walkthrough

Walkthrough

Changes

OAuth rejection reporting

Layer / File(s)	Summary
Validation error classification backend/internal/oauth/oauth2/tokenservice/error_constants.go, backend/internal/oauth/oauth2/tokenservice/validator.go	Adds sentinel errors and propagates them for expiration, untrusted issuers, and audience rejection.
Grant error mapping and coverage backend/internal/oauth/oauth2/granthandlers/jwt_bearer.go, backend/internal/oauth/oauth2/granthandlers/token_exchange.go, backend/internal/oauth/oauth2/granthandlers/*_test.go	Maps assertion, subject-token, and actor-token validation failures to specific OAuth descriptions, with table-driven tests covering each reason and generic fallbacks.
Token exchange rejection contract docs/content/guides/protocols/oauth-oidc/token-exchange.mdx, tests/integration/oauth/token/tokenexchange_test.go	Documents and verifies reason-specific token exchange rejection descriptions.

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

Sequence Diagram(s)

sequenceDiagram
  participant JWTBearerGrantHandler
  participant TokenExchangeGrantHandler
  participant TokenValidator
  participant OAuthClient
  JWTBearerGrantHandler->>TokenValidator: ValidateIDJAGAssertion
  TokenExchangeGrantHandler->>TokenValidator: ValidateSubjectToken or actor_token
  TokenValidator-->>JWTBearerGrantHandler: Classified validation error
  TokenValidator-->>TokenExchangeGrantHandler: Classified validation error
  JWTBearerGrantHandler-->>OAuthClient: invalid_grant with rejection description
  TokenExchangeGrantHandler-->>OAuthClient: invalid_request or server_error response

Loading

Possibly related PRs

• thunder-id/thunderid#3727: Adds revocation deny-list enforcement related to token exchange rejection handling.
• thunder-id/thunderid#3803: Introduces centralized revocation enforcement used by token validation outcomes.
• thunder-id/thunderid#3893: Introduces ID-JAG grant handling refined by this error mapping.

Suggested reviewers: thamindudilshan, rajithacharith

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	Concise and specific; it clearly matches the main change to improve token exchange error reporting.
Description check	✅ Passed	Covers Purpose, Approach, and Related Issues, but omits the checklist and security sections from the template.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@backend/internal/oauth/oauth2/tokenservice/validator.go`:
- Around line 265-266: Extend validator_test.go to cover each new issuer,
audience, and expiry classification path, asserting errors.Is against the
corresponding sentinel error even when the underlying errors are mocked.
Preserve the existing validation setup and verify the returned errors retain the
expected sentinel classification.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: b680b00c-9ff5-416e-890f-40c5402280fd

📥 Commits

Reviewing files that changed from the base of the PR and between 52141fb and 0680089.

📒 Files selected for processing (7)

• backend/internal/oauth/oauth2/granthandlers/jwt_bearer.go
• backend/internal/oauth/oauth2/granthandlers/jwt_bearer_test.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange_test.go
• backend/internal/oauth/oauth2/tokenservice/error_constants.go
• backend/internal/oauth/oauth2/tokenservice/validator.go
• docs/content/guides/protocols/oauth-oidc/token-exchange.mdx

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/content/guides/protocols/oauth-oidc/token-exchange.mdx`:
- Around line 111-118: Revise the token rejection documentation around the
subject_token validation table to scope the invalid_request response to
token-validation failures only. Add a note that revocation enforcement being
unavailable causes the handler to return server_error instead, so the section
does not imply every rejected subject_token produces invalid_request.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 708a4e3b-8432-4048-b84f-91a7d2d84fdf

📥 Commits

Reviewing files that changed from the base of the PR and between 0680089 and 7b7ed80.

📒 Files selected for processing (8)

• backend/internal/oauth/oauth2/granthandlers/jwt_bearer.go
• backend/internal/oauth/oauth2/granthandlers/jwt_bearer_test.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange_test.go
• backend/internal/oauth/oauth2/tokenservice/error_constants.go
• backend/internal/oauth/oauth2/tokenservice/validator.go
• docs/content/guides/protocols/oauth-oidc/token-exchange.mdx
• tests/integration/oauth/token/tokenexchange_test.go

🚧 Files skipped from review as they are similar to previous changes (6)

• backend/internal/oauth/oauth2/tokenservice/error_constants.go
• backend/internal/oauth/oauth2/granthandlers/jwt_bearer.go
• backend/internal/oauth/oauth2/tokenservice/validator.go
• backend/internal/oauth/oauth2/granthandlers/jwt_bearer_test.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange_test.go
• backend/internal/oauth/oauth2/granthandlers/token_exchange.go

[codecov]

Codecov Report

❌ Patch coverage is 98.50746% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...nd/internal/oauth/oauth2/tokenservice/validator.go	91.66%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!