fix(auth): make PKCE optional per ADR-003
PKCE was incorrectly required in the /authorize endpoint, contradicting ADR-003 which defers PKCE to v1.1.0. Changes: - PKCE parameters are now optional in /authorize - If code_challenge provided, validates method is S256 - Defaults to S256 if method not specified - Logs when clients don't use PKCE for monitoring - Updated tests for optional PKCE behavior This fixes authentication for clients that don't implement PKCE. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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docs/designs/bugfix-pkce-optional-v1.0.0.md
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docs/designs/bugfix-pkce-optional-v1.0.0.md
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# Design: Make PKCE Optional in v1.0.0 (Bug Fix)
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Date: 2025-12-17
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Status: Ready for Implementation
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Priority: P0 (Blocking)
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## Problem Statement
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The `/authorize` endpoint currently **requires** PKCE parameters (`code_challenge` and `code_challenge_method`), which contradicts ADR-003 that explicitly states PKCE is deferred to v1.1.0.
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**Current behavior (lines 325-343 in authorization.py):**
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```python
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# Validate code_challenge (PKCE required)
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if not code_challenge:
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return {"error": "invalid_request", "error_description": "code_challenge is required (PKCE)"}
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if code_challenge_method != "S256":
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return {"error": "invalid_request", "error_description": "code_challenge_method must be S256"}
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```
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**Expected v1.0.0 behavior per ADR-003:**
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- PKCE parameters should be **optional**
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- Clients without PKCE should be able to authenticate
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- PKCE validation is deferred to v1.1.0
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This bug is blocking real-world IndieAuth clients that do not use PKCE.
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## Design Overview
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The fix is straightforward: remove the mandatory PKCE checks from the authorization endpoint while preserving the ability to accept and store PKCE parameters for forward compatibility.
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### Principle: Minimal Change
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This is a bug fix, not a feature. The change should be minimal and surgical:
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1. Remove the two error-returning conditionals
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2. Add validation only when PKCE parameters ARE provided
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3. Preserve all existing storage behavior
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## Detailed Changes
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### Change 1: Remove Mandatory PKCE Check
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**Location:** `/src/gondulf/routers/authorization.py`, lines 325-343
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**Current Code (to be removed):**
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```python
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# Validate code_challenge (PKCE required)
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if not code_challenge:
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error_params = {
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"error": "invalid_request",
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"error_description": "code_challenge is required (PKCE)",
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"state": state or ""
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}
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redirect_url = f"{redirect_uri}?{urlencode(error_params)}"
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return RedirectResponse(url=redirect_url, status_code=302)
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# Validate code_challenge_method
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if code_challenge_method != "S256":
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error_params = {
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"error": "invalid_request",
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"error_description": "code_challenge_method must be S256",
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"state": state or ""
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}
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redirect_url = f"{redirect_uri}?{urlencode(error_params)}"
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return RedirectResponse(url=redirect_url, status_code=302)
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```
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**New Code (replacement):**
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```python
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# PKCE validation (optional in v1.0.0, per ADR-003)
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# If code_challenge is provided, validate the method
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if code_challenge:
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if code_challenge_method and code_challenge_method != "S256":
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error_params = {
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"error": "invalid_request",
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"error_description": "code_challenge_method must be S256",
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"state": state or ""
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}
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redirect_url = f"{redirect_uri}?{urlencode(error_params)}"
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return RedirectResponse(url=redirect_url, status_code=302)
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# If code_challenge provided without method, default to S256
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if not code_challenge_method:
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code_challenge_method = "S256"
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else:
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# Log for future monitoring (per ADR-003 recommendation)
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logger.info(f"Client {client_id} not using PKCE")
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```
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### Change 2: Handle None Values in Session Storage
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The `AuthSessionService.create_session()` already accepts these parameters, and the database schema likely allows NULL values. No changes needed to the service layer.
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**Verification:** The auth_session.py already uses these parameters directly:
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```python
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"code_challenge": code_challenge,
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"code_challenge_method": code_challenge_method,
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```
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If `code_challenge` is `None`, this will store NULL in the database, which is the desired behavior.
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### Change 3: Update Template Context (Optional Cleanup)
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The templates already receive `code_challenge` and `code_challenge_method` - they will now sometimes be `None` or empty. This should not cause issues as Jinja2 handles None values gracefully in form hidden fields.
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## Behavior Matrix
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| code_challenge | code_challenge_method | Result |
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|----------------|----------------------|--------|
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| None | None | Proceed without PKCE |
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| None | "S256" | Proceed without PKCE (method ignored) |
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| "abc123..." | None | Proceed with PKCE, default to S256 |
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| "abc123..." | "S256" | Proceed with PKCE |
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| "abc123..." | "plain" | ERROR: method must be S256 |
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## What NOT to Change
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1. **Token endpoint** - Already handles PKCE correctly (optional, logged but not validated per ADR-003 lines 200-203)
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2. **POST /authorize** - Already handles PKCE correctly (optional, logged but not validated per lines 856-858)
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3. **Auth session service** - Already accepts optional code_challenge parameters
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4. **Database schema** - Likely already allows NULL for these fields
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## Security Considerations
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**No security regression:**
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- ADR-003 explicitly accepted this risk for v1.0.0
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- HTTPS enforcement mitigates code interception
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- 10-minute code lifetime limits attack window
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- Single-use codes prevent replay
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**Forward compatibility:**
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- PKCE parameters are still stored when provided
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- v1.1.0 can enable validation without schema changes
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- Clients using PKCE today will work in v1.1.0
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## Testing Strategy
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### Unit Tests
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1. **Test authorization without PKCE:**
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- Call `/authorize` without `code_challenge` - should succeed
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- Verify session is created with NULL code_challenge
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2. **Test authorization with PKCE:**
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- Call `/authorize` with valid `code_challenge` and `code_challenge_method=S256` - should succeed
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- Verify session stores the code_challenge
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3. **Test PKCE with default method:**
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- Call `/authorize` with `code_challenge` but no `code_challenge_method`
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- Should succeed, default to S256
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4. **Test invalid PKCE method:**
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- Call `/authorize` with `code_challenge` and `code_challenge_method=plain`
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- Should return error (only S256 supported)
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5. **End-to-end flow without PKCE:**
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- Complete full authorization flow without PKCE parameters
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- Verify token can be obtained
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### Manual Testing
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1. Use a real IndieAuth client that does NOT send PKCE
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2. Verify authentication completes successfully
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## Acceptance Criteria
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1. Clients without PKCE can complete authorization flow
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2. Clients with PKCE continue to work unchanged
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3. Invalid PKCE method (not S256) is rejected
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4. PKCE parameters are stored in auth session when provided
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5. All existing tests continue to pass
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6. New tests cover optional PKCE behavior
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## Implementation Notes
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### For the Developer
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The fix is contained to a single location in `authorization.py`. The key insight is:
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1. **DELETE** the two blocks that return errors for missing PKCE
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2. **ADD** a simpler block that only validates method IF code_challenge is provided
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3. **ADD** a log statement for clients not using PKCE (monitoring per ADR-003)
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The rest of the codebase already handles optional PKCE correctly. This was an error in the GET /authorize validation logic only.
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### Estimated Effort
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**S (Small)** - 1-2 hours including tests
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### Files to Modify
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1. `/src/gondulf/routers/authorization.py` - Remove mandatory PKCE checks (~20 lines changed)
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### Files to Add
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1. Tests for optional PKCE behavior (or add to existing authorization tests)
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## References
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- ADR-003: `/docs/decisions/ADR-003-pkce-deferred-to-v1-1-0.md`
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- W3C IndieAuth: https://www.w3.org/TR/indieauth/ (PKCE is not mentioned, making it optional)
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- RFC 7636: https://datatracker.ietf.org/doc/html/rfc7636 (PKCE specification)
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