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+# Proposal: Probe-Based Authentication Dispatch
+
+Author: Harbor Community
+
+Discussion: [goharbor/harbor#1572](https://github.com/goharbor/harbor/issues/1572), [goharbor/harbor#13372](https://github.com/goharbor/harbor/issues/13372), [goharbor/harbor#21300](https://github.com/goharbor/harbor/issues/21300), [goharbor/harbor#23458](https://github.com/goharbor/harbor/pull/23458)
+
+## Abstract
+
+Replace Harbor's single-backend `auth_mode` configuration with a flexible per-backend matching system. This fixes three critical production issues: admin lockout when OIDC provider is unavailable, security vulnerability where locked users authenticate as nil, and registry proxy bypassing per-user permissions. Enables fallback authentication chains while maintaining full backward compatibility.
+
+## Background
+
+Harbor currently uses a global `auth_mode` setting to select a single authentication backend:
+
+```
+auth_mode = "db"        # Database only
+auth_mode = "ldap"      # LDAP only
+auth_mode = "oidc"      # OIDC only
+auth_mode = "http_auth" # HTTP Auth Proxy only
+auth_mode = "uaa"       # UAA only
+```
+
+This design has caused multiple production issues:
+
+### Issue 1: Admin Lockout in OIDC Mode
+- OIDC provider becomes unavailable (misconfigured, IdP outage, network issue)
+- No fallback to local database authentication
+- Admin account becomes completely inaccessible
+- Recovery requires Harbor restart and manual config editing
+
+### Issue 2: Security Bug — Locked Users Authenticate as nil
+- User account locked due to repeated login failures
+- `Login()` returns `(nil, nil)` instead of error
+- HTTP handlers treat nil as successful login with no user
+- Locked accounts silently authenticate and bypass authorization checks
+
+### Issue 3: Registry Proxy Ignores User Credentials
+- Registry proxy always forwards shared service-account credential to upstream
+- Per-user permissions completely bypassed
+- Read-only user can push images (proxied as service account)
+- No way to enforce repository-level access control
+
+### Issue 4: HTTP Auth Proxy Mode Silently Broken
+- `http_auth` mode missing from authentication dispatch entirely
+- All authentication falls through to database only
+- HTTP Auth Proxy configuration non-functional
+
+## Proposal
+
+### Core Solution
+
+Replace single-backend dispatch with per-backend `Match()` method. Each backend reports whether it's configured and should be tried. The dispatcher tries backends in priority order; first success wins.
+
+**Per-backend Match() interface:**
+```
+Match(ctx) → (bool, error)
+  - (true, nil) = backend configured, should be tried
+  - (false, nil) = backend not configured, skip
+  - (false, err) = backend misconfigured, fatal error
+```
+
+### Authentication Dispatch Flows
+
+**OIDC Mode** (with fallback):
+```
+1. Try OIDC
+   ✓ Success → return user
+   ✗ Fail → continue
+2. Try Database (fallback)
+   ✓ Success → return user (admin always accessible)
+   ✗ Fail → reject login
+```
+
+**LDAP Mode** (no fallback):
+```
+1. Try LDAP
+   ✓ Success → return user
+   ✗ Fail → reject (clear error)
+```
+
+**HTTP Auth Proxy Mode** (new support):
+```
+1. Try HTTP Auth Proxy
+   ✓ Success → create/update local user, return
+   ✗ Fail → reject (clear error)
+```
+
+**Database Mode** (unchanged):
+```
+1. Try Database only
+   ✓ Success → return user
+   ✗ Fail → reject
+```
+
+### Security Fixes
+
+**Locked User Fix:**
+- `Login()` now returns `ErrAuth` for locked users instead of `(nil, nil)`
+- Locked accounts properly rejected from all auth paths
+- HTTP handlers can distinguish between auth failure and unauthenticated access
+
+**Registry Proxy User Credential Passthrough:**
+- Probe upstream registry to detect auth method (Bearer or Basic)
+- Exchange user credentials for scoped Bearer token via token service
+- Cache tokens per-repository scope (not globally; prevents privilege escalation)
+- Fall back to probed credentials for unauthenticated requests
+- Handle token service failures gracefully
+
+### Registry Proxy Auth Probe
+
+```
+GET /v2/ with no credentials
+  ↓
+If 401 + Bearer realm header:
+  → Use token service exchange for Bearer tokens
+If 401 + Basic realm header:
+  → Use Basic auth passthrough
+If 200 (no auth required):
+  → Allow anonymous access
+```
+
+## Non-Goals
+
+- OAuth 2.0 as authentication mode (separate enhancement)
+- Automatic auth backend selection (requires explicit configuration)
+- Role-based auth backend routing (out of scope)
+- User migration between auth backends (handled separately)
+- Authentication performance optimization (orthogonal)
+
+## Rationale
+
+**Why per-backend Match()?** Allows each backend to report readiness independently, enabling smart fallback without complex conditional logic.
+
+**Why OIDC + DB fallback?** Admin access is critical. If OIDC provider fails, ops need a way back in without restarting Harbor.
+
+**Why probe-based registry auth?** Different registries use different auth methods. Probing detects the method automatically; no manual configuration needed.
+
+**Why per-scope token caching?** Prevents privilege escalation. A token scoped to `repo1` cannot be reused for `repo2`.
+
+**Why fix locked-user bug?** Nil authentication creates security gap. Locked accounts should be locked, not silently pass through.
+
+## Compatibility
+
+**No configuration changes required.** All existing `auth_mode` values work:
+
+| auth_mode | Behavior | Change |
+|-----------|----------|--------|
+| `db` | Database auth only | No change |
+| `ldap` | LDAP auth only | No change |
+| `oidc` | OIDC + DB fallback | **New**: fallback added (was OIDC-only) |
+| `http_auth` | HTTP Auth Proxy | **Fix**: now functional (was broken) |
+| `uaa` | UAA auth only | No change |
+
+**Backward Compatibility:**
+- No database migrations required
+- No API changes
+- Existing authentication methods continue working
+- Existing robot accounts continue working
+- Configuration files need no updates
+
+## Implementation
+
+### Code Changes
+
+**Core Authentication** (`src/core/auth/`):
+- `authenticator.go` — New `Match()` interface; multi-backend dispatch; locked-user security fix
+- `authproxy/auth.go`, `db/db.go`, `ldap/ldap.go`, `oidc/oidc.go`, `uaa/uaa.go` — Per-backend `Match()` implementations
+
+**Registry Proxy** (`src/server/registry/`):
+- `proxy.go` — Probe-based auth detection; credential passthrough; per-scope token caching
+- `proxy_test.go` — 372 lines of test coverage
+
+**V2 Auth Middleware** (`src/server/middleware/v2auth/`):
+- `auth.go` — Correct Bearer challenge for Basic-auth clients; graceful token service error handling
+
+**Config & UI** (`src/lib/config/`, `src/portal/`):
+- Remove unused `AUTH_MODE` UI strings
+- Remove context helpers `WithAuthMode` / `AuthModeFrom`
+- Update i18n files
+
+### Testing
+
+**Unit Tests:**
+- Registry proxy auth probing (3 cases)
+- Token caching and expiry (2 cases)
+- Scope extraction from request paths (8 cases)
+- Basic auth exchange (1 case)
+- Bearer challenge generation (1 case)
+
+**Integration Tests:**
+- Multi-backend dispatch with actual backends
+- Registry proxy with upstream probe
+- Token exchange with token service
+- Fallback behavior across all `auth_mode` values
+
+**E2E Tests:**
+- Docker login with each `auth_mode`
+- Registry push/pull with proxy and passthrough auth
+- OIDC provider unavailability + admin fallback
+- Locked user rejected from all paths
+- HTTP Auth Proxy external endpoint integration
+
+### Implementation PR Organization
+
+The Harbor implementation PR [#23458](https://github.com/goharbor/harbor/pull/23458) is organized into 16 commits across 3 logical units:
+
+**Unit 1 — Security fixes** (4 commits)
+- Locked user returns `ErrAuth` instead of nil
+- Nil pointer dereference guards
+
+**Unit 2 — Registry proxy** (5 commits)
+- Probe-based auth detection
+- User credential passthrough
+- Per-scope token caching
+- Test coverage
+
+**Unit 3 — Core refactor** (7 commits)
+- Per-backend `Match()` dispatch
+- Remove global auth_mode switch
+- Portal UI updates
+- Config cleanup
+
+These can be reviewed separately if preferred; Unit 2 (registry proxy) is independent of Unit 3.
+
+## Known Limitations
+
+- **Token cache in-memory only**: Distributed deployments re-exchange tokens per instance; future: distributed cache support
+- **Per-scope tokens only**: Repository-level granularity not yet supported; future work for per-action (pull/push/delete) scoping
+- **No auto-fallback config**: Operators must explicitly set `auth_mode = "oidc"` for fallback; no automatic selection
+- **Token service errors**: Registry proxy degrades to Basic challenge on token service failure; tokens not cached
+
+## Potential Future Enhancements
+
+- Distributed token cache (Redis/Memcached) for multi-instance Harbor
+- Per-action token scoping (pull vs push vs delete)
+- Scheduled token cleanup from cache
+- Auth backend success/failure metrics
+- Detailed audit logging of auth backend decisions
+- Role-based auth backend routing
+- OAuth 2.0 support as native authentication mode
+
+## Issues Resolved
+
+| Issue | Title | Resolution |
+|-------|-------|-----------|
+| #1572 | Lack of brute force / account locking mechanism | **FIXED** — locked users now return ErrAuth |
+| #13372 | Combine multiple authentication modes | **ADDRESSES** — multi-mode support via fallback dispatch |
+| #21300 | Passthrough Authentication in Proxy Cache | **IMPROVES** — user credentials now passed to token service |
+| #7965 | Support local user creation in http_auth mode | **FIXES** — http_auth mode now fully functional |
+| #7964 | Config option for token login in http_auth mode | **FIXES** — http_auth mode now fully functional |
+| #21853 | OIDC and LDAP at the same time as auth mode | **ADDRESSES** — OIDC + DB fallback achieves some co-existence |
+
+## References
+
+- **Implementation**: [goharbor/harbor#23458](https://github.com/goharbor/harbor/pull/23458)
+- **Related Issues**: #1572, #13372, #21300, #7965, #7964, #21853
+- **Code locations**: `src/core/auth/`, `src/server/registry/`, `src/server/middleware/v2auth/`
+- **Related work**: Docker Registry V2 auth probe, Kubernetes bearer token handling, GitHub OAuth fallback chains