headroom/plugins/headroom-oauth2/SPEC.md

Spec — headroom-oauth2 (generic OAuth2 client-credentials upstream auth)

Summary

A vendor-neutral proxy extension (registers on Headroom's headroom.proxy_extension seam) that mints an OAuth2 client-credentials (RFC 6749 §4.4) bearer from a configured token endpoint and injects it as the upstream Authorization on every proxied request. Lets Headroom front any gateway that requires a minted machine token (not a static API key) — with zero core changes and no vendor specifics (the gateway is entirely config/env).

It complements #510 (env-var auth), which assumes a long-lived static key; this covers the "mint-then-refresh a short-lived token" case.

API surface (config / CLI / env)

Opt-in only, via Headroom's existing flags — no new CLI flags:

headroom proxy --backend litellm-openai --proxy-extension oauth2
# or HEADROOM_PROXY_EXTENSIONS=oauth2

All configuration is env (12-factor; nothing baked in):

Env var	Required	Meaning
HEADROOM_OAUTH2_TOKEN_URL	yes (else no-op)	OAuth2 token endpoint; must be https (loopback http allowed for tests)
HEADROOM_OAUTH2_CLIENT_ID / _CLIENT_SECRET	yes	client credentials
HEADROOM_OAUTH2_SCOPES	no	space/comma-separated scopes
HEADROOM_OAUTH2_AUDIENCE	no	audience form param
HEADROOM_OAUTH2_RESOURCE	no	RFC 8707 target resource form param
HEADROOM_OAUTH2_GRANT_TYPE	no	default client_credentials
HEADROOM_OAUTH2_AUTH_STYLE	no	post (form creds) or basic (HTTP Basic)
HEADROOM_OAUTH2_HEADERS	no	static upstream headers, K=V,K2=V2 (control chars rejected)
HEADROOM_OAUTH2_TIMEOUT / _SKEW	no	token request timeout / pre-expiry refresh skew (s)
HEADROOM_OAUTH2_ALLOW_INSECURE	no	1 to allow a non-loopback http token_url (discouraged)

Public Python API: OAuth2ClientCredentials, OAuth2Middleware, OAuth2Error, install, provider_from_env, parse_headers.

Changes to existing behavior / defaults / compatibility

• None unless explicitly enabled. The entry point is dormant until --proxy-extension oauth2 is passed, and even then a no-op unless HEADROOM_OAUTH2_TOKEN_URL is set.
• When active, it overwrites the request Authorization header with the minted bearer before the backend runs. The client's own Authorization/x-api-key is intentionally replaced (the proxy authenticates to the gateway on the client's behalf). Compatibility note: because the request then carries a bearer, Headroom classifies it as OAuth-mode auth — same as supplying a bearer yourself; no new classification path.
• No change to defaults, the request/response body, model routing, or compression.

User stories (Given / When / Then)

• Golden path — Given a proxy started with --proxy-extension oauth2 and valid TOKEN_URL/CLIENT_ID/CLIENT_SECRET, When a client sends /v1/messages, Then the extension mints (or reuses a cached) bearer and the upstream receives Authorization: Bearer <minted> plus any static headers; the client never sees the secret.
• Edge: token endpoint down — Given an unreachable/erroring TOKEN_URL, When a request arrives, Then the proxy returns 502 upstream_auth_error (no upstream call, no secret/body leak) and stays up; the next request retries.
• Edge: wrong backend — Given --backend bedrock (env-auth), When the extension installs, Then it logs a loud warning that the injected bearer will have no effect and to use an OpenAI-compatible/passthrough backend.

Failure modes & recovery

Failure	Behavior
Missing/invalid config at startup	install() raises -> proxy fails closed (won't start mis-auth'd)
Token endpoint unreachable / non-2xx / non-JSON / no access_token	OAuth2Error -> 502, per-request, proxy stays up, retried next request
expires_in = 0/negative/absent	clamped to a positive TTL (never stale, never per-request mint)
Concurrent first requests	single-flight lock -> exactly one mint per refresh
Malformed HEADROOM_OAUTH2_HEADERS (CR/LF)	offending pair dropped with a warning (no header injection)

Resilience (Docker / native / wrappers / providers / multi-process)

• Native & Docker: identical; pure env-driven, std-lib only. Token minted via urllib against the system cert store, so a corporate-injected CA is trusted with no bundled roots (works in SSL-inspection networks).
• Wrappers (headroom wrap, agent hooks): the extension lives at the proxy layer, so anything routed through the proxy inherits it transparently.
• Providers: effective for OpenAI-compatible / passthrough litellm backends (those that forward the request bearer upstream). bedrock/vertex/sagemaker authenticate from env and ignore the bearer -> the extension warns and is a no-op there.
• Multi-process (multiple workers): the token cache is per-process; each worker mints/refreshes independently. Acceptable for client-credentials (idempotent, low rate); no shared state, no cross-process lock needed. Documented so operators can size token-endpoint rate limits.

Security & privacy

• Secrets are env-only; never logged and never returned to the client.
• The IdP error body is drained, not surfaced (may echo sensitive context).
• token_url is https-enforced (loopback exception for tests; explicit opt-out env).
• The minted bearer is sent only to the configured upstream; the client's inbound credential is replaced, not forwarded onward.

Observability / logging / telemetry

• INFO on install (token_url + auth style, no secrets) and on each mint (ttl, scopes).
• WARNING on mint failure, env-auth-backend no-op, and dropped malformed static headers.
• No metrics/telemetry emitted; piggybacks on Headroom's existing request logging. (A future counter for mint/refresh/failure could be added if maintainers want it.)

Rollback / migration

• No migration — additive and opt-in; existing deployments are unaffected.
• Instant rollback: drop --proxy-extension oauth2 (or unset HEADROOM_PROXY_EXTENSIONS), or uninstall the package. No state to clean up, no config format changes.

Dependencies

• Runtime: standard library only (no new core dependency). litellm is touched only if HEADROOM_OAUTH2_HEADERS is set, and it is already a Headroom backend dependency — declared here as the optional [litellm] extra, not a hard requirement.