mcp-chassis/auth/jwt.go

// Package auth provides the Dex/Authentik-JWT + static-Bearer authentication
// primitives shared by every Mathias-owned MCP server (gitea-mcp, brain-mcp /
// ingestion, future MCPs spawned from template-go-agent).
//
// Replaces ~80 LOC of near-identical jwt.go in each consumer, ~50 LOC of
// Bearer middleware, and ~25 LOC of RFC 9728 protected-resource metadata
// handler. See `gitea.d-ma.be/mathias/infra` docs/superpowers/handoffs/
// 2026-05-22-mcp-chassis-spike.md for the design rationale and the
// abort-criterion check.
//
// A validator trusts a LIST of OIDC issuers (infra ADR-0011): one MCP server can
// accept Authentik JWTs (interactive/web) AND k3s cluster ServiceAccount tokens
// (in-cluster, audience-bound, kubelet-rotated) at once.
package auth

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"time"

	"github.com/lestrrat-go/jwx/v2/jwk"
	"github.com/lestrrat-go/jwx/v2/jwt"
)

// ErrUnavailable indicates JWT validation could not be COMPLETED because the
// JWKS / issuer endpoint was unreachable — a transient condition — as opposed to
// the token being present and invalid. Callers (e.g. BearerMiddleware) map it
// to HTTP 503 temporarily_unavailable rather than a generic 401, so an issuer
// outage is distinguishable from a bad token (gitea-mcp#6).
var ErrUnavailable = errors.New("jwt validation temporarily unavailable")

// IssuerConfig names one trusted OIDC issuer and its (optional) required
// audience. Trusting a list of these is what lets a single MCP server accept
// tokens from Authentik (D3/D4) and the k3s cluster OIDC issuer (D1 SA tokens)
// simultaneously — infra ADR-0011 Decision 5.
type IssuerConfig struct {
	IssuerURL string
	Audience  string // "" = skip audience validation for this issuer
}

// issuerEntry is a resolved IssuerConfig: OIDC discovery has run and jwks_uri is
// registered in the shared cache.
type issuerEntry struct {
	issuer   string
	audience string
	jwksURI  string
}

// JWTValidator validates Bearer JWTs against one or more trusted OIDC issuers.
//
// A nil *JWTValidator behaves as "JWT auth disabled" — Validate returns an
// error without panicking. Callers can construct one validator at startup keyed
// on whether any issuer is configured, and pass nil through the rest of the
// codebase without further branching.
type JWTValidator struct {
	entries []issuerEntry
	cache   *jwk.Cache
}

// NewJWTValidator builds a single-issuer validator — the common case, and
// backward compatible with every existing caller. Empty issuerURL returns
// (nil, nil) so callers can use one constructor regardless of whether an issuer
// is configured.
func NewJWTValidator(ctx context.Context, issuerURL, audience string) (*JWTValidator, error) {
	if issuerURL == "" {
		return nil, nil
	}
	return NewMultiJWTValidator(ctx, []IssuerConfig{{IssuerURL: issuerURL, Audience: audience}})
}

// NewMultiJWTValidator builds a validator that trusts every issuer in the list.
// For each, it fetches the OIDC discovery document, registers jwks_uri in a
// shared cache, and warms it. Entries with an empty IssuerURL are skipped; an
// empty/nil resulting set returns (nil, nil) — "JWT auth disabled".
func NewMultiJWTValidator(ctx context.Context, issuers []IssuerConfig) (*JWTValidator, error) {
	cache := jwk.NewCache(ctx)
	var entries []issuerEntry
	for _, ic := range issuers {
		if ic.IssuerURL == "" {
			continue
		}
		jwksURI, err := discoverJWKSURI(ctx, ic.IssuerURL)
		if err != nil {
			return nil, fmt.Errorf("issuer %s: %w", ic.IssuerURL, err)
		}
		if err := cache.Register(jwksURI, jwk.WithMinRefreshInterval(time.Hour)); err != nil {
			return nil, fmt.Errorf("register jwks cache (%s): %w", ic.IssuerURL, err)
		}
		if _, err := cache.Refresh(ctx, jwksURI); err != nil {
			return nil, fmt.Errorf("initial jwks fetch (%s): %w", ic.IssuerURL, err)
		}
		entries = append(entries, issuerEntry{issuer: ic.IssuerURL, audience: ic.Audience, jwksURI: jwksURI})
	}
	if len(entries) == 0 {
		return nil, nil
	}
	return &JWTValidator{entries: entries, cache: cache}, nil
}

// discoverJWKSURI fetches the OIDC discovery document from issuerURL and returns
// its jwks_uri.
func discoverJWKSURI(ctx context.Context, issuerURL string) (string, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet,
		issuerURL+"/.well-known/openid-configuration", nil)
	if err != nil {
		return "", fmt.Errorf("build oidc discovery request: %w", err)
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return "", fmt.Errorf("fetch oidc discovery: %w", err)
	}
	defer func() { _ = resp.Body.Close() }()
	if resp.StatusCode != http.StatusOK {
		return "", fmt.Errorf("oidc discovery: status %d", resp.StatusCode)
	}

	var doc struct {
		JWKSURI string `json:"jwks_uri"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&doc); err != nil {
		return "", fmt.Errorf("decode oidc discovery: %w", err)
	}
	if doc.JWKSURI == "" {
		return "", fmt.Errorf("oidc discovery: empty jwks_uri")
	}
	return doc.JWKSURI, nil
}

// Validate parses rawToken against each trusted issuer and returns the subject
// claim on the first success. If no issuer accepts it: when EVERY issuer's JWKS
// was unreachable the error wraps ErrUnavailable (caller answers 503); otherwise
// at least one issuer reached a signature/claims verdict and rejected it, so the
// error is a definitive rejection (401). A nil receiver returns errDisabled.
func (v *JWTValidator) Validate(ctx context.Context, rawToken string) (string, error) {
	if v == nil {
		return "", errDisabled
	}

	var lastErr error
	sawDecision := false // at least one issuer reached a real verify verdict
	for _, e := range v.entries {
		keySet, err := v.cache.Get(ctx, e.jwksURI)
		if err != nil {
			// This issuer's JWKS is unreachable — transient. Try the next; only
			// if ALL issuers are unreachable do we surface ErrUnavailable.
			lastErr = fmt.Errorf("%w: get jwks (%s): %v", ErrUnavailable, e.issuer, err)
			continue
		}

		opts := []jwt.ParseOption{
			jwt.WithKeySet(keySet),
			jwt.WithValidate(true),
			jwt.WithIssuer(e.issuer),
		}
		if e.audience != "" {
			opts = append(opts, jwt.WithAudience(e.audience))
		}

		tok, err := jwt.ParseString(rawToken, opts...)
		if err == nil {
			return tok.Subject(), nil
		}
		sawDecision = true
		lastErr = fmt.Errorf("validate jwt: %w", err)
	}

	if lastErr == nil {
		lastErr = errDisabled
	}
	if !sawDecision {
		// Every issuer's JWKS was unreachable — lastErr already wraps ErrUnavailable.
		return "", lastErr
	}
	return "", lastErr
}

// errDisabled is the sentinel returned by Validate on a nil receiver.
// Not exported because callers care about "not authorized" not "why";
// this lets consumers treat any err the same way without depending on
// the specific value.
var errDisabled = fmt.Errorf("jwt auth disabled")