feat: Add Kerberos support (#4640)

Signed-off-by: Ivan Zvyagintsev <ivan.zvyagintsev@flant.com>

Author: Jabejixo

connector/ldap/kerberos.go

@@ -0,0 +1,398 @@
+// Package ldap implements strategies for authenticating using the LDAP protocol.
+// This file contains Kerberos/SPNEGO authentication support for LDAP connector.
+package ldap
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/go-ldap/ldap/v3"
+	"github.com/jcmturner/goidentity/v6"
+	"github.com/jcmturner/gokrb5/v8/credentials"
+	"github.com/jcmturner/gokrb5/v8/keytab"
+	"github.com/jcmturner/gokrb5/v8/service"
+	"github.com/jcmturner/gokrb5/v8/spnego"
+
+	"github.com/dexidp/dex/connector"
+)
+
+// bufferedResponse is a minimal http.ResponseWriter that buffers status,
+// headers, and body in memory so the SPNEGO middleware's response can be
+// inspected and conditionally forwarded. spnego.SPNEGOKRB5Authenticate is a
+// "terminal" middleware (it commits failure responses directly to the
+// ResponseWriter), but Dex needs to layer policy on top — FallbackToPassword
+// may want to discard a 401 so the password form can render, and
+// ExpectedRealm checks happen after a successful auth. Buffering decouples
+// the middleware's output from the eventual client response.
+type bufferedResponse struct {
+	header      http.Header
+	body        bytes.Buffer
+	code        int
+	wroteHeader bool
+}
+
+func newBufferedResponse() *bufferedResponse {
+	return &bufferedResponse{
+		header: make(http.Header),
+		code:   http.StatusOK,
+	}
+}
+
+func (r *bufferedResponse) Header() http.Header {
+	return r.header
+}
+
+// WriteHeader mirrors net/http: the first call wins, subsequent calls are
+// silently ignored (stdlib emits a "superfluous WriteHeader" warning and
+// keeps the original status; we just drop the override).
+func (r *bufferedResponse) WriteHeader(code int) {
+	if r.wroteHeader {
+		return
+	}
+	r.code = code
+	r.wroteHeader = true
+}
+
+// Write mirrors net/http: a Write before any WriteHeader implicitly commits
+// status 200, locking the status against later overrides.
+func (r *bufferedResponse) Write(b []byte) (int, error) {
+	if !r.wroteHeader {
+		r.WriteHeader(http.StatusOK)
+	}
+	return r.body.Write(b)
+}
+
+// krbState holds Kerberos/SPNEGO configuration bound to a ldapConnector.
+//
+// The authenticate field is the SPNEGO HTTP middleware factory: it wraps an
+// inner http.Handler so that inner runs only after successful SPNEGO auth,
+// with an authenticated *credentials.Credentials attached to the request
+// context under goidentity.CTXKey. In production it delegates to
+// spnego.SPNEGOKRB5Authenticate. Unit tests replace it with a stub that
+// injects a fake credential without any real Kerberos exchange.
+type krbState struct {
+	authenticate func(inner http.Handler) http.Handler
+}
+
+// loadKerberosState validates the keytab and returns a krbState wired to
+// spnego.SPNEGOKRB5Authenticate with the requested service settings.
+func loadKerberosState(cfg kerberosConfig) (*krbState, error) {
+	fi, err := os.Stat(cfg.KeytabPath)
+	if err != nil {
+		return nil, fmt.Errorf("keytab file not found: %w", err)
+	}
+	if fi.IsDir() {
+		return nil, fmt.Errorf("keytab path is a directory: %s", cfg.KeytabPath)
+	}
+	kt, err := keytab.Load(cfg.KeytabPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to load keytab: %w", err)
+	}
+
+	settings := []func(*service.Settings){service.DecodePAC(false)}
+	if cfg.SPN != "" {
+		settings = append(settings, service.SName(cfg.SPN))
+	}
+	if cfg.KeytabPrincipal != "" {
+		settings = append(settings, service.KeytabPrincipal(cfg.KeytabPrincipal))
+	}
+	if cfg.MaxClockSkew > 0 {
+		settings = append(settings, service.MaxClockSkew(time.Duration(cfg.MaxClockSkew)*time.Second))
+	}
+
+	return &krbState{
+		authenticate: func(inner http.Handler) http.Handler {
+			return spnego.SPNEGOKRB5Authenticate(inner, kt, settings...)
+		},
+	}, nil
+}
+
+// mapPrincipal builds the LDAP-side username from the Kerberos credentials
+// according to the configured mapping. Inputs come from gokrb5:
+// username is credentials.UserName() (bare, no realm); realm is Domain().
+//
+//   - "userprincipalname":                    "username@realm".
+//   - "localpart"/"samaccountname" (default): bare username.
+//
+// Case of the output is preserved; callers rely on the LDAP server's
+// attribute matching rules (typically case-insensitive) for comparisons.
+func mapPrincipal(username, realm, mapping string) string {
+	// Defensive: if username somehow carries an '@', trim to before it; we
+	// always derive the realm from the separate Domain() field.
+	if i := strings.IndexByte(username, '@'); i >= 0 {
+		username = username[:i]
+	}
+	if strings.EqualFold(mapping, "userprincipalname") {
+		if realm == "" {
+			return username
+		}
+		return username + "@" + realm
+	}
+	return username
+}
+
+// TrySPNEGO attempts SPNEGO authentication against the LDAP connector's keytab.
+//
+// Behavior:
+//   - Kerberos disabled: returns (nil, false, nil) so the caller renders the
+//     password form.
+//   - FallbackToPassword=true with no Authorization header: a cookie probe
+//     gives the browser exactly one round to negotiate. The first such
+//     request sets a short-lived "tried" cookie and forwards the middleware's
+//     401 Negotiate challenge so a Kerberos-aware client can respond. If the
+//     follow-up request still has no Authorization header (cookie present),
+//     we treat the client as unable to SPNEGO and render the password form.
+//   - FallbackToPassword=true with an Authorization header that the
+//     middleware rejects: render the password form (the client tried and
+//     failed; do not loop 401s).
+//   - FallbackToPassword=false: forward the middleware's response verbatim.
+//   - On success, the authenticated principal is resolved in LDAP and a
+//     connector.Identity is returned. Any prior probe cookie is cleared.
+func (c *ldapConnector) TrySPNEGO(ctx context.Context, s connector.Scopes, w http.ResponseWriter, r *http.Request) (*connector.Identity, connector.Handled, error) {
+	if c.krb == nil {
+		return nil, false, nil
+	}
+
+	hasNegotiate := strings.HasPrefix(r.Header.Get("Authorization"), "Negotiate ")
+
+	// Cookie probe: see the package-level doc on spnegoProbeCookieName. Only
+	// applies when fallback is enabled and the client has not (yet) sent a
+	// SPNEGO token; "Authorization present" paths bypass the probe entirely.
+	if c.krbConf.FallbackToPassword && !hasNegotiate {
+		if hasSPNEGOProbeCookie(r) {
+			c.logger.Info("kerberos: SPNEGO probe cookie present and no Negotiate header; falling back to password form")
+			return nil, false, nil
+		}
+		setSPNEGOProbeCookie(w, r)
+	}
+
+	// Run the SPNEGO middleware with a capturing recorder so we can decide
+	// whether to forward its response or fall back to the password form.
+	var (
+		id       *credentials.Credentials
+		innerErr error
+	)
+	inner := http.HandlerFunc(func(_ http.ResponseWriter, rr *http.Request) {
+		ident := goidentity.FromHTTPRequestContext(rr)
+		if ident == nil {
+			innerErr = fmt.Errorf("kerberos: no identity in request context after SPNEGO")
+			return
+		}
+		creds, ok := ident.(*credentials.Credentials)
+		if !ok {
+			innerErr = fmt.Errorf("kerberos: unexpected identity type %T", ident)
+			return
+		}
+		id = creds
+	})
+
+	rec := newBufferedResponse()
+	c.krb.authenticate(inner).ServeHTTP(rec, r)
+
+	if innerErr != nil {
+		c.logger.Error("kerberos: SPNEGO middleware completed with unusable credentials", "err", innerErr)
+		return nil, true, innerErr
+	}
+
+	if id == nil {
+		// Client offered a token and the middleware rejected it: under
+		// fallback semantics this is "tried and failed" — render the form
+		// rather than looping 401s.
+		if c.krbConf.FallbackToPassword && hasNegotiate {
+			c.logger.Info("kerberos: SPNEGO rejected client token; falling back to password form")
+			return nil, false, nil
+		}
+		// Otherwise (fallback=false, OR fallback=true probe round): forward
+		// the middleware-authored response (challenge token, error payload,
+		// etc.) verbatim — the protocol decided to reject and we preserve
+		// its wire details.
+		c.logger.Info("kerberos: SPNEGO did not authenticate; forwarding middleware response", "status", rec.code)
+		copyBuffered(rec, w)
+		return nil, true, nil
+	}
+
+	if c.krbConf.ExpectedRealm != "" && !strings.EqualFold(c.krbConf.ExpectedRealm, id.Domain()) {
+		c.logger.Info("kerberos: realm mismatch", "expected", c.krbConf.ExpectedRealm, "actual", id.Domain())
+		if c.krbConf.FallbackToPassword {
+			// Intentionally do NOT clear the probe cookie here: the
+			// client's TGT is for a realm we will never accept. Clearing
+			// would re-arm a probe round on the next no-Authorization GET,
+			// re-issue 401 Negotiate, the browser would resend the same
+			// wrong-realm token, and we'd land back here — an infinite
+			// flap. Keeping the cookie pins the client to the password
+			// form until the cookie's Max-Age elapses.
+			return nil, false, nil
+		}
+		// SPNEGO authenticated successfully, but our ExpectedRealm policy
+		// rejects it. The middleware's buffered response is irrelevant here
+		// (it represents the post-success path through the inner handler);
+		// emit our own bare Negotiate challenge so the client knows to
+		// retry with a different realm.
+		writeBareNegotiateChallenge(w)
+		return nil, true, nil
+	}
+
+	mapped := mapPrincipal(id.UserName(), id.Domain(), c.krbConf.UsernameFromPrincipal)
+	c.logger.Info("kerberos: principal authenticated",
+		"principal", id.UserName(),
+		"realm", id.Domain(),
+		"auth_time", id.AuthTime(),
+		"mapped_username", mapped,
+	)
+
+	userEntry, err := c.lookupKerberosUser(ctx, mapped)
+	if err != nil {
+		c.logger.Error("kerberos: LDAP user lookup failed",
+			"principal", id.UserName(), "mapped", mapped, "err", err)
+		return nil, true, fmt.Errorf("ldap: user lookup failed for kerberos principal %q: %w", id.UserName(), err)
+	}
+	c.logger.Info("kerberos: LDAP user found", "dn", userEntry.DN)
+
+	ident, err := c.identityFromEntry(userEntry)
+	if err != nil {
+		c.logger.Error("kerberos: failed to build identity from LDAP entry", "err", err)
+		return nil, true, err
+	}
+	if s.Groups {
+		groups, err := c.groups(ctx, userEntry)
+		if err != nil {
+			c.logger.Error("kerberos: failed to query groups", "err", err)
+			return nil, true, fmt.Errorf("ldap: failed to query groups: %w", err)
+		}
+		ident.Groups = groups
+	}
+
+	// No user-bind has happened; only materialize ConnectorData when refresh
+	// needs the LDAP entry later (OfflineAccess). Mirror (*ldapConnector).Login:
+	// a marshal failure here would silently break a subsequent Refresh call
+	// (Unmarshal on nil), so fail the login instead of letting that happen.
+	if s.OfflineAccess {
+		refresh := refreshData{Username: mapped, Entry: userEntry}
+		data, mErr := json.Marshal(refresh)
+		if mErr != nil {
+			c.logger.Error("kerberos: failed to marshal refresh data", "err", mErr)
+			return nil, true, fmt.Errorf("ldap: marshal refresh data: %w", mErr)
+		}
+		ident.ConnectorData = data
+	}
+
+	// If the client carried a stale probe cookie from a prior fallback
+	// round, clear it so a future logout/re-login starts negotiation fresh.
+	if hasSPNEGOProbeCookie(r) {
+		clearSPNEGOProbeCookie(w, r)
+	}
+
+	c.logger.Info("kerberos: SPNEGO login succeeded",
+		"username", ident.Username, "email", ident.Email, "groups_count", len(ident.Groups))
+	return &ident, true, nil
+}
+
+// copyBuffered forwards a buffered handler response to the real client.
+// Used when we want the middleware-authored bytes to reach the user agent
+// unchanged (e.g. SPNEGO continuation/reject tokens).
+func copyBuffered(rec *bufferedResponse, w http.ResponseWriter) {
+	for k, vv := range rec.header {
+		for _, v := range vv {
+			w.Header().Add(k, v)
+		}
+	}
+	w.WriteHeader(rec.code)
+	_, _ = w.Write(rec.body.Bytes())
+}
+
+// writeBareNegotiateChallenge emits an unsolicited "WWW-Authenticate:
+// Negotiate" 401 directly to the client. This is reserved for cases where
+// Dex itself rejects an otherwise-successful SPNEGO exchange (currently
+// only ExpectedRealm mismatch); in those cases the middleware's buffered
+// output does not represent the answer we want to send.
+func writeBareNegotiateChallenge(w http.ResponseWriter) {
+	w.Header().Set("WWW-Authenticate", "Negotiate")
+	w.WriteHeader(http.StatusUnauthorized)
+}
+
+// spnegoProbeCookieName names the short-lived cookie that records "we
+// already issued a 401 Negotiate challenge to this client". It implements
+// the fallback-to-password semantics: the very first GET without an
+// Authorization header gets a real Negotiate challenge so a Kerberos-aware
+// browser can SSO; if the client comes back without a token we treat that
+// as "client cannot/will not negotiate" and render the password form
+// instead of looping 401s. The cookie is bounded by a small Max-Age so a
+// later visit gets a fresh chance to negotiate.
+const (
+	spnegoProbeCookieName = "dex_spnego_tried"
+	spnegoProbeMaxAge     = 60 // seconds; long enough for one challenge round-trip
+)
+
+func hasSPNEGOProbeCookie(r *http.Request) bool {
+	_, err := r.Cookie(spnegoProbeCookieName)
+	return err == nil
+}
+
+// newSPNEGOProbeCookie returns a probe-cookie carrier with the attributes
+// shared between "set" and "clear" forms (Path, HttpOnly, Secure, SameSite),
+// so the two callers cannot accidentally drift apart.
+func newSPNEGOProbeCookie(r *http.Request, value string, maxAge int) *http.Cookie {
+	return &http.Cookie{
+		Name:  spnegoProbeCookieName,
+		Value: value,
+		// Scope to "/" so the cookie reaches every Dex auth endpoint
+		// regardless of issuer prefix; the cookie carries no secret and
+		// expires within spnegoProbeMaxAge seconds, so the broad path is
+		// not a privacy or security boundary concern.
+		Path:     "/",
+		MaxAge:   maxAge,
+		HttpOnly: true,
+		Secure:   isSecureRequest(r),
+		SameSite: http.SameSiteLaxMode,
+	}
+}
+
+func setSPNEGOProbeCookie(w http.ResponseWriter, r *http.Request) {
+	http.SetCookie(w, newSPNEGOProbeCookie(r, "1", spnegoProbeMaxAge))
+}
+
+func clearSPNEGOProbeCookie(w http.ResponseWriter, r *http.Request) {
+	http.SetCookie(w, newSPNEGOProbeCookie(r, "", -1))
+}
+
+// isSecureRequest reports whether the original client connection is HTTPS.
+// It honors X-Forwarded-Proto so the cookie's Secure attribute is correct
+// when Dex sits behind a TLS-terminating proxy. The header is treated as
+// advisory: misbehaving/spoofed values can only flip Secure to true on a
+// plain-HTTP setup (which makes browsers refuse to echo the cookie back —
+// a graceful no-op for the probe), never to false on a real HTTPS link.
+func isSecureRequest(r *http.Request) bool {
+	if r.TLS != nil {
+		return true
+	}
+	return strings.EqualFold(r.Header.Get("X-Forwarded-Proto"), "https")
+}
+
+// lookupKerberosUser resolves an LDAP user entry by username. When
+// krbLookupUserHook is non-nil it is used exclusively and the real LDAP
+// search is skipped entirely — this keeps unit tests hermetic.
+func (c *ldapConnector) lookupKerberosUser(ctx context.Context, username string) (ldap.Entry, error) {
+	if c.krbLookupUserHook != nil {
+		return c.krbLookupUserHook(ctx, c, username)
+	}
+
+	var userEntry ldap.Entry
+	err := c.do(ctx, func(conn *ldap.Conn) error {
+		entry, found, err := c.userEntry(conn, username)
+		if err != nil {
+			return err
+		}
+		if !found {
+			return fmt.Errorf("user not found for principal")
+		}
+		userEntry = entry
+		return nil
+	})
+	return userEntry, err
+}