hscontrol: gate /key on supported capability version

/key handed out the Noise public key for any v>=39, a floor unrelated
to the handshake's capver.MinSupportedCapabilityVersion. Reject below
the supported floor, matching /ts2021, and drop the stale constant.

Fixes #3380

hscontrol/handlers.go

@@ -19,17 +19,6 @@ import (
 )
 
 const (
-	// NoiseCapabilityVersion is used by Tailscale clients to indicate
-	// their codebase version. Tailscale clients can communicate over TS2021
-	// from CapabilityVersion 28, but we only have good support for it
-	// since https://github.com/tailscale/tailscale/pull/4323 (Noise in any HTTPS port).
-	//
-	// Related to this change, there is https://github.com/tailscale/tailscale/pull/5379,
-	// where CapabilityVersion 39 is introduced to indicate #4323 was merged.
-	//
-	// See also https://github.com/tailscale/tailscale/blob/main/tailcfg/tailcfg.go
-	NoiseCapabilityVersion = 39
-
 	reservedResponseHeaderSize = 4
 )
 
@@ -199,21 +188,28 @@ func (h *Headscale) KeyHandler(
 		return
 	}
 
-	// TS2021 (Tailscale v2 protocol) requires to have a different key
-	if capVer >= NoiseCapabilityVersion {
-		resp := tailcfg.OverTLSPublicKeyResponse{
-			PublicKey: h.noisePrivateKey.Public(),
-		}
-
-		writer.Header().Set("Content-Type", "application/json")
-
-		err := json.NewEncoder(writer).Encode(resp)
-		if err != nil {
-			log.Error().Err(err).Msg("failed to encode public key response")
-		}
-
+	// Only disclose the Noise public key to clients this server can
+	// actually complete a handshake with. Gating on the same floor the
+	// Noise handshake enforces (capver.MinSupportedCapabilityVersion, see
+	// isSupportedVersion in noise.go) keeps /key consistent with /ts2021:
+	// versions the handshake would reject get a clear rejection here
+	// instead of a key that only serves as a version-boundary oracle.
+	// See https://github.com/juanfont/headscale/issues/3380.
+	if !isSupportedVersion(capVer) {
+		httpError(writer, NewHTTPError(http.StatusBadRequest, "unsupported client version", unsupportedClientError(capVer)))
 		return
 	}
+
+	resp := tailcfg.OverTLSPublicKeyResponse{
+		PublicKey: h.noisePrivateKey.Public(),
+	}
+
+	writer.Header().Set("Content-Type", "application/json")
+
+	err = json.NewEncoder(writer).Encode(resp)
+	if err != nil {
+		log.Error().Err(err).Msg("failed to encode public key response")
+	}
 }
 
 func (h *Headscale) HealthHandler(

hscontrol/handlers_test.go

@@ -5,12 +5,14 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
 	"strings"
 	"testing"
 
+	"github.com/juanfont/headscale/hscontrol/capver"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -116,6 +118,52 @@ func TestVerifyHandler_SuccessSetsJSONContentType(t *testing.T) {
 		"successful /verify response must advertise application/json")
 }
 
+// TestKeyHandler_UnsupportedCapVerDoesNotLeakKey reproduces
+// https://github.com/juanfont/headscale/issues/3380. The /key handler
+// must gate key disclosure on the same floor the Noise handshake
+// enforces (capver.MinSupportedCapabilityVersion). A capability version
+// below that floor can never complete a handshake, so it must be
+// rejected rather than handed the server's Noise public key, which would
+// otherwise serve only as a fingerprint / version-boundary oracle.
+func TestKeyHandler_UnsupportedCapVerDoesNotLeakKey(t *testing.T) {
+	t.Parallel()
+
+	noise := key.NewMachine()
+	h := &Headscale{noisePrivateKey: &noise}
+
+	unsupported := capver.MinSupportedCapabilityVersion - 1
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(
+		context.Background(),
+		http.MethodGet,
+		fmt.Sprintf("/key?v=%d", unsupported),
+		nil,
+	)
+
+	h.KeyHandler(rec, req)
+
+	assert.Equal(t, http.StatusBadRequest, rec.Code,
+		"a client below the supported floor must be rejected")
+	assert.NotContains(t, rec.Body.String(), noise.Public().String(),
+		"must not disclose Noise public key to a client below the supported floor")
+
+	// A supported client still receives the key.
+	recOK := httptest.NewRecorder()
+	reqOK := httptest.NewRequestWithContext(
+		context.Background(),
+		http.MethodGet,
+		fmt.Sprintf("/key?v=%d", capver.MinSupportedCapabilityVersion),
+		nil,
+	)
+
+	h.KeyHandler(recOK, reqOK)
+
+	assert.Equal(t, http.StatusOK, recOK.Code)
+	assert.Contains(t, recOK.Body.String(), noise.Public().String(),
+		"a supported client must receive the Noise public key")
+}
+
 // errorAsHTTPError is a small local helper that unwraps an [HTTPError]
 // from an error chain.
 func errorAsHTTPError(err error) (HTTPError, bool) {