diff --git a/pkg/client/client.go b/pkg/client/client.go
index 2768f40..005df6a 100644
--- a/pkg/client/client.go
+++ b/pkg/client/client.go
@@ -799,7 +799,17 @@ func (c *Client) processFrame(roomID string, info roomView, data []byte, handler
 	if err != nil {
 		return
 	}
-	if info.Policy.SignMsgs && f.Sig != nil {
+	// A room with SignMsgs REQUIRES a signature, so an unsigned frame is
+	// unauthenticated and must be dropped — not silently accepted. The previous
+	// `&& f.Sig != nil` guard verified the signature only when one was present, so
+	// an attacker with data-plane access could publish a frame with Sig==nil and a
+	// forged Sender and have the receiver accept it as authentic in a room that
+	// demands signatures (audit N3, report 0006). Requiring the signature first
+	// closes that spoof.
+	if info.Policy.SignMsgs {
+		if f.Sig == nil {
+			return // signature required by room policy but absent: drop
+		}
 		pub, err := c.signerPub(roomID, f.Sender)
 		if err != nil || !cs.VerifyEd25519(pub, f.SigningBytes(), f.Sig) {
 			return // unauthenticated frame: drop
diff --git a/pkg/client/sig_nil_spoof_test.go b/pkg/client/sig_nil_spoof_test.go
new file mode 100644
index 0000000..8d1baa5
--- /dev/null
+++ b/pkg/client/sig_nil_spoof_test.go
@@ -0,0 +1,154 @@
+package client_test
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/enmanuel/unibus/pkg/client"
+	"github.com/enmanuel/unibus/pkg/frame"
+	"github.com/enmanuel/unibus/pkg/room"
+	"github.com/nats-io/nats.go"
+)
+
+// TestReaudit_SigNilSpoof ports the re-auditor's N3 (Alto) finding: in a room
+// that REQUIRES per-message signatures, an attacker with data-plane access
+// publishes a raw frame with Sig==nil and a forged Sender. Before the fix
+// processFrame verified the signature only when one was present
+// (`SignMsgs && f.Sig != nil`), so the receiver accepted the unsigned, forged
+// frame as authentic. The fix drops any unsigned frame in a SignMsgs room.
+//
+// Coverage:
+//   - golden: a properly signed frame from a real member IS delivered;
+//   - error : an unsigned frame with a forged Sender in a SignMsgs room is DROPPED;
+//   - edge  : a room WITHOUT SignMsgs still delivers an unsigned frame (the drop
+//     is specific to signed rooms, not a blanket reject of unsigned frames).
+func TestReaudit_SigNilSpoof(t *testing.T) {
+	h := newHarness(t)
+	waitHealth(t, h.ctrlURL)
+
+	alice, err := client.New(h.natsURL, h.ctrlURL, mustIdentity(t))
+	if err != nil {
+		t.Fatalf("connect alice: %v", err)
+	}
+	defer alice.Close()
+	bob, err := client.New(h.natsURL, h.ctrlURL, mustIdentity(t))
+	if err != nil {
+		t.Fatalf("connect bob: %v", err)
+	}
+	defer bob.Close()
+
+	// A signed-but-NOT-encrypted room: SignMsgs enforces authorship, and the lack
+	// of encryption is exactly the case the auditor flagged as Alto (any peer with
+	// the subject can forge a sender if signatures are not strictly required).
+	const subject = "room.signed.spoof"
+	signedPolicy := room.Policy{Encrypt: false, Persist: false, SignMsgs: true}
+	roomID, err := alice.CreateRoom(subject, signedPolicy)
+	if err != nil {
+		t.Fatalf("alice create signed room: %v", err)
+	}
+	if err := alice.Invite(roomID, bob.Endpoint()); err != nil {
+		t.Fatalf("alice invite bob: %v", err)
+	}
+	if err := bob.Join(roomID); err != nil {
+		t.Fatalf("bob join: %v", err)
+	}
+
+	var mu sync.Mutex
+	var got []string
+	sub, err := bob.Subscribe(roomID, func(_ frame.Frame, plaintext []byte) {
+		mu.Lock()
+		got = append(got, string(plaintext))
+		mu.Unlock()
+	})
+	if err != nil {
+		t.Fatalf("bob subscribe: %v", err)
+	}
+	defer sub.Unsubscribe()
+	time.Sleep(150 * time.Millisecond)
+
+	// Attacker: a raw NATS connection (the dev harness leaves the data plane open),
+	// no identity, forged Sender, NO signature.
+	const spoofMsg = "I am totally the victim"
+	rawAtk, err := nats.Connect(h.natsURL)
+	if err != nil {
+		t.Fatalf("attacker raw connect: %v", err)
+	}
+	defer rawAtk.Close()
+	spoof := frame.Frame{
+		Type:    frame.PUB,
+		Subject: subject,
+		Sender:  "victim-forged-endpoint",
+		MsgID:   "spoof-1",
+		Epoch:   1,
+		Payload: []byte(spoofMsg),
+		// Sig intentionally nil — this is the attack.
+	}
+	sb, err := spoof.Marshal()
+	if err != nil {
+		t.Fatalf("marshal spoof: %v", err)
+	}
+	if err := rawAtk.Publish(subject, sb); err != nil {
+		t.Fatalf("attacker publish: %v", err)
+	}
+	_ = rawAtk.Flush()
+
+	// Golden: alice's properly signed frame must be delivered.
+	const goodMsg = "authentic from alice"
+	if err := alice.Publish(roomID, []byte(goodMsg)); err != nil {
+		t.Fatalf("alice publish: %v", err)
+	}
+	if !waitFor(&mu, &got, func(rs []string) bool {
+		for _, r := range rs {
+			if r == goodMsg {
+				return true
+			}
+		}
+		return false
+	}, 2*time.Second) {
+		t.Fatalf("a properly signed frame should be delivered; got %v", snapshot(&mu, &got))
+	}
+
+	// Error path: the unsigned, forged frame must NEVER reach the handler.
+	for _, r := range snapshot(&mu, &got) {
+		if r == spoofMsg {
+			t.Fatalf("SIG-NIL SPOOF: receiver accepted an unsigned frame with a forged Sender in a SignMsgs room")
+		}
+	}
+
+	// Edge: a room WITHOUT SignMsgs still delivers an unsigned raw frame, proving
+	// the drop is scoped to signed rooms and did not break the plain-NATS path.
+	const subjectOpen = "room.open.nosig"
+	openRoom, err := alice.CreateRoom(subjectOpen, room.ModeNATS)
+	if err != nil {
+		t.Fatalf("alice create open room: %v", err)
+	}
+	openCol := subscribeCollect(t, alice, openRoom)
+	defer openCol.sub.Unsubscribe()
+	time.Sleep(150 * time.Millisecond)
+
+	const openMsg = "unsigned but allowed here"
+	openFrame := frame.Frame{
+		Type:    frame.PUB,
+		Subject: subjectOpen,
+		Sender:  "anyone",
+		MsgID:   "open-1",
+		Payload: []byte(openMsg),
+		// no Sig — fine in a non-signed room
+	}
+	ob, _ := openFrame.Marshal()
+	if err := rawAtk.Publish(subjectOpen, ob); err != nil {
+		t.Fatalf("publish open frame: %v", err)
+	}
+	_ = rawAtk.Flush()
+	if !waitFor(&openCol.mu, &openCol.msgs, func(rs []string) bool {
+		for _, r := range rs {
+			if r == openMsg {
+				return true
+			}
+		}
+		return false
+	}, 2*time.Second) {
+		t.Fatalf("an unsigned frame in a non-signed room should be delivered; got %v", snapshot(&openCol.mu, &openCol.msgs))
+	}
+}