diff --git a/src/node/handlers/session.rs b/src/node/handlers/session.rs
index 3bd6230..16dc005 100644
--- a/src/node/handlers/session.rs
+++ b/src/node/handlers/session.rs
@@ -835,6 +835,31 @@ impl Node {
                 return;
             }
 
+            // The rekey must come from the peer the session was established
+            // with. Compare x-only keys: the stored key's parity may be a
+            // synthesized even parity (npubs carry no parity), while the
+            // handshake learns the true point.
+            let rekey_pubkey = match handshake.remote_static() {
+                Some(pk) => *pk,
+                None => {
+                    debug!("No remote static key after processing rekey XK msg3");
+                    entry.abandon_rekey();
+                    self.sessions.insert(*src_addr, entry);
+                    return;
+                }
+            };
+            if rekey_pubkey.x_only_public_key().0 != entry.remote_pubkey().x_only_public_key().0 {
+                warn!(
+                    src = %self.peer_display_name(src_addr),
+                    "FSP rekey: initiator static key differs from the established peer key"
+                );
+                entry.abandon_rekey();
+                self.sessions.insert(*src_addr, entry);
+                self.stats_mut()
+                    .record_reject(RejectReason::Session(SessionReject::RekeyKeyMismatch));
+                return;
+            }
+
             // Complete the handshake → store as pending new session
             let session = match handshake.into_session() {
                 Ok(s) => s,
@@ -884,6 +909,21 @@ impl Node {
             }
         };
 
+        // The claimed source address must be derivable from the key we just
+        // authenticated, or the peer is opening a session under another
+        // node's address.
+        let derived_addr = NodeAddr::from_pubkey(&remote_pubkey.x_only_public_key().0);
+        if derived_addr != *src_addr {
+            warn!(
+                src = %self.peer_display_name(src_addr),
+                derived = %derived_addr,
+                "SessionMsg3 source address does not match the authenticated static key"
+            );
+            self.stats_mut()
+                .record_reject(RejectReason::Session(SessionReject::AddrMismatch));
+            return; // Entry was already removed
+        }
+
         // Register the initiator's identity for future TUN → session routing
         self.register_identity(*src_addr, remote_pubkey);
 
diff --git a/src/node/reject.rs b/src/node/reject.rs
index d2a44cf..89451f3 100644
--- a/src/node/reject.rs
+++ b/src/node/reject.rs
@@ -177,7 +177,8 @@ pub enum HandshakeReject {
 /// FSP session rejection reasons.
 ///
 /// `UnknownSession` and `BadState` cover the session unknown-session
-/// and state-machine cluster.
+/// and state-machine cluster. `AddrMismatch` and `RekeyKeyMismatch`
+/// cover the peer-identity binding checks on the XK msg3 receive path.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[non_exhaustive]
 pub enum SessionReject {
@@ -196,6 +197,18 @@ pub enum SessionReject {
     /// is not `AwaitingMsg3`. Tracked via
     /// [`SessionStats::bad_state`](crate::node::stats::SessionStats).
     BadState,
+    /// Inbound XK msg3 completed the handshake, but the initiator's
+    /// static key does not derive the source address the datagram
+    /// claimed — the peer is opening a session under another node's
+    /// address. Tracked via
+    /// [`SessionStats::addr_mismatch`](crate::node::stats::SessionStats).
+    AddrMismatch,
+    /// Inbound XK msg3 completed a responder-side rekey, but the
+    /// initiator's static key differs from the key the session was
+    /// established with — the rekey is not from the established peer.
+    /// Tracked via
+    /// [`SessionStats::rekey_key_mismatch`](crate::node::stats::SessionStats).
+    RekeyKeyMismatch,
 }
 
 /// MMP rejection reasons.
diff --git a/src/node/stats.rs b/src/node/stats.rs
index 5edfe5d..5d84ce8 100644
--- a/src/node/stats.rs
+++ b/src/node/stats.rs
@@ -32,6 +32,14 @@ pub struct SessionStats {
     /// before Established; SessionAck outside Initiating; SessionMsg3
     /// outside AwaitingMsg3).
     pub bad_state: u64,
+    /// Inbound XK msg3 whose initiator static key does not derive the
+    /// source address the datagram claimed. The half-open session is
+    /// dropped and no identity is registered.
+    pub addr_mismatch: u64,
+    /// Inbound rekey XK msg3 whose initiator static key differs from
+    /// the key the session was established with. The rekey is
+    /// abandoned and the existing session is left intact.
+    pub rekey_key_mismatch: u64,
 }
 
 impl SessionStats {
@@ -39,6 +47,8 @@ impl SessionStats {
         SessionStatsSnapshot {
             unknown_session: self.unknown_session,
             bad_state: self.bad_state,
+            addr_mismatch: self.addr_mismatch,
+            rekey_key_mismatch: self.rekey_key_mismatch,
         }
     }
 
@@ -46,6 +56,8 @@ impl SessionStats {
         match reason {
             SessionReject::UnknownSession => self.unknown_session += 1,
             SessionReject::BadState => self.bad_state += 1,
+            SessionReject::AddrMismatch => self.addr_mismatch += 1,
+            SessionReject::RekeyKeyMismatch => self.rekey_key_mismatch += 1,
         }
     }
 }
@@ -303,6 +315,8 @@ pub struct BloomStatsSnapshot {
 pub struct SessionStatsSnapshot {
     pub unknown_session: u64,
     pub bad_state: u64,
+    pub addr_mismatch: u64,
+    pub rekey_key_mismatch: u64,
 }
 
 #[derive(Clone, Debug, Default, Serialize)]
@@ -359,6 +373,35 @@ mod tests {
         assert_eq!(stats.unknown_session, 0);
     }
 
+    #[test]
+    fn session_stats_record_reject_addr_mismatch() {
+        let mut stats = SessionStats::default();
+        stats.record_reject(SessionReject::AddrMismatch);
+        stats.record_reject(SessionReject::AddrMismatch);
+        assert_eq!(stats.addr_mismatch, 2);
+        assert_eq!(stats.rekey_key_mismatch, 0);
+        assert_eq!(stats.unknown_session, 0);
+    }
+
+    #[test]
+    fn session_stats_record_reject_rekey_key_mismatch() {
+        let mut stats = SessionStats::default();
+        stats.record_reject(SessionReject::RekeyKeyMismatch);
+        assert_eq!(stats.rekey_key_mismatch, 1);
+        assert_eq!(stats.addr_mismatch, 0);
+    }
+
+    #[test]
+    fn session_stats_snapshot_carries_identity_binding_counters() {
+        let mut stats = SessionStats::default();
+        stats.record_reject(SessionReject::AddrMismatch);
+        stats.record_reject(SessionReject::RekeyKeyMismatch);
+        stats.record_reject(SessionReject::RekeyKeyMismatch);
+        let snap = stats.snapshot();
+        assert_eq!(snap.addr_mismatch, 1);
+        assert_eq!(snap.rekey_key_mismatch, 2);
+    }
+
     #[test]
     fn node_stats_record_reject_dispatches_to_session() {
         let mut stats = NodeStats::new();
diff --git a/src/node/tests/session.rs b/src/node/tests/session.rs
index 25d12c5..5f5b1cb 100644
--- a/src/node/tests/session.rs
+++ b/src/node/tests/session.rs
@@ -6,7 +6,7 @@ use crate::node::tests::spanning_tree::{
     TestNode, cleanup_nodes, generate_random_edges, lock_large_network_test,
     process_available_packets, run_tree_test, run_tree_test_with_mtus, verify_tree_convergence,
 };
-use crate::protocol::{SessionAck, SessionDatagram};
+use crate::protocol::{SessionAck, SessionDatagram, SessionMsg3};
 
 /// Populate all nodes' coordinate caches with each other's coords.
 ///
@@ -2391,3 +2391,226 @@ fn test_handle_path_mtu_notification_no_session_no_op() {
         "PathMtuNotification with no session must not touch path_mtu_lookup"
     );
 }
+
+// ============================================================================
+// Session identity binding: XK msg3 source address / static key
+// ============================================================================
+
+/// Helper: drive a full XK exchange against `responder_identity` and return
+/// the responder's half-completed handshake plus the initiator's msg3.
+///
+/// The msg3 is cryptographically valid for the responder and carries
+/// `initiator_identity`'s static key, which is exactly the shape of the
+/// defect: a peer that completes a real handshake while the datagram claims
+/// somebody else's source address.
+fn drive_xk_to_msg3(
+    initiator_identity: &Identity,
+    responder_identity: &Identity,
+) -> (crate::noise::HandshakeState, Vec<u8>) {
+    use crate::noise::HandshakeState;
+
+    let mut initiator = HandshakeState::new_xk_initiator(
+        initiator_identity.keypair(),
+        responder_identity.pubkey_full(),
+    );
+    let mut responder = HandshakeState::new_xk_responder(responder_identity.keypair());
+
+    let mut init_epoch = [0u8; 8];
+    rand::Rng::fill_bytes(&mut rand::rng(), &mut init_epoch);
+    initiator.set_local_epoch(init_epoch);
+    let mut resp_epoch = [0u8; 8];
+    rand::Rng::fill_bytes(&mut rand::rng(), &mut resp_epoch);
+    responder.set_local_epoch(resp_epoch);
+
+    let msg1 = initiator.write_xk_message_1().unwrap();
+    responder.read_xk_message_1(&msg1).unwrap();
+    let msg2 = responder.write_xk_message_2().unwrap();
+    initiator.read_xk_message_2(&msg2).unwrap();
+    let msg3 = initiator.write_xk_message_3().unwrap();
+
+    (responder, msg3)
+}
+
+/// Helper: generate an identity whose full public key has odd parity.
+fn generate_odd_parity_identity() -> Identity {
+    loop {
+        let id = Identity::generate();
+        if id.pubkey_full().serialize()[0] == 0x03 {
+            return id;
+        }
+    }
+}
+
+#[tokio::test]
+async fn test_session_msg3_rejects_spoofed_source_address() {
+    let mut node = make_node();
+    let attacker = Identity::generate();
+    let victim = Identity::generate();
+    let victim_addr = *victim.node_addr();
+
+    let (responder, msg3) = drive_xk_to_msg3(&attacker, node.identity());
+
+    // Half-open session recorded under the victim's address, as
+    // handle_session_setup would have done from the claimed source.
+    let entry = crate::node::session::SessionEntry::new(
+        victim_addr,
+        node.identity().pubkey_full(),
+        EndToEndState::AwaitingMsg3(responder),
+        1000,
+        false,
+    );
+    node.sessions.insert(victim_addr, entry);
+
+    node.handle_session_payload(&victim_addr, &SessionMsg3::new(msg3).encode(), 1280, false)
+        .await;
+
+    assert_eq!(
+        node.session_count(),
+        0,
+        "session must not be installed under an address the peer's key does not derive"
+    );
+    assert_eq!(
+        node.identity_cache_len(),
+        0,
+        "identity cache must not be poisoned with the spoofed address"
+    );
+    assert_eq!(node.stats().session.addr_mismatch, 1);
+}
+
+#[tokio::test]
+async fn test_session_msg3_accepts_matching_source_address() {
+    let mut node = make_node();
+    let peer = Identity::generate();
+    let peer_addr = *peer.node_addr();
+
+    let (responder, msg3) = drive_xk_to_msg3(&peer, node.identity());
+
+    let entry = crate::node::session::SessionEntry::new(
+        peer_addr,
+        node.identity().pubkey_full(),
+        EndToEndState::AwaitingMsg3(responder),
+        1000,
+        false,
+    );
+    node.sessions.insert(peer_addr, entry);
+
+    node.handle_session_payload(&peer_addr, &SessionMsg3::new(msg3).encode(), 1280, false)
+        .await;
+
+    assert!(
+        node.sessions
+            .get(&peer_addr)
+            .is_some_and(|e| e.is_established()),
+        "an honest initiator using its own address must still establish"
+    );
+    assert_eq!(node.identity_cache_len(), 1);
+    assert_eq!(node.stats().session.addr_mismatch, 0);
+}
+
+#[tokio::test]
+async fn test_rekey_msg3_rejects_different_static_key() {
+    let mut node = make_node();
+    let legit = Identity::generate();
+    let attacker = Identity::generate();
+    let peer_addr = *legit.node_addr();
+
+    let session = make_noise_session(node.identity(), &legit);
+    let mut entry = crate::node::session::SessionEntry::new(
+        peer_addr,
+        legit.pubkey_full(),
+        EndToEndState::Established(session),
+        1000,
+        true,
+    );
+    entry.mark_established(1000);
+
+    // Responder-side rekey armed, but driven by a different identity.
+    let (responder, msg3) = drive_xk_to_msg3(&attacker, node.identity());
+    entry.set_rekey_state(responder, false);
+    node.sessions.insert(peer_addr, entry);
+
+    node.handle_session_payload(&peer_addr, &SessionMsg3::new(msg3).encode(), 1280, false)
+        .await;
+
+    let entry = node
+        .sessions
+        .get(&peer_addr)
+        .expect("existing session must survive a spoofed rekey");
+    assert!(entry.is_established());
+    assert!(
+        entry.pending_new_session().is_none(),
+        "a rekey from a different static key must not become the pending session"
+    );
+    assert!(!entry.has_rekey_in_progress());
+    assert_eq!(node.stats().session.rekey_key_mismatch, 1);
+}
+
+#[tokio::test]
+async fn test_rekey_msg3_accepts_established_peer_key() {
+    let mut node = make_node();
+    let legit = Identity::generate();
+    let peer_addr = *legit.node_addr();
+
+    let session = make_noise_session(node.identity(), &legit);
+    let mut entry = crate::node::session::SessionEntry::new(
+        peer_addr,
+        legit.pubkey_full(),
+        EndToEndState::Established(session),
+        1000,
+        true,
+    );
+    entry.mark_established(1000);
+
+    let (responder, msg3) = drive_xk_to_msg3(&legit, node.identity());
+    entry.set_rekey_state(responder, false);
+    node.sessions.insert(peer_addr, entry);
+
+    node.handle_session_payload(&peer_addr, &SessionMsg3::new(msg3).encode(), 1280, false)
+        .await;
+
+    let entry = node.sessions.get(&peer_addr).expect("session present");
+    assert!(entry.pending_new_session().is_some());
+    assert_eq!(node.stats().session.rekey_key_mismatch, 0);
+}
+
+#[tokio::test]
+async fn test_rekey_msg3_accepts_odd_parity_peer_stored_as_even() {
+    let mut node = make_node();
+    let legit = generate_odd_parity_identity();
+    let peer_addr = *legit.node_addr();
+
+    // The stored key is the even-parity synthesis an npub-sourced peer
+    // identity produces; the peer's real key has odd parity. This must
+    // still be accepted, or every peer-initiated rekey against roughly
+    // half of all peers would be rejected.
+    let stored_pubkey = crate::identity::PeerIdentity::from_pubkey(legit.pubkey()).pubkey_full();
+    assert_ne!(
+        stored_pubkey,
+        legit.pubkey_full(),
+        "test fixture must actually differ in parity"
+    );
+
+    let session = make_noise_session(node.identity(), &legit);
+    let mut entry = crate::node::session::SessionEntry::new(
+        peer_addr,
+        stored_pubkey,
+        EndToEndState::Established(session),
+        1000,
+        true,
+    );
+    entry.mark_established(1000);
+
+    let (responder, msg3) = drive_xk_to_msg3(&legit, node.identity());
+    entry.set_rekey_state(responder, false);
+    node.sessions.insert(peer_addr, entry);
+
+    node.handle_session_payload(&peer_addr, &SessionMsg3::new(msg3).encode(), 1280, false)
+        .await;
+
+    let entry = node.sessions.get(&peer_addr).expect("session present");
+    assert!(
+        entry.pending_new_session().is_some(),
+        "a parity-normalized stored key must not reject a legitimate rekey"
+    );
+    assert_eq!(node.stats().session.rekey_key_mismatch, 0);
+}