diff --git a/packages/core/CHANGELOG.md b/packages/core/CHANGELOG.md
index 17b077b..4d4d8d4 100644
--- a/packages/core/CHANGELOG.md
+++ b/packages/core/CHANGELOG.md
@@ -18,6 +18,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Prevent nonce poisoning by deferring nonce persistence until after successful decryption ([#69](https://github.com/MetaMask/mobile-wallet-protocol/pull/69))
+- Guard against `NaN` in nonce storage ([#69](https://github.com/MetaMask/mobile-wallet-protocol/pull/69))
 - Fix `SessionStore` race conditions and fire-and-forget garbage collection ([#71](https://github.com/MetaMask/mobile-wallet-protocol/pull/71))
 - Guard against `NaN` in session expiry timestamps ([#70](https://github.com/MetaMask/mobile-wallet-protocol/pull/70))
 
diff --git a/packages/core/src/base-client.ts b/packages/core/src/base-client.ts
index ede243f..b6a0ee0 100644
--- a/packages/core/src/base-client.ts
+++ b/packages/core/src/base-client.ts
@@ -46,7 +46,12 @@ export abstract class BaseClient extends EventEmitter {
 		this.transport.on("message", async (payload) => {
 			if (!this.session?.keyPair.privateKey) return;
 			const message = await this.decryptMessage(payload.data);
-			if (message) this.handleMessage(message);
+			if (message) {
+				// Confirm the nonce only after successful decryption to prevent
+				// attackers from poisoning the nonce tracker with invalid messages.
+				await payload.confirmNonce?.();
+				this.handleMessage(message);
+			}
 		});
 	}
 
diff --git a/packages/core/src/domain/transport.ts b/packages/core/src/domain/transport.ts
index b295b4c..47395aa 100644
--- a/packages/core/src/domain/transport.ts
+++ b/packages/core/src/domain/transport.ts
@@ -40,7 +40,7 @@ export interface ITransport {
 	 * @param event The name of the event to listen for.
 	 * @param handler The callback function to execute.
 	 */
-	on(event: "message", handler: (payload: { channel: string; data: string }) => void): void;
+	on(event: "message", handler: (payload: { channel: string; data: string; confirmNonce?: () => Promise<void> }) => void): void;
 	on(event: "connecting" | "connected" | "disconnected", handler: () => void): void;
 	on(event: "error", handler: (error: Error) => void): void;
 
diff --git a/packages/core/src/transport/websocket/index.integration.test.ts b/packages/core/src/transport/websocket/index.integration.test.ts
index b063e95..293b3d3 100644
--- a/packages/core/src/transport/websocket/index.integration.test.ts
+++ b/packages/core/src/transport/websocket/index.integration.test.ts
@@ -189,7 +189,8 @@ t.describe.each(testModes)("WebSocketTransport with $name", ({ useSharedConnecti
 			const received = await messagePromise;
 
 			// The subscriber transport should unwrap the envelope and emit the original payload.
-			t.expect(received).toEqual({ channel, data: payload });
+			t.expect(received).toMatchObject({ channel, data: payload });
+			t.expect(received.confirmNonce).toBeTypeOf("function");
 
 			await publisher.disconnect();
 		});
@@ -431,16 +432,18 @@ t.describe.each(testModes)("WebSocketTransport with $name", ({ useSharedConnecti
 			const messagePayload = "dedup-test-message";
 			let messageCount = 0;
 
-			subscriber.on("message", ({ data }) => {
+			subscriber.on("message", ({ data, confirmNonce }) => {
 				if (data === messagePayload) {
 					messageCount++;
+					confirmNonce?.();
 				}
 			});
 
 			// Send the message once using normal publish
 			const firstMessagePromise = waitFor(subscriber, "message");
 			await rawPublisher.publish(channel, messagePayload);
-			await firstMessagePromise;
+			const firstMsg = await firstMessagePromise;
+			await firstMsg.confirmNonce?.();
 			t.expect(messageCount).toBe(1);
 
 			// Create the exact same message envelope that was sent
@@ -791,8 +794,11 @@ t.describe.each(testModes)("WebSocketTransport with $name", ({ useSharedConnecti
 				await publisher.publish(channel, `message-${i}`);
 			}
 
-			// Wait for all messages to be received
-			await Promise.all(messagePromises);
+			// Wait for all messages to be received and confirm nonces so they persist
+			const receivedMsgs = await Promise.all(messagePromises);
+			for (const msg of receivedMsgs) {
+				await msg.confirmNonce?.();
+			}
 
 			// Verify storage has accumulated data
 			const storage = (transport as any).storage;
diff --git a/packages/core/src/transport/websocket/index.ts b/packages/core/src/transport/websocket/index.ts
index 8e7be2d..799a7a9 100644
--- a/packages/core/src/transport/websocket/index.ts
+++ b/packages/core/src/transport/websocket/index.ts
@@ -51,6 +51,19 @@ type TransportState = "disconnected" | "connecting" | "connected";
 
 /** The maximum number of messages to fetch from history upon a new subscription. */
 const HISTORY_FETCH_LIMIT = 50;
+/**
+ * Maximum allowed nonce jump from a known sender. Messages with a nonce that
+ * jumps more than this from the last confirmed nonce are rejected as suspicious.
+ * Does not apply to the first message from a new sender (baseline is 0). This
+ * is safe because a spoofed first message would fail decryption and its nonce
+ * would never be confirmed to storage.
+ *
+ * Trade-off: if the receiver goes offline and misses more than this many
+ * messages from a known sender, legitimate messages will be permanently
+ * blocked. In practice this is unlikely given low message rates in MWP
+ * sessions, but worth noting.
+ */
+const MAX_NONCE_JUMP = 100;
 /** The maximum number of retry attempts for publishing a message. */
 const MAX_RETRY_ATTEMPTS = 5;
 /** The base delay in milliseconds for exponential backoff between publish retries. */
@@ -65,6 +78,7 @@ export class WebSocketTransport extends EventEmitter implements ITransport {
 	private readonly centrifuge: Centrifuge | SharedCentrifuge;
 	private readonly storage: WebSocketTransportStorage;
 	private readonly queue: QueuedItem[] = [];
+	private readonly pendingNonces = new Map<string, Set<number>>();
 	private isProcessingQueue = false;
 	private state: TransportState = "disconnected";
 
@@ -214,6 +228,9 @@ export class WebSocketTransport extends EventEmitter implements ITransport {
 	 */
 	public async clear(channel: string): Promise<void> {
 		await this.storage.clear(channel);
+		for (const key of this.pendingNonces.keys()) {
+			if (key.startsWith(`${channel}:`)) this.pendingNonces.delete(key);
+		}
 		const sub = this.centrifuge.getSubscription(channel);
 		if (sub) this.centrifuge.removeSubscription(sub as Subscription);
 	}
@@ -229,6 +246,11 @@ export class WebSocketTransport extends EventEmitter implements ITransport {
 
 	/**
 	 * Parses an incoming raw message, checks for duplicates, and emits it.
+	 *
+	 * The nonce is checked for deduplication but NOT persisted here. The emitted
+	 * payload includes a `confirmNonce` callback that the consumer (BaseClient)
+	 * must call after successful decryption. This prevents an attacker from
+	 * poisoning the nonce tracker with high-nonce messages that fail decryption.
 	 */
 	private async _handleIncomingMessage(channel: string, rawData: string): Promise<void> {
 		try {
@@ -246,13 +268,44 @@ export class WebSocketTransport extends EventEmitter implements ITransport {
 			const latestNonces = await this.storage.getLatestNonces(channel);
 			const latestNonce = latestNonces.get(message.clientId) || 0;
 
-			if (message.nonce > latestNonce) {
-				// This is a new message, update the latest nonce and emit the message.
-				latestNonces.set(message.clientId, message.nonce);
-				await this.storage.setLatestNonces(channel, latestNonces);
-				this.emit("message", { channel, data: message.payload });
+			if (message.nonce <= latestNonce) {
+				return;
 			}
-			// If message.nonce <= latestNonce, it's a duplicate and we ignore it.
+
+			// Reject suspiciously large nonce jumps (but allow first message from a new sender).
+			if (latestNonce > 0 && message.nonce - latestNonce > MAX_NONCE_JUMP) {
+				this.emit("error", new TransportError(ErrorCode.TRANSPORT_PARSE_FAILED, `Nonce jump too large: ${latestNonce} -> ${message.nonce}`));
+				return;
+			}
+
+			// Guard against duplicate processing between emit and confirm.
+			// Without this, a message arriving via both live publication and
+			// _fetchHistory could pass the storage-based dedup check twice
+			// because the nonce hasn't been confirmed yet.
+			const pendingKey = `${channel}:${message.clientId}`;
+			const pending = this.pendingNonces.get(pendingKey);
+			if (pending?.has(message.nonce)) {
+				return;
+			}
+			if (!pending) {
+				this.pendingNonces.set(pendingKey, new Set([message.nonce]));
+			} else {
+				pending.add(message.nonce);
+			}
+
+			const confirmNonce = async () => {
+				try {
+					await this.storage.confirmNonce(channel, message.clientId, message.nonce);
+				} catch (error) {
+					this.emit("error", new TransportError(ErrorCode.UNKNOWN, `Failed to confirm nonce: ${error instanceof Error ? error.message : String(error)}`));
+				}
+				const p = this.pendingNonces.get(pendingKey);
+				if (p) {
+					p.delete(message.nonce);
+					if (p.size === 0) this.pendingNonces.delete(pendingKey);
+				}
+			};
+			this.emit("message", { channel, data: message.payload, confirmNonce });
 		} catch (error) {
 			this.emit("error", new TransportError(ErrorCode.TRANSPORT_PARSE_FAILED, `Failed to parse incoming message: ${error instanceof Error ? error.message : "Unknown error"}`));
 		}
diff --git a/packages/core/src/transport/websocket/store.test.ts b/packages/core/src/transport/websocket/store.test.ts
index 871e1a8..a58bcbe 100644
--- a/packages/core/src/transport/websocket/store.test.ts
+++ b/packages/core/src/transport/websocket/store.test.ts
@@ -116,6 +116,52 @@ t.describe("WebSocketTransportStorage", () => {
 			t.expect(nextNonce).toBe(6);
 			t.expect(await kvstore.get(newStorage.getNonceKey(channel))).toBe("6");
 		});
+
+		t.test("should recover from NaN nonce value in storage", async () => {
+			const channel = "session:nan-channel";
+
+			// Corrupt the stored nonce value
+			await kvstore.set(storage.getNonceKey(channel), "not-a-number");
+
+			const nonce = await storage.getNextNonce(channel);
+			t.expect(nonce).toBe(1);
+			t.expect(await kvstore.get(storage.getNonceKey(channel))).toBe("1");
+		});
+	});
+
+	t.describe("Nonce Confirmation", () => {
+		let storage: WebSocketTransportStorage;
+
+		t.beforeEach(async () => {
+			storage = await WebSocketTransportStorage.create(kvstore);
+		});
+
+		t.test("should save nonce via confirmNonce", async () => {
+			const channel = "session:confirm-channel";
+			await storage.confirmNonce(channel, "sender-1", 5);
+
+			const nonces = await storage.getLatestNonces(channel);
+			t.expect(nonces.get("sender-1")).toBe(5);
+		});
+
+		t.test("should not regress nonce on confirmNonce with lower value", async () => {
+			const channel = "session:confirm-channel";
+			await storage.confirmNonce(channel, "sender-1", 10);
+			await storage.confirmNonce(channel, "sender-1", 3);
+
+			const nonces = await storage.getLatestNonces(channel);
+			t.expect(nonces.get("sender-1")).toBe(10);
+		});
+
+		t.test("should track nonces independently per sender", async () => {
+			const channel = "session:confirm-channel";
+			await storage.confirmNonce(channel, "sender-1", 5);
+			await storage.confirmNonce(channel, "sender-2", 8);
+
+			const nonces = await storage.getLatestNonces(channel);
+			t.expect(nonces.get("sender-1")).toBe(5);
+			t.expect(nonces.get("sender-2")).toBe(8);
+		});
 	});
 
 	t.describe("Latest Nonces Management", () => {
diff --git a/packages/core/src/transport/websocket/store.ts b/packages/core/src/transport/websocket/store.ts
index a7a7263..10362e9 100644
--- a/packages/core/src/transport/websocket/store.ts
+++ b/packages/core/src/transport/websocket/store.ts
@@ -1,3 +1,4 @@
+import { Mutex } from "async-mutex";
 import { v4 as uuid } from "uuid";
 import type { IKVStore } from "../../domain/kv-store";
 
@@ -8,6 +9,7 @@ import type { IKVStore } from "../../domain/kv-store";
 export class WebSocketTransportStorage {
 	private readonly kvstore: IKVStore;
 	private readonly clientId: string;
+	private readonly nonceMutex = new Mutex();
 
 	/**
 	 * Creates a new WebSocketTransportStorage instance with a persistent client ID.
@@ -42,12 +44,28 @@ export class WebSocketTransportStorage {
 	async getNextNonce(channel: string): Promise<number> {
 		const key = this.getNonceKey(channel);
 		const value = await this.kvstore.get(key);
-		const currentNonce = value ? parseInt(value, 10) : 0;
+		let currentNonce = value ? parseInt(value, 10) : 0;
+		if (Number.isNaN(currentNonce)) currentNonce = 0;
 		const nextNonce = currentNonce + 1;
 		await this.kvstore.set(key, nextNonce.toString());
 		return nextNonce;
 	}
 
+	/**
+	 * Confirms a received nonce after the message has been successfully processed
+	 * (e.g., decrypted). Only updates if the nonce is higher than the current value.
+	 */
+	async confirmNonce(channel: string, clientId: string, nonce: number): Promise<void> {
+		await this.nonceMutex.runExclusive(async () => {
+			const latestNonces = await this.getLatestNonces(channel);
+			const current = latestNonces.get(clientId) || 0;
+			if (nonce > current) {
+				latestNonces.set(clientId, nonce);
+				await this.setLatestNonces(channel, latestNonces);
+			}
+		});
+	}
+
 	/**
 	 * Retrieves the latest received nonces from all senders on the specified channel.
 	 * Used for message deduplication - only messages with nonces greater than the