diff --git a/spec/unit/rust-crypto/rust-crypto.spec.ts b/spec/unit/rust-crypto/rust-crypto.spec.ts
index 36387e6cfc..48be04ea37 100644
--- a/spec/unit/rust-crypto/rust-crypto.spec.ts
+++ b/spec/unit/rust-crypto/rust-crypto.spec.ts
@@ -77,6 +77,7 @@ import encryptAESSecretStorageItem from "../../../src/utils/encryptAESSecretStor
 import { type CryptoStore, type SecretStorePrivateKeys } from "../../../src/crypto/store/base";
 import { CryptoEvent } from "../../../src/crypto-api/index.ts";
 import { RustBackupManager } from "../../../src/rust-crypto/backup.ts";
+import { flushPromises } from "../../test-utils/flushPromises.ts";
 
 const TEST_USER = "@alice:example.com";
 const TEST_DEVICE_ID = "TEST_DEVICE";
@@ -1139,6 +1140,11 @@ describe("RustCrypto", () => {
                 RustSdkCryptoJs.ShieldStateCode.VerificationViolation,
                 EventShieldReason.VERIFICATION_VIOLATION,
             ],
+            [
+                "Mismatched sender",
+                RustSdkCryptoJs.ShieldStateCode.MismatchedSender,
+                EventShieldReason.MISMATCHED_SENDER,
+            ],
         ])("gets the right shield reason (%s)", async (rustReason, rustCode, expectedReason) => {
             // suppress the warning from the unknown shield reason
             jest.spyOn(console, "warn").mockImplementation(() => {});
@@ -2302,6 +2308,8 @@ describe("RustCrypto", () => {
             });
 
             const rustCrypto = await makeTestRustCrypto(makeMatrixHttpApi(), undefined, undefined, secretStorage);
+            await flushPromises();
+
             // We have a key backup
             expect(await rustCrypto.getActiveSessionBackupVersion()).not.toBeNull();
 
diff --git a/src/crypto-api/index.ts b/src/crypto-api/index.ts
index 173ff31764..a7acbf90af 100644
--- a/src/crypto-api/index.ts
+++ b/src/crypto-api/index.ts
@@ -1355,6 +1355,12 @@ export enum EventShieldReason {
      * The sender was previously verified but changed their identity.
      */
     VERIFICATION_VIOLATION,
+
+    /**
+     * The `sender` field on the event does not match the owner of the device
+     * that established the Megolm session.
+     */
+    MISMATCHED_SENDER,
 }
 
 /** The result of a call to {@link CryptoApi.getOwnDeviceKeys} */
diff --git a/src/rust-crypto/rust-crypto.ts b/src/rust-crypto/rust-crypto.ts
index 9cb8522d38..863a0e8e93 100644
--- a/src/rust-crypto/rust-crypto.ts
+++ b/src/rust-crypto/rust-crypto.ts
@@ -2277,6 +2277,12 @@ function rustEncryptionInfoToJsEncryptionInfo(
         case RustSdkCryptoJs.ShieldStateCode.VerificationViolation:
             shieldReason = EventShieldReason.VERIFICATION_VIOLATION;
             break;
+        case RustSdkCryptoJs.ShieldStateCode.MismatchedSender:
+            shieldReason = EventShieldReason.MISMATCHED_SENDER;
+            break;
+        default:
+            shieldReason = EventShieldReason.UNKNOWN;
+            break;
     }
 
     return { shieldColour, shieldReason };