diff --git a/packages/server/src/authentication/verifyAuthenticationResponse.test.ts b/packages/server/src/authentication/verifyAuthenticationResponse.test.ts
index f74fe579..b52c2e01 100644
--- a/packages/server/src/authentication/verifyAuthenticationResponse.test.ts
+++ b/packages/server/src/authentication/verifyAuthenticationResponse.test.ts
@@ -578,6 +578,266 @@ Deno.test('should return user verified flag after successful auth', async () =>
   assertFalse(verification.authenticationInfo?.userVerified);
 });
 
+Deno.test('should verify when crossOrigin is true, topOrigin is missing, and expectedTopOrigin is not specified (Safari workaround)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+      },
+    ]),
+  );
+
+  try {
+    const verification = await verifyAuthenticationResponse({
+      response: assertionResponse,
+      expectedChallenge: assertionChallenge,
+      expectedOrigin: assertionOrigin,
+      expectedRPID: 'dev.dontneeda.pw',
+      credential,
+      requireUserVerification: false,
+    });
+
+    assertEquals(verification.verified, true);
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should verify when crossOrigin is true, topOrigin is missing, but expectedTopOrigin is specified (Safari workaround)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+      },
+    ]),
+  );
+
+  try {
+    const verification = await verifyAuthenticationResponse({
+      response: assertionResponse,
+      expectedChallenge: assertionChallenge,
+      expectedOrigin: assertionOrigin,
+      expectedRPID: 'dev.dontneeda.pw',
+      expectedTopOrigin: 'https://top.origin.com',
+      credential,
+      requireUserVerification: false,
+    });
+
+    assertEquals(verification.verified, true);
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should verify when crossOrigin is true and topOrigin matches expectedTopOrigin (string)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+        topOrigin: 'https://top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    const verification = await verifyAuthenticationResponse({
+      response: assertionResponse,
+      expectedChallenge: assertionChallenge,
+      expectedOrigin: assertionOrigin,
+      expectedRPID: 'dev.dontneeda.pw',
+      expectedTopOrigin: 'https://top.origin.com',
+      credential,
+      requireUserVerification: false,
+    });
+
+    assertEquals(verification.verified, true);
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should throw when crossOrigin is true and topOrigin does not match expectedTopOrigin (string)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+        topOrigin: 'https://wrong.top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    await assertRejects(
+      () =>
+        verifyAuthenticationResponse({
+          response: assertionResponse,
+          expectedChallenge: assertionChallenge,
+          expectedOrigin: assertionOrigin,
+          expectedRPID: 'dev.dontneeda.pw',
+          expectedTopOrigin: 'https://top.origin.com',
+          credential,
+        }),
+      Error,
+      'Unexpected cross-origin authentication response top origin of "https://wrong.top.origin.com", expected: https://top.origin.com',
+    );
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should verify when crossOrigin is true and topOrigin matches one of expectedTopOrigin (array)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+        topOrigin: 'https://top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    const verification = await verifyAuthenticationResponse({
+      response: assertionResponse,
+      expectedChallenge: assertionChallenge,
+      expectedOrigin: assertionOrigin,
+      expectedRPID: 'dev.dontneeda.pw',
+      expectedTopOrigin: ['https://other.origin.com', 'https://top.origin.com'],
+      credential,
+      requireUserVerification: false,
+    });
+
+    assertEquals(verification.verified, true);
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should throw when crossOrigin is true and topOrigin does not match any of expectedTopOrigin (array)', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+        topOrigin: 'https://wrong.top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    await assertRejects(
+      () =>
+        verifyAuthenticationResponse({
+          response: assertionResponse,
+          expectedChallenge: assertionChallenge,
+          expectedOrigin: assertionOrigin,
+          expectedRPID: 'dev.dontneeda.pw',
+          expectedTopOrigin: ['https://top.origin.com', 'https://other.origin.com'],
+          credential,
+        }),
+      Error,
+      'Unexpected cross-origin authentication response top origin of "https://wrong.top.origin.com", expected one of: https://top.origin.com, https://other.origin.com',
+    );
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should throw when crossOrigin is true but expectedTopOrigin is not specified', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: true,
+        topOrigin: 'https://top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    await assertRejects(
+      () =>
+        verifyAuthenticationResponse({
+          response: assertionResponse,
+          expectedChallenge: assertionChallenge,
+          expectedOrigin: assertionOrigin,
+          expectedRPID: 'dev.dontneeda.pw',
+          credential,
+        }),
+      Error,
+      'Detected cross-origin authentication response from top origin of "https://top.origin.com", but a value for `expectedTopOrigin` was not specified when calling `verifyAuthenticationResponse()`',
+    );
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
+Deno.test('should throw when topOrigin is set despite crossOrigin being false', async () => {
+  const mockDecodeClientData = stub(
+    _decodeClientDataJSONInternals,
+    'stubThis',
+    returnsNext([
+      {
+        type: 'webauthn.get',
+        origin: assertionOrigin,
+        challenge: assertionChallenge,
+        crossOrigin: false,
+        topOrigin: 'https://some.top.origin.com',
+      },
+    ]),
+  );
+
+  try {
+    await assertRejects(
+      () =>
+        verifyAuthenticationResponse({
+          response: assertionResponse,
+          expectedChallenge: assertionChallenge,
+          expectedOrigin: assertionOrigin,
+          expectedRPID: 'dev.dontneeda.pw',
+          credential,
+          requireUserVerification: false,
+        }),
+      Error,
+      'Unexpected top origin of "https://some.top.origin.com" within a non-cross-origin authentication response. This error should be reported to the browser vendor as a WebAuthn specification violation with a link to https://w3c.github.io/webauthn/#dom-collectedclientdata-toporigin',
+    );
+  } finally {
+    mockDecodeClientData.restore();
+  }
+});
+
 /**
  * Assertion examples below
  */
diff --git a/packages/server/src/authentication/verifyAuthenticationResponse.ts b/packages/server/src/authentication/verifyAuthenticationResponse.ts
index 86fce0ef..97e80263 100644
--- a/packages/server/src/authentication/verifyAuthenticationResponse.ts
+++ b/packages/server/src/authentication/verifyAuthenticationResponse.ts
@@ -42,6 +42,7 @@ export async function verifyAuthenticationResponse(
     expectedRPID: string | string[];
     credential: WebAuthnCredential;
     expectedType?: string | string[];
+    expectedTopOrigin?: string | string[];
     requireUserVerification?: boolean;
     advancedFIDOConfig?: {
       userVerification?: UserVerificationRequirement;
@@ -54,6 +55,7 @@ export async function verifyAuthenticationResponse(
     expectedOrigin,
     expectedRPID,
     expectedType,
+    expectedTopOrigin,
     credential,
     requireUserVerification = true,
     advancedFIDOConfig,
@@ -87,7 +89,7 @@ export async function verifyAuthenticationResponse(
 
   const clientDataJSON = decodeClientDataJSON(assertionResponse.clientDataJSON);
 
-  const { type, origin, challenge, tokenBinding } = clientDataJSON;
+  const { type, origin, challenge, tokenBinding, crossOrigin, topOrigin } = clientDataJSON;
 
   // Make sure we're handling an authentication
   if (Array.isArray(expectedType)) {
@@ -120,6 +122,50 @@ export async function verifyAuthenticationResponse(
     );
   }
 
+  // Check that the authentication response is within an expected iframe
+  if (crossOrigin) {
+    /**
+     * TODO: Since Safari doesn't support `topOrigin` as of May 2026, only check this when
+     * `topOrigin` is available for now.
+     */
+    if (topOrigin) {
+      if (!expectedTopOrigin) {
+        /**
+         * If `expectedTopOrigin` is not set, this should be considered an unexpected cross-origin
+         * request. Reject the response while helping the RP understand how they need to update this
+         * method call if they want to support verification of such WebAuthn authentication requests.
+         */
+        throw new Error(
+          `Detected cross-origin authentication response from top origin of "${topOrigin}", but a value for \`expectedTopOrigin\` was not specified when calling \`verifyAuthenticationResponse()\``,
+        );
+      }
+      
+      if (Array.isArray(expectedTopOrigin)) {
+        if (!expectedTopOrigin.includes(topOrigin)) {
+          const joinedExpectedTopOrigin = expectedTopOrigin.join(', ');
+          throw new Error(
+            `Unexpected cross-origin authentication response top origin of "${topOrigin}", expected one of: ${joinedExpectedTopOrigin}`,
+          );
+        }
+      } else {
+        if (topOrigin !== expectedTopOrigin) {
+          throw new Error(
+            `Unexpected cross-origin authentication response top origin of "${topOrigin}", expected: ${expectedTopOrigin}`,
+          );
+        }
+      }
+    }
+  } else {
+    if (topOrigin) {
+      /**
+       * If `topOrigin` is set despite `crossOrigin` being false, this is an unexpected request.
+       * 
+       * See https://w3c.github.io/webauthn/#dom-collectedclientdata-toporigin.
+       */
+      throw new Error(`Unexpected top origin of "${topOrigin}" within a non-cross-origin authentication response. This error should be reported to the browser vendor as a WebAuthn specification violation with a link to https://w3c.github.io/webauthn/#dom-collectedclientdata-toporigin`);
+    }
+  }
+
   // Check that the origin is our site
   if (Array.isArray(expectedOrigin)) {
     if (!expectedOrigin.includes(origin)) {
diff --git a/packages/server/src/helpers/decodeClientDataJSON.test.ts b/packages/server/src/helpers/decodeClientDataJSON.test.ts
index eefecb0d..cd946995 100644
--- a/packages/server/src/helpers/decodeClientDataJSON.test.ts
+++ b/packages/server/src/helpers/decodeClientDataJSON.test.ts
@@ -15,3 +15,18 @@ Deno.test('should convert base64url-encoded attestation clientDataJSON to JSON',
     },
   );
 });
+
+Deno.test('should convert base64url-encoded clientDataJSON with crossOrigin and topOrigin to JSON', () => {
+  assertEquals(
+    decodeClientDataJSON(
+      'eyJ0eXBlIjoid2ViYXV0aG4uZ2V0IiwiY2hhbGxlbmdlIjoiY2hhbGxlbmdlIiwib3JpZ2luIjoiaHR0cHM6Ly9vcmlnaW4uY29tIiwiY3Jvc3NPcmlnaW4iOnRydWUsInRvcE9yaWdpbiI6Imh0dHBzOi8vdG9wLm9yaWdpbi5jb20ifQ',
+    ),
+    {
+      type: 'webauthn.get',
+      challenge: 'challenge',
+      origin: 'https://origin.com',
+      crossOrigin: true,
+      topOrigin: 'https://top.origin.com',
+    },
+  );
+});
diff --git a/packages/server/src/helpers/decodeClientDataJSON.ts b/packages/server/src/helpers/decodeClientDataJSON.ts
index 22fef212..171ae297 100644
--- a/packages/server/src/helpers/decodeClientDataJSON.ts
+++ b/packages/server/src/helpers/decodeClientDataJSON.ts
@@ -16,6 +16,7 @@ export type ClientDataJSON = {
   challenge: string;
   origin: string;
   crossOrigin?: boolean;
+  topOrigin?: string;
   tokenBinding?: {
     id?: string;
     status: 'present' | 'supported' | 'not-supported';