diff --git a/apps/browser/src/vault/fido2/content/page-script.ts b/apps/browser/src/vault/fido2/content/page-script.ts
index 42d695b235b..1194e0a4e4c 100644
--- a/apps/browser/src/vault/fido2/content/page-script.ts
+++ b/apps/browser/src/vault/fido2/content/page-script.ts
@@ -54,6 +54,14 @@ const browserCredentials = {
 
 const messenger = Messenger.forDOMCommunication(window);
 
+function isSameOriginWithAncestors() {
+  try {
+    return window.self === window.top;
+  } catch {
+    return false;
+  }
+}
+
 navigator.credentials.create = async (
   options?: CredentialCreationOptions,
   abortController?: AbortController
@@ -64,14 +72,15 @@ navigator.credentials.create = async (
     (options?.publicKey?.authenticatorSelection.authenticatorAttachment !== "platform" &&
       browserNativeWebauthnSupport);
   try {
+    const isNotIframe = isSameOriginWithAncestors();
+
     const response = await messenger.request(
       {
         type: MessageType.CredentialCreationRequest,
-        // TODO: Fix sameOriginWithAncestors!
         data: WebauthnUtils.mapCredentialCreationOptions(
           options,
           window.location.origin,
-          true,
+          isNotIframe,
           fallbackSupported
         ),
       },
@@ -99,6 +108,8 @@ navigator.credentials.get = async (
   const fallbackSupported = browserNativeWebauthnSupport;
 
   try {
+    const isNotIframe = isSameOriginWithAncestors();
+
     if (options?.mediation && options.mediation !== "optional") {
       throw new FallbackRequestedError();
     }
@@ -106,11 +117,10 @@ navigator.credentials.get = async (
     const response = await messenger.request(
       {
         type: MessageType.CredentialGetRequest,
-        // TODO: Fix sameOriginWithAncestors!
         data: WebauthnUtils.mapCredentialRequestOptions(
           options,
           window.location.origin,
-          true,
+          isNotIframe,
           fallbackSupported
         ),
       },
diff --git a/libs/common/src/vault/services/fido2/fido2-client.service.spec.ts b/libs/common/src/vault/services/fido2/fido2-client.service.spec.ts
index 8f8141df9c7..a19afa6aaa1 100644
--- a/libs/common/src/vault/services/fido2/fido2-client.service.spec.ts
+++ b/libs/common/src/vault/services/fido2/fido2-client.service.spec.ts
@@ -342,6 +342,18 @@ describe("FidoAuthenticatorService", () => {
         const rejects = expect(result).rejects;
         await rejects.toThrow(FallbackRequestedError);
       });
+
+      // Spec: If sameOriginWithAncestors is false, return a "NotAllowedError" DOMException.
+      it("should throw error if sameOriginWithAncestors is false", async () => {
+        const params = createParams();
+        params.sameOriginWithAncestors = false; // Simulating the falsey value
+
+        const result = async () => await client.assertCredential(params);
+
+        const rejects = expect(result).rejects;
+        await rejects.toMatchObject({ name: "NotAllowedError" });
+        await rejects.toBeInstanceOf(DOMException);
+      });
     });
 
     describe("assert non-discoverable credential", () => {
diff --git a/libs/common/src/vault/services/fido2/fido2-client.service.ts b/libs/common/src/vault/services/fido2/fido2-client.service.ts
index 4733010944a..55a62949465 100644
--- a/libs/common/src/vault/services/fido2/fido2-client.service.ts
+++ b/libs/common/src/vault/services/fido2/fido2-client.service.ts
@@ -200,6 +200,13 @@ export class Fido2ClientService implements Fido2ClientServiceAbstraction {
       throw new FallbackRequestedError();
     }
 
+    if (!params.sameOriginWithAncestors) {
+      this.logService?.warning(
+        `[Fido2Client] Invalid 'sameOriginWithAncestors' value: ${params.sameOriginWithAncestors}`
+      );
+      throw new DOMException("Invalid 'sameOriginWithAncestors' value", "NotAllowedError");
+    }
+
     const { domain: effectiveDomain } = parse(params.origin, { allowPrivateDomains: true });
     if (effectiveDomain == undefined) {
       this.logService?.warning(`[Fido2Client] Invalid origin: ${params.origin}`);