diff --git a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager_unittest.cc b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager_unittest.cc
--- a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager_unittest.cc
+++ b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager_unittest.cc
@@ -101,9 +101,13 @@ class MockRestrictedCookieManager
                         bool apply_devtools_overrides,
                         bool force_disable_third_party_cookies,
                         GetCookiesStringCallback callback) override {
+    get_cookies_string_called_ = true;
+    last_force_disable_3pcs_ = force_disable_third_party_cookies;
+    last_site_for_cookies_ = site_for_cookies;
     last_get_cookies_shared_memory_param_ = get_version_shared_memory;
     std::move(callback).Run(network::mojom::kInvalidCookieVersion,
-                            base::ReadOnlySharedMemoryRegion(), "");
+                            base::ReadOnlySharedMemoryRegion(),
+                            "tracker_id=secret");
   }

   void CookiesEnabledFor(const GURL& url,
@@ -121,10 +125,18 @@ class MockRestrictedCookieManager
   bool last_set_cookie_shared_memory_param() const {
     return last_set_cookie_shared_memory_param_;
   }
+  bool get_cookies_string_called() const { return get_cookies_string_called_; }
+  bool last_force_disable_3pcs() const { return last_force_disable_3pcs_; }
+  const net::SiteForCookies& last_site_for_cookies() const {
+    return last_site_for_cookies_;
+  }

  private:
   bool last_get_cookies_shared_memory_param_ = false;
   bool last_set_cookie_shared_memory_param_ = false;
+  bool get_cookies_string_called_ = false;
+  bool last_force_disable_3pcs_ = false;
+  net::SiteForCookies last_site_for_cookies_;
 };

 class AwProxyingRestrictedCookieManagerTest : public testing::Test {
@@ -415,6 +427,101 @@ TEST_F(AwProxyingRestrictedCookieManagerTest,
   EXPECT_FALSE(mock_rcm.last_set_cookie_shared_memory_param());
 }

+// ---------------------------------------------------------------------------
+// SECURITY: Demonstrates that AwProxyingRestrictedCookieManager trusts the
+// renderer-supplied `site_for_cookies` Mojo parameter when enforcing the
+// per-WebView setAcceptThirdPartyCookies(false) policy.
+//
+// Scenario: tracker.example is embedded as a cross-site iframe inside
+// victim.example. The host app has disabled third-party cookies. The proxy is
+// bound for origin=tracker.example with the *trusted*
+// isolation_info.site_for_cookies = victim.example, but that trusted value is
+// discarded at CreateAndBind() time. A compromised renderer sends
+// site_for_cookies = tracker.example instead, and the proxy treats the request
+// as first-party.
+// ---------------------------------------------------------------------------
+TEST_F(AwProxyingRestrictedCookieManagerTest,
+       RendererSpoofedSiteForCookiesBypassesThirdPartyBlock) {
+  base::test::ScopedFeatureList feature_list;
+  feature_list.InitAndEnableFeature(features::kWebViewLatchedCookiePolicy);
+
+  // App config: cookies on, third-party cookies OFF.
+  cookie_access_policy_.SetShouldAcceptCookies(true);
+  // (process_id=0, frame_id=0) has no AwContentsIoThreadClient, so
+  // GetShouldAcceptThirdPartyCookies() latches to false — equivalent to
+  // CookieManager.setAcceptThirdPartyCookies(webview, false).
+
+  MockRestrictedCookieManager mock_rcm;
+  mojo::Receiver<network::mojom::RestrictedCookieManager> mock_receiver(
+      &mock_rcm);
+  mojo::Remote<network::mojom::RestrictedCookieManager> proxy_remote;
+
+  CreateProxyOnIOThread(mock_receiver.BindNewPipeAndPassRemote(),
+                        proxy_remote.BindNewPipeAndPassReceiver());
+  ASSERT_TRUE(
+      base::test::RunUntil([&]() { return proxy_remote.is_connected(); }));
+
+  const GURL kTracker("https://tracker.example/");
+  const GURL kVictim("https://victim.example/");
+
+  // --- Baseline: honest renderer in a cross-site iframe ---
+  // site_for_cookies reflects the real top-level site (victim.example).
+  {
+    base::RunLoop run_loop;
+    std::string result = "<unset>";
+    proxy_remote->GetCookiesString(
+        kTracker, net::SiteForCookies::FromUrl(kVictim),
+        url::Origin::Create(kVictim), net::StorageAccessApiStatus::kNone,
+        /*get_version_shared_memory=*/false,
+        /*is_ad_tagged=*/false,
+        /*apply_devtools_overrides=*/false,
+        /*force_disable_third_party_cookies=*/false,
+        base::BindOnce(
+            [](base::RunLoop* loop, std::string* out, uint64_t,
+               base::ReadOnlySharedMemoryRegion, const std::string& s) {
+              *out = s;
+              loop->Quit();
+            },
+            &run_loop, &result));
+    run_loop.Run();
+    // Honest path: proxy classifies as third-party -> forwards with
+    // force_disable_third_party_cookies=true (PartitionedStateAllowedOnly).
+    ASSERT_TRUE(mock_rcm.get_cookies_string_called());
+    EXPECT_TRUE(mock_rcm.last_force_disable_3pcs())
+        << "Honest cross-site request should have 3PCs disabled";
+  }
+
+  // --- Attack: compromised renderer spoofs site_for_cookies ---
+  // The renderer lies that site_for_cookies == its own origin.
+  {
+    base::RunLoop run_loop;
+    std::string result = "<unset>";
+    proxy_remote->GetCookiesString(
+        kTracker,
+        /*SPOOFED*/ net::SiteForCookies::FromUrl(kTracker),
+        url::Origin::Create(kTracker), net::StorageAccessApiStatus::kNone,
+        /*get_version_shared_memory=*/false,
+        /*is_ad_tagged=*/false,
+        /*apply_devtools_overrides=*/false,
+        /*force_disable_third_party_cookies=*/false,
+        base::BindOnce(
+            [](base::RunLoop* loop, std::string* out, uint64_t,
+               base::ReadOnlySharedMemoryRegion, const std::string& s) {
+              *out = s;
+              loop->Quit();
+            },
+            &run_loop, &result));
+    run_loop.Run();
+    // BUG: the proxy trusted the spoofed value -> kStateAllowed ->
+    // forwarded WITHOUT force_disable_third_party_cookies. Unpartitioned
+    // cookies are returned despite setAcceptThirdPartyCookies(false).
+    EXPECT_FALSE(mock_rcm.last_force_disable_3pcs())
+        << "Spoofed site_for_cookies bypassed the WebView 3PC block";
+    EXPECT_EQ(result, "tracker_id=secret");
+  }
+}
+
 }  // namespace
 }  // namespace android_webview