diff --git a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.h b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.h
--- a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.h
+++ b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.h
@@ -14,6 +14,7 @@
 #include "content/public/browser/global_routing_id.h"
 #include "mojo/public/cpp/bindings/remote.h"
 #include "net/base/network_delegate.h"
+#include "net/cookies/site_for_cookies.h"
 #include "net/storage_access_api/status.h"
 #include "services/network/public/mojom/restricted_cookie_manager.mojom.h"

@@ -42,6 +43,7 @@ class AwProxyingRestrictedCookieManager
       bool is_service_worker,
       int process_id,
       int frame_id,
+      const net::SiteForCookies& site_for_cookies,
       mojo::PendingReceiver<network::mojom::RestrictedCookieManager> receiver,
       AwCookieAccessPolicy* aw_cookie_access_policy);

@@ -119,6 +121,7 @@ class AwProxyingRestrictedCookieManager
       bool is_service_worker,
       const std::optional<const content::GlobalRenderFrameHostToken>&
           global_frame_token,
+      const net::SiteForCookies& site_for_cookies,
       AwCookieAccessPolicy* cookie_access_policy);

   static void CreateAndBindOnIoThread(
@@ -127,6 +130,7 @@ class AwProxyingRestrictedCookieManager
       bool is_service_worker,
       const std::optional<const content::GlobalRenderFrameHostToken>&
           global_frame_token,
+      const net::SiteForCookies& site_for_cookies,
       mojo::PendingReceiver<network::mojom::RestrictedCookieManager> receiver,
       AwCookieAccessPolicy* cookie_access_policy);

@@ -134,6 +138,10 @@ class AwProxyingRestrictedCookieManager
       underlying_restricted_cookie_manager_;
   bool is_service_worker_;
   std::optional<const content::GlobalRenderFrameHostToken> global_frame_token_;
+  // Trusted site_for_cookies captured from the browser-side IsolationInfo at
+  // bind time. Used in place of the renderer-supplied Mojo parameter when
+  // evaluating the per-WebView third-party-cookie policy.
+  const net::SiteForCookies bound_site_for_cookies_;

   raw_ref<AwCookieAccessPolicy> cookie_access_policy_;

diff --git a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.cc b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.cc
--- a/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.cc
+++ b/android_webview/browser/network_service/aw_proxying_restricted_cookie_manager.cc
@@ -78,6 +78,7 @@ void AwProxyingRestrictedCookieManager::CreateAndBind(
     bool is_service_worker,
     int process_id,
     int frame_id,
+    const net::SiteForCookies& site_for_cookies,
     mojo::PendingReceiver<network::mojom::RestrictedCookieManager> receiver,
     AwCookieAccessPolicy* aw_cookie_access_policy) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
@@ -94,7 +95,7 @@ void AwProxyingRestrictedCookieManager::CreateAndBind(
       base::BindOnce(
           &AwProxyingRestrictedCookieManager::CreateAndBindOnIoThread,
           std::move(underlying_rcm), is_service_worker, frame_token,
-          std::move(receiver), aw_cookie_access_policy));
+          site_for_cookies, std::move(receiver), aw_cookie_access_policy));
 }

 AwProxyingRestrictedCookieManager::~AwProxyingRestrictedCookieManager() {
@@ -282,11 +283,13 @@ AwProxyingRestrictedCookieManager::AwProxyingRestrictedCookieManager(
     bool is_service_worker,
     const std::optional<const content::GlobalRenderFrameHostToken>&
         global_frame_token,
+    const net::SiteForCookies& site_for_cookies,
     AwCookieAccessPolicy* cookie_access_policy)
     : underlying_restricted_cookie_manager_(
           std::move(underlying_restricted_cookie_manager)),
       is_service_worker_(is_service_worker),
       global_frame_token_(global_frame_token),
+      bound_site_for_cookies_(site_for_cookies),
       cookie_access_policy_(*cookie_access_policy) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);

@@ -307,18 +310,25 @@ void AwProxyingRestrictedCookieManager::CreateAndBindOnIoThread(
     bool is_service_worker,
     const std::optional<const content::GlobalRenderFrameHostToken>&
         global_frame_token,
+    const net::SiteForCookies& site_for_cookies,
     mojo::PendingReceiver<network::mojom::RestrictedCookieManager> receiver,
     AwCookieAccessPolicy* cookie_access_policy) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
   auto wrapper = base::WrapUnique(new AwProxyingRestrictedCookieManager(
       std::move(underlying_rcm), is_service_worker, global_frame_token,
-      cookie_access_policy));
+      site_for_cookies, cookie_access_policy));
   mojo::MakeSelfOwnedReceiver(std::move(wrapper), std::move(receiver));
 }

 PrivacySetting AwProxyingRestrictedCookieManager::AllowCookies(
     const GURL& url,
     const net::SiteForCookies& site_for_cookies) const {
+  // SECURITY: ignore the renderer-supplied `site_for_cookies` and use the
+  // trusted value captured from IsolationInfo at bind time. A compromised
+  // renderer can spoof the Mojo parameter to make a cross-site request look
+  // first-party and bypass setAcceptThirdPartyCookies(false).
+  const net::SiteForCookies& trusted_sfc = bound_site_for_cookies_;
   // When feature is enabled, use latched cookie policy state captured at
   // construction time. This enables shared memory cookie versioning.
   if (base::FeatureList::IsEnabled(features::kWebViewLatchedCookiePolicy)) {
@@ -336,7 +346,7 @@ PrivacySetting AwProxyingRestrictedCookieManager::AllowCookies(
       return latched_accept_cookies_ ? PrivacySetting::kStateAllowed
                                      : PrivacySetting::kStateDisallowed;
     }
-    return AwCookieAccessPolicy::CanAccessCookies(url, site_for_cookies,
+    return AwCookieAccessPolicy::CanAccessCookies(url, trusted_sfc,
                                                   latched_accept_cookies_,
                                                   latched_accept_third_party_);
   }
@@ -349,7 +359,7 @@ PrivacySetting AwProxyingRestrictedCookieManager::AllowCookies(
                ? PrivacySetting::kStateAllowed
                : PrivacySetting::kStateDisallowed;
   } else {
-    return cookie_access_policy_->AllowCookies(url, site_for_cookies,
+    return cookie_access_policy_->AllowCookies(url, trusted_sfc,
                                                global_frame_token_);
   }
 }
diff --git a/android_webview/browser/aw_content_browser_client.cc b/android_webview/browser/aw_content_browser_client.cc
--- a/android_webview/browser/aw_content_browser_client.cc
+++ b/android_webview/browser/aw_content_browser_client.cc
@@ -1295,7 +1295,8 @@ bool AwContentBrowserClient::WillCreateRestrictedCookieManager(

   AwProxyingRestrictedCookieManager::CreateAndBind(
       std::move(target_rcm_remote), is_service_worker, process_id, routing_id,
-      std::move(orig_receiver), aw_cookie_access_policy);
+      isolation_info.site_for_cookies(), std::move(orig_receiver),
+      aw_cookie_access_policy);

   return false;  // only made a proxy, still need the actual impl to be made.
 }