diff --git a/Source/WebCore/loader/DocumentLoader.cpp b/Source/WebCore/loader/DocumentLoader.cpp
index 9ee9f3231e57..d757709c2535 100644
--- a/Source/WebCore/loader/DocumentLoader.cpp
+++ b/Source/WebCore/loader/DocumentLoader.cpp
@@ -1142,6 +1142,16 @@ void DocumentLoader::continueAfterContentPolicy(PolicyAction policy)
             return;
         }
 
+        // Defense-in-depth: refuse to download a data: URL through a top-frame navigation that
+        // wasn't initiated by the user or the API client, mirroring the existing check in the
+        // PolicyAction::Use branch. The primary defense lives in the UI process; this guards
+        // ports / future flows that don't share that boundary.
+        if (disallowDataRequest()) {
+            protect(frameLoader())->policyChecker().cannotShowMIMEType(m_response);
+            stopLoadingForPolicyChange();
+            return;
+        }
+
         if (RefPtr mainResourceLoader = this->mainResourceLoader())
             InspectorInstrumentation::continueWithPolicyDownload(*frame, *mainResourceLoader->identifier(), *this, m_response);
 
diff --git a/Source/WebKit/UIProcess/API/APINavigation.h b/Source/WebKit/UIProcess/API/APINavigation.h
index 75076f101b82..69ca01776969 100644
--- a/Source/WebKit/UIProcess/API/APINavigation.h
+++ b/Source/WebKit/UIProcess/API/APINavigation.h
@@ -132,6 +132,7 @@ class Navigation : public ObjectImpl<Object::Type::Navigation> {
 
     bool wasUserInitiated() const { return m_lastNavigationAction && !!m_lastNavigationAction->userGestureTokenIdentifier; }
     bool NODELETE isRequestFromClientOrUserInput() const;
+    bool isFromAPIClientRequest() const { return m_requestIsFromClientInput; }
     void NODELETE markRequestAsFromClientInput();
     void markAsFromLoadData() { m_isFromLoadData = true; }
     bool isFromLoadData() const { return m_isFromLoadData; }
diff --git a/Source/WebKit/UIProcess/WebPageProxy.cpp b/Source/WebKit/UIProcess/WebPageProxy.cpp
index 572ea8d6537f..ceee9ead3799 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.cpp
+++ b/Source/WebKit/UIProcess/WebPageProxy.cpp
@@ -5924,6 +5924,19 @@ void WebPageProxy::receivedNavigationResponsePolicyDecision(WebCore::PolicyActio
     if (!hasRunningProcess())
         return completionHandler(PolicyDecision { });
 
+    // Refuse to convert a navigation response into a download when the request is a data: URL,
+    // unless the navigation was driven by the API client (e.g. -loadRequest: with a data: URL).
+    // Otherwise a compromised Web Content process could navigate to a data: URL with an unshowable
+    // MIME type and rely on the navigation delegate's stock "download unshowable responses"
+    // behavior to write attacker-controlled bytes to disk without user interaction. Legitimate
+    // downloads of data: URLs go through the navigation action policy (e.g. <a href="data:..." download>)
+    // or the explicit download API, neither of which reaches this code path.
+    if (action == PolicyAction::Download && request.url().protocolIsData()
+        && (!navigation || !navigation->isFromAPIClientRequest())) {
+        WEBPAGEPROXY_RELEASE_LOG(Loading, "receivedNavigationResponsePolicyDecision: refusing to download data: URL not initiated by API client");
+        action = PolicyAction::Ignore;
+    }
+
     Ref pageLoadState = internals().pageLoadState;
     auto transaction = pageLoadState->transaction();