diff --git a/content/browser/devtools/devtools_session.cc b/content/browser/devtools/devtools_session.cc
index 22c21058b4b23..5815a733fd4a4 100644
--- a/content/browser/devtools/devtools_session.cc
+++ b/content/browser/devtools/devtools_session.cc
@@ -556,7 +556,14 @@ void DevToolsSession::DispatchProtocolResponseOrNotification(
     blink::mojom::DevToolsMessagePtr message,
     const std::string& session_id,
     const bool& is_notification) {
-  base::span<const uint8_t> message_span = message->data;
+  // SECURITY: |message->data| is a mojo_base::BigBuffer and may be backed by
+  // shared memory that the (potentially compromised) renderer can still write
+  // to. Copy it into browser-owned memory before validating, and forward the
+  // *copy*, so the bytes ValidateMessage() inspects are exactly the bytes the
+  // DevTools client receives (no TOCTOU window).
+  const std::vector<uint8_t> message_copy(message->data.begin(),
+                                          message->data.end());
+  const base::span<const uint8_t> message_span(message_copy);
   if (!ValidateMessage(session_id, /*expected_has_id=*/!is_notification,
                        message_span)) {
     if (RenderProcessHost* process_host = agent_host->GetProcessHost()) {
@@ -565,7 +572,7 @@ void DevToolsSession::DispatchProtocolResponseOrNotification(
     }
     return;
   }
-  client->DispatchProtocolMessage(agent_host, message->data);
+  client->DispatchProtocolMessage(agent_host, message_span);
 }
 
 void DevToolsSession::DispatchProtocolResponse(