From b40ca4aa9abe852c75ebf0afaa348459b4161dd9 Mon Sep 17 00:00:00 2001
From: Simon Zünd <szuend@chromium.org>
Date: Fri, 28 Aug 2026 06:09:31 +0000
Subject: [PATCH] [inspector] Disallow proxy values in JSONML for custom formatters

In substituteObjectTags, JS proxies could masquerade as objects or
arrays and bypass validation, since v8::Value::IsArray() returns false
for JSProxy instances and attributes were not checked for proxies.

This CL adds explicit checks for v8::Value::IsProxy() in
substituteObjectTags and reports an error if encountered, ensuring
proxies are rejected both as JSONML elements and as attribute
dictionaries.

TAG=agy CONV=8316c18a-958e-4e9f-b927-e516a24655c4 R=dsv@chromium.org

Fixed: 533095855
Change-Id: Ic2afc2e0f82581169cd876634188e6099d482c98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/8312289
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#109553}
---
diff --git a/src/inspector/custom-preview.cc b/src/inspector/custom-preview.cc
index 5553068..469705c 100644
--- a/src/inspector/custom-preview.cc
+++ b/src/inspector/custom-preview.cc
@@ -84,6 +84,10 @@
       reportError(context, tryCatch, "attributes should be an Object");
       return false;
     }
+    if (attributesValue->IsProxy()) {
+      reportError(context, tryCatch, "Proxy values are not allowed in JSONML");
+      return false;
+    }
     v8::Local<v8::Object> attributes = attributesValue.As<v8::Object>();
     v8::Local<v8::Value> originValue;
     if (!attributes->Get(context, objectLiteral).ToLocal(&originValue)) {
@@ -146,6 +150,11 @@
         reportError(context, tryCatch);
         return false;
       }
+      if (value->IsProxy()) {
+        reportError(context, tryCatch,
+                    "Proxy values are not allowed in JSONML");
+        return false;
+      }
       if (value->IsArray() && value.As<v8::Array>()->Length() > 0 &&
           !substituteObjectTags(sessionId, groupName, context,
                                 value.As<v8::Array>(), maxDepth - 1)) {
diff --git a/test/inspector/runtime/custom-preview-proxy-expected.txt b/test/inspector/runtime/custom-preview-proxy-expected.txt
new file mode 100644
index 0000000..60cdf68
--- /dev/null
+++ b/test/inspector/runtime/custom-preview-proxy-expected.txt
@@ -0,0 +1,91 @@
+Checks that proxies in JSONML are rejected by custom formatters
+Test proxy child in header JSONML..
+{
+    method : Runtime.consoleAPICalled
+    params : {
+        args : [
+            [0] : {
+                type : string
+                value : Custom Formatter Failed: Uncaught Proxy values are not allowed in JSONML
+            }
+        ]
+        executionContextId : <executionContextId>
+        timestamp : <timestamp>
+        type : error
+    }
+}
+No customPreview generated
+Test proxy attributes in header JSONML..
+{
+    method : Runtime.consoleAPICalled
+    params : {
+        args : [
+            [0] : {
+                type : string
+                value : Custom Formatter Failed: Uncaught Proxy values are not allowed in JSONML
+            }
+        ]
+        executionContextId : <executionContextId>
+        timestamp : <timestamp>
+        type : error
+    }
+}
+No customPreview generated
+Test proxy array child masquerading as tag..
+{
+    method : Runtime.consoleAPICalled
+    params : {
+        args : [
+            [0] : {
+                type : string
+                value : Custom Formatter Failed: Uncaught Proxy values are not allowed in JSONML
+            }
+        ]
+        executionContextId : <executionContextId>
+        timestamp : <timestamp>
+        type : error
+    }
+}
+No customPreview generated
+Test proxy attributes in object tag..
+{
+    method : Runtime.consoleAPICalled
+    params : {
+        args : [
+            [0] : {
+                type : string
+                value : Custom Formatter Failed: Uncaught Proxy values are not allowed in JSONML
+            }
+        ]
+        executionContextId : <executionContextId>
+        timestamp : <timestamp>
+        type : error
+    }
+}
+No customPreview generated
+Test proxy child in body JSONML..
+customPreview: ["span",{},"Header ok"]
+{
+    method : Runtime.consoleAPICalled
+    params : {
+        args : [
+            [0] : {
+                type : string
+                value : Custom Formatter Failed: Uncaught Proxy values are not allowed in JSONML
+            }
+        ]
+        executionContextId : <executionContextId>
+        timestamp : <timestamp>
+        type : error
+    }
+}
+{
+    id : <messageId>
+    result : {
+        result : {
+            type : undefined
+        }
+    }
+}
+Test formatting a proxy object itself (allowed)..
+customPreview: ["span",{},"Proxy formatted: ","proxyObject"]
diff --git a/test/inspector/runtime/custom-preview-proxy.js b/test/inspector/runtime/custom-preview-proxy.js
new file mode 100644
index 0000000..c525d84
--- /dev/null
+++ b/test/inspector/runtime/custom-preview-proxy.js
@@ -0,0 +1,115 @@
+// Copyright 2026 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+const { session, contextGroup, Protocol } =
+  InspectorTest.start('Checks that proxies in JSONML are rejected by custom formatters');
+
+(async function test() {
+  Protocol.Runtime.enable();
+  Protocol.Runtime.setCustomObjectFormatterEnabled({ enabled: true });
+  Protocol.Runtime.onConsoleAPICalled(m => InspectorTest.logMessage(m));
+
+  InspectorTest.log('Test proxy child in header JSONML..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      this.devtoolsFormatters = [{
+        header: () => ['span', {}, 'Header with proxy child', new Proxy([], {})],
+        hasBody: () => false
+      }];
+    `
+  });
+  let result = await Protocol.Runtime.evaluate({ expression: '({})', generatePreview: true });
+  dumpCustomPreview(result);
+
+  InspectorTest.log('Test proxy attributes in header JSONML..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      this.devtoolsFormatters = [{
+        header: () => ['span', new Proxy({}, {}), 'Header with proxy attributes'],
+        hasBody: () => false
+      }];
+    `
+  });
+  result = await Protocol.Runtime.evaluate({ expression: '({})', generatePreview: true });
+  dumpCustomPreview(result);
+
+  InspectorTest.log('Test proxy array child masquerading as tag..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      this.devtoolsFormatters = [{
+        header: () => ['span', {}, new Proxy(['object', {object: {}}], {})],
+        hasBody: () => false
+      }];
+    `
+  });
+  result = await Protocol.Runtime.evaluate({ expression: '({})', generatePreview: true });
+  dumpCustomPreview(result);
+
+  InspectorTest.log('Test proxy attributes in object tag..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      this.devtoolsFormatters = [{
+        header: () => ['span', {}, ['object', new Proxy({object: {}}, {})]],
+        hasBody: () => false
+      }];
+    `
+  });
+  result = await Protocol.Runtime.evaluate({ expression: '({})', generatePreview: true });
+  dumpCustomPreview(result);
+
+  InspectorTest.log('Test proxy child in body JSONML..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      this.devtoolsFormatters = [{
+        header: () => ['span', {}, 'Header ok'],
+        hasBody: () => true,
+        body: () => ['span', {}, 'Body with proxy child', new Proxy([], {})]
+      }];
+    `
+  });
+  result = await Protocol.Runtime.evaluate({ expression: '({})', generatePreview: true });
+  await dumpCustomPreviewWithBody(result);
+
+  InspectorTest.log('Test formatting a proxy object itself (allowed)..');
+  await Protocol.Runtime.evaluate({
+    expression: `
+      var targetProxy = new Proxy({name: 'proxyObject'}, {});
+      this.devtoolsFormatters = [{
+        header: (x) => x === targetProxy ? ['span', {}, 'Proxy formatted: ', x.name] : null,
+        hasBody: () => false
+      }];
+    `
+  });
+  result = await Protocol.Runtime.evaluate({ expression: 'targetProxy', generatePreview: true });
+  dumpCustomPreview(result);
+
+  InspectorTest.completeTest();
+})();
+
+function dumpCustomPreview(result) {
+  const remoteObject = result.result.result;
+  if (remoteObject.customPreview) {
+    InspectorTest.log('customPreview: ' + remoteObject.customPreview.header);
+  } else {
+    InspectorTest.log('No customPreview generated');
+  }
+}
+
+async function dumpCustomPreviewWithBody(result) {
+  const remoteObject = result.result.result;
+  if (remoteObject.customPreview) {
+    InspectorTest.log('customPreview: ' + remoteObject.customPreview.header);
+    if (remoteObject.customPreview.bodyGetterId) {
+      const body = await Protocol.Runtime.callFunctionOn({
+        objectId: remoteObject.objectId,
+        functionDeclaration: 'function(bodyGetter) { return bodyGetter.call(this); }',
+        arguments: [{ objectId: remoteObject.customPreview.bodyGetterId }],
+        returnByValue: true
+      });
+      InspectorTest.logMessage(body);
+    }
+  } else {
+    InspectorTest.log('No customPreview generated');
+  }
+}