From 67ed1f75de8df62e337bef6e7725a085ad045e36 Mon Sep 17 00:00:00 2001
From: Rohan Raja <roraja@microsoft.com>
Date: Sat, 24 Jan 2026 09:30:40 -0800
Subject: [PATCH] Add IsRendererPasteAllowed checks to clipboard custom format methods in ClipboardHostImpl

This CL fixes a security vulnerability where
ReadAvailableCustomAndStandardFormats() and
ReadUnsanitizedCustomFormat() bypass permission checks, allowing
compromised renderers to read web custom clipboard formats without user
activation.

Changes Made:
- Added IsRendererPasteAllowed() check to ReadAvailableCustomAndStandardFormats()
  Returns empty vector if permission denied

- Added IsRendererPasteAllowed() check to ReadUnsanitizedCustomFormat()
  Returns empty BigBuffer if permission denied

- Fixed all early return paths in ReadUnsanitizedCustomFormat() to properly invoke
  callbacks, preventing callback leaks

Risk Assessment: Low
- Follows established pattern from existing
clipboard methods.
- No functional changes to legitimate use cases
requiring user activation

Bug: 474763968
Change-Id: I555360a1d3f2309f2a90b8da9709b014480606ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7509483
Reviewed-by: Sambamurthy Bandaru <sambamurthy.bandaru@microsoft.com>
Reviewed-by: Dan Clark <daniec@microsoft.com>
Reviewed-by: Shweta Bindal <shwetabindal@microsoft.com>
Commit-Queue: Rohan Raja <roraja@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1574177}
---

diff --git a/content/browser/renderer_host/clipboard_host_impl.cc b/content/browser/renderer_host/clipboard_host_impl.cc
index be6eb7b5..24f0fddc 100644
--- a/content/browser/renderer_host/clipboard_host_impl.cc
+++ b/content/browser/renderer_host/clipboard_host_impl.cc
@@ -605,6 +605,12 @@
 
 void ClipboardHostImpl::ReadAvailableCustomAndStandardFormats(
     ReadAvailableCustomAndStandardFormatsCallback callback) {
+  if (!IsRendererPasteAllowed(ui::ClipboardBuffer::kCopyPaste,
+                              render_frame_host())) {
+    std::move(callback).Run(std::vector<std::u16string>());
+    return;
+  }
+
   std::vector<std::u16string> format_types =
       ui::Clipboard::GetForCurrentThread()
           ->ReadAvailableStandardAndCustomFormatNames(
@@ -615,10 +621,18 @@
 void ClipboardHostImpl::ReadUnsanitizedCustomFormat(
     const std::u16string& format,
     ReadUnsanitizedCustomFormatCallback callback) {
+  if (!IsRendererPasteAllowed(ui::ClipboardBuffer::kCopyPaste,
+                              render_frame_host())) {
+    std::move(callback).Run(mojo_base::BigBuffer());
+    return;
+  }
+
   // `kMaxFormatSize` includes the null terminator as well so we check if
   // the `format` size is strictly less than `kMaxFormatSize` or not.
-  if (format.length() >= blink::mojom::ClipboardHost::kMaxFormatSize)
+  if (format.length() >= blink::mojom::ClipboardHost::kMaxFormatSize) {
+    std::move(callback).Run(mojo_base::BigBuffer());
     return;
+  }
 
   // Extract the custom format names and then query the web custom format
   // corresponding to the MIME type.
@@ -630,15 +644,19 @@
   std::string web_custom_format_string;
   if (custom_format_names.find(format_name) != custom_format_names.end())
     web_custom_format_string = custom_format_names[format_name];
-  if (web_custom_format_string.empty())
+  if (web_custom_format_string.empty()) {
+    std::move(callback).Run(mojo_base::BigBuffer());
     return;
+  }
 
   std::string result;
   ui::Clipboard::GetForCurrentThread()->ReadData(
       ui::ClipboardFormatType::CustomPlatformType(web_custom_format_string),
       data_endpoint.get(), &result);
-  if (result.size() >= blink::mojom::ClipboardHost::kMaxDataSize)
+  if (result.size() >= blink::mojom::ClipboardHost::kMaxDataSize) {
+    std::move(callback).Run(mojo_base::BigBuffer());
     return;
+  }
   base::span<const uint8_t> span = base::as_byte_span(result);
   mojo_base::BigBuffer buffer = mojo_base::BigBuffer(span);
   std::move(callback).Run(std::move(buffer));
diff --git a/content/browser/renderer_host/clipboard_host_impl_unittest.cc b/content/browser/renderer_host/clipboard_host_impl_unittest.cc
index f6eda8e..5b12e63 100644
--- a/content/browser/renderer_host/clipboard_host_impl_unittest.cc
+++ b/content/browser/renderer_host/clipboard_host_impl_unittest.cc
@@ -24,6 +24,8 @@
 #include "content/public/test/browser_task_environment.h"
 #include "content/public/test/navigation_simulator.h"
 #include "content/public/test/test_renderer_host.h"
+#include "content/public/test/test_utils.h"
+#include "content/test/test_content_browser_client.h"
 #include "mojo/public/cpp/bindings/remote.h"
 #include "mojo/public/cpp/system/message_pipe.h"
 #include "skia/ext/skia_utils_base.h"
@@ -51,6 +53,26 @@
 }
 namespace content {
 
+// Custom ContentBrowserClient for testing clipboard paste permissions.
+class ClipboardPasteAllowedBrowserClient : public TestContentBrowserClient {
+ public:
+  ClipboardPasteAllowedBrowserClient() = default;
+  ~ClipboardPasteAllowedBrowserClient() override = default;
+
+  void set_is_clipboard_paste_allowed(bool allowed) {
+    is_clipboard_paste_allowed_ = allowed;
+  }
+
+  // ContentBrowserClient:
+  bool IsClipboardPasteAllowed(
+      content::RenderFrameHost* render_frame_host) override {
+    return is_clipboard_paste_allowed_;
+  }
+
+ private:
+  bool is_clipboard_paste_allowed_ = true;
+};
+
 class ClipboardHostImplTest : public RenderViewHostTestHarness {
  protected:
   ClipboardHostImplTest() { ui::TestClipboard::CreateForCurrentThread(); }
@@ -920,4 +942,102 @@
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_F(ClipboardHostImplTest,
+       ReadUnsanitizedCustomFormat_WithoutUserActivation) {
+  // Setup: Custom browser client that denies clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(false);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write custom format to clipboard
+  std::string test_data = "confidential_custom_data";
+  {
+    ui::ScopedClipboardWriter writer(ui::ClipboardBuffer::kCopyPaste);
+    writer.WriteData(u"web text/custom",
+                     mojo_base::BigBuffer(base::as_byte_span(test_data)));
+  }
+
+  // Test: Try to read custom format without user activation
+  base::test::TestFuture<mojo_base::BigBuffer> future;
+  mojo_clipboard()->ReadUnsanitizedCustomFormat(u"web text/custom",
+                                                future.GetCallback());
+
+  // Verify: Should return empty buffer due to permission check failure
+  EXPECT_EQ(0u, future.Get().size());
+}
+
+TEST_F(ClipboardHostImplTest,
+       ReadAvailableCustomAndStandardFormats_WithUserActivation) {
+  // Setup: Custom browser client that allows clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(true);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write some standard format data that TestClipboard can handle
+  mojo_clipboard()->WriteText(u"test text");
+  mojo_clipboard()->CommitWrite();
+  base::RunLoop().RunUntilIdle();
+
+  // Test: Read available formats with permission allowed
+  base::test::TestFuture<const std::vector<std::u16string>&> future;
+  mojo_clipboard()->ReadAvailableCustomAndStandardFormats(future.GetCallback());
+
+  // Verify: With permission allowed, the call completes successfully.
+  // TestClipboard should return standard formats like "text/plain".
+  const auto& formats = future.Get();
+  EXPECT_TRUE(std::ranges::contains(formats, u"text/plain"));
+}
+
+TEST_F(ClipboardHostImplTest, ReadUnsanitizedCustomFormat_WithUserActivation) {
+  // Setup: Custom browser client that allows clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(true);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write custom format data using ScopedClipboardWriter which properly
+  // handles web custom format metadata
+  std::string test_data = "test_custom_data";
+  {
+    ui::ScopedClipboardWriter writer(ui::ClipboardBuffer::kCopyPaste);
+    writer.WriteData(u"text/custom",
+                     mojo_base::BigBuffer(base::as_byte_span(test_data)));
+  }

diff --git a/content/browser/renderer_host/clipboard_host_impl_unittest.cc b/content/browser/renderer_host/clipboard_host_impl_unittest.cc
index f6eda8e..5b12e63 100644
--- a/content/browser/renderer_host/clipboard_host_impl_unittest.cc
+++ b/content/browser/renderer_host/clipboard_host_impl_unittest.cc
@@ -24,6 +24,8 @@
 #include "content/public/test/browser_task_environment.h"
 #include "content/public/test/navigation_simulator.h"
 #include "content/public/test/test_renderer_host.h"
+#include "content/public/test/test_utils.h"
+#include "content/test/test_content_browser_client.h"
 #include "mojo/public/cpp/bindings/remote.h"
 #include "mojo/public/cpp/system/message_pipe.h"
 #include "skia/ext/skia_utils_base.h"
@@ -51,6 +53,26 @@
 }
 namespace content {
 
+// Custom ContentBrowserClient for testing clipboard paste permissions.
+class ClipboardPasteAllowedBrowserClient : public TestContentBrowserClient {
+ public:
+  ClipboardPasteAllowedBrowserClient() = default;
+  ~ClipboardPasteAllowedBrowserClient() override = default;
+
+  void set_is_clipboard_paste_allowed(bool allowed) {
+    is_clipboard_paste_allowed_ = allowed;
+  }
+
+  // ContentBrowserClient:
+  bool IsClipboardPasteAllowed(
+      content::RenderFrameHost* render_frame_host) override {
+    return is_clipboard_paste_allowed_;
+  }
+
+ private:
+  bool is_clipboard_paste_allowed_ = true;
+};
+
 class ClipboardHostImplTest : public RenderViewHostTestHarness {
  protected:
   ClipboardHostImplTest() { ui::TestClipboard::CreateForCurrentThread(); }
@@ -920,4 +942,102 @@
   base::RunLoop().RunUntilIdle();
 }
 
+TEST_F(ClipboardHostImplTest,
+       ReadUnsanitizedCustomFormat_WithoutUserActivation) {
+  // Setup: Custom browser client that denies clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(false);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write custom format to clipboard
+  std::string test_data = "confidential_custom_data";
+  {
+    ui::ScopedClipboardWriter writer(ui::ClipboardBuffer::kCopyPaste);
+    writer.WriteData(u"web text/custom",
+                     mojo_base::BigBuffer(base::as_byte_span(test_data)));
+  }
+
+  // Test: Try to read custom format without user activation
+  base::test::TestFuture<mojo_base::BigBuffer> future;
+  mojo_clipboard()->ReadUnsanitizedCustomFormat(u"web text/custom",
+                                                future.GetCallback());
+
+  // Verify: Should return empty buffer due to permission check failure
+  EXPECT_EQ(0u, future.Get().size());
+}
+
+TEST_F(ClipboardHostImplTest,
+       ReadAvailableCustomAndStandardFormats_WithUserActivation) {
+  // Setup: Custom browser client that allows clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(true);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write some standard format data that TestClipboard can handle
+  mojo_clipboard()->WriteText(u"test text");
+  mojo_clipboard()->CommitWrite();
+  base::RunLoop().RunUntilIdle();
+
+  // Test: Read available formats with permission allowed
+  base::test::TestFuture<const std::vector<std::u16string>&> future;
+  mojo_clipboard()->ReadAvailableCustomAndStandardFormats(future.GetCallback());
+
+  // Verify: With permission allowed, the call completes successfully.
+  // TestClipboard should return standard formats like "text/plain".
+  const auto& formats = future.Get();
+  EXPECT_TRUE(std::ranges::contains(formats, u"text/plain"));
+}
+
+TEST_F(ClipboardHostImplTest, ReadUnsanitizedCustomFormat_WithUserActivation) {
+  // Setup: Custom browser client that allows clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(true);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write custom format data using ScopedClipboardWriter which properly
+  // handles web custom format metadata
+  std::string test_data = "test_custom_data";
+  {
+    ui::ScopedClipboardWriter writer(ui::ClipboardBuffer::kCopyPaste);
+    writer.WriteData(u"text/custom",
+                     mojo_base::BigBuffer(base::as_byte_span(test_data)));
+  }
+
+  // Test: Read custom format with permission allowed
+  // Note: Need to prepend "web " prefix to match how ExtractCustomPlatformNames
+  // works
+  base::test::TestFuture<mojo_base::BigBuffer> future;
+  mojo_clipboard()->ReadUnsanitizedCustomFormat(u"web text/custom",
+                                                future.GetCallback());
+
+  // Verify: With permission allowed, the data should be successfully retrieved
+  const auto& result = future.Get();
+  EXPECT_GT(result.size(), 0u);
+
+  // Verify the content matches what was written
+  std::string retrieved_data(result.begin(), result.end());
+  EXPECT_EQ(retrieved_data, test_data);
+}
+
+TEST_F(ClipboardHostImplTest,
+       ReadAvailableCustomAndStandardFormats_TextWithoutUserActivation) {
+  // Setup: Custom browser client that denies clipboard paste
+  ClipboardPasteAllowedBrowserClient browser_client;
+  browser_client.set_is_clipboard_paste_allowed(false);
+  ScopedContentBrowserClientSetting browser_client_setting(&browser_client);
+
+  // Write standard text format to clipboard
+  {
+    ui::ScopedClipboardWriter writer(ui::ClipboardBuffer::kCopyPaste);
+    writer.WriteText(u"test text");
+  }
+
+  // Test: Try to read available formats without permission
+  base::test::TestFuture<const std::vector<std::u16string>&> future;
+  mojo_clipboard()->ReadAvailableCustomAndStandardFormats(future.GetCallback());
+
+  // Verify: Should return empty vector due to permission check failure
+  EXPECT_EQ(0u, future.Get().size());
+}
+
 }  // namespace content