Source: https://code.google.com/p/google-security-research/issues/detail?id=664

There is an overflow in the ui::PlatformCursor WebCursor::GetPlatformCursor method. In src/content/common/cursors/webcursor_aurax11.cc&q=webcursor_aurax11.cc, there is the following code:

bitmap.allocN32Pixels(custom_size_.width(), custom_size_.height());
memcpy(bitmap.getAddr32(0, 0), custom_data_.data(), custom_data_.size());

The bitmap buffer is allocated based on the width and height of the custom_size_, but the memcpy is performed using the size of the custom_data_.

These values are set during WebCursor deserialization in src/content/common/cursors/webcursor.cc in WebCursor::Deserialize.

custom_size_ is set from two integers that a deserialized from a message and can be between 0 and 1024. custom_data_ is set from a vector that is deserialized, and can be any size, unrelated to the width and height. The custom_data_ is verified not to be smaller than the expected pixel buffer based on the width and height, but can be longer.

GetPlatformCursor is called indirectly by RenderWidgetHostImpl::OnSetCursor, which is called in response to a  ViewHostMsg_SetCursor message from the renderer.

The issue above is in the x11 implementation, but it appears also affect other platform-specific implementations other than the Windows one, which instead reads out of bounds.

I recommend this issue be fixed by changing the check in WebCursor::Deserialize:

if (size_x * size_y * 4 > data_len)
    return false;

to

if (size_x * size_y * 4 != data_len)
    return false;

to prevent the issue in all platform-specific implementations.
 
To reproduce the issue replace WebCursor::Serialize with:

bool WebCursor::Serialize(base::Pickle* pickle) const {

  if(type_ == WebCursorInfo::TypeCustom){
  LOG(WARNING) << "IN SERIALIZE\n";
  if (!pickle->WriteInt(type_) ||
      !pickle->WriteInt(hotspot_.x()) ||
      !pickle->WriteInt(hotspot_.y()) ||
      !pickle->WriteInt(2) ||
      !pickle->WriteInt(1) ||
      !pickle->WriteFloat(custom_scale_))
     return false;
   }else{

     if (!pickle->WriteInt(type_) ||
      !pickle->WriteInt(hotspot_.x()) ||
      !pickle->WriteInt(hotspot_.y()) ||
      !pickle->WriteInt(custom_size_.width()) ||
      !pickle->WriteInt(custom_size_.height()) ||
      !pickle->WriteFloat(custom_scale_))
    return false;

  }
  const char* data = NULL;
  if (!custom_data_.empty())
    data = &custom_data_[0];
  if (!pickle->WriteData(data, custom_data_.size()))
    return false;

  return SerializePlatformData(pickle);
}

and visit the attached html page, with the attached image in the same directory.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39039.zip