Mozilla Compatible Silverlight 4 Plugin Requires Loading DLLs from CWD

chrome-silverlight-agcore-missingI visited a site yesterday in Chrome that tried to load Silverlight to provide a video player. I have KB2264107 installed and have globally disabled loading of DLLs from the current working directory in order to mitigate luring attacks against apps that use the default insecure DLL loading behavior of LoadLibrary(). Just like the Java plugin for Mozilla, Chrome generated a big fat bonk dialog trying to load the DLLs that the Silverlight plugin uses. The specific missing file is agcore.dll, which is found in “C:\Program Files (x86)\Microsoft Silverlight\4.0.50524.0” on my system.

I tried creating a symlink to agcore,dll so that agcore.dll is in the same directory as Chrome.exe, which fixes the bonk but Silverlight doesn’t work. I just end up with a black box where the movie player should be. I also tried adding the Silverlight directory to $env:path which removed the bonk but, instead, I got the “Install Microsoft Silverlight” button. I tried various combinations of symlinking DLLs and messing with the $env:path but I didn’t arrive at a combination that can actually work.

The only solution that I found is to dial the CWDIllegalInDllSearch value for Chrome and Firefox to 2 (DLLs not allowed to load from CWD if CWD is any remote, network location) instead of 0xffffffff (it also works to change this globally). I then have to hope that Firefox and Chrome are careful about how they are using CWD. I hope they are setting CWD just for loading the installed plugins in “Prgram Files” but cannot be lured into loading some evil DLL from a spurious location when doing something like opening an HTML document on a USB stick.


PS> Get-ItemProperty chrome.exe, firefox.exe | select pspath,cwdillegalindllsearch | fl


PSPath                : Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVer
                        sion\Image File Execution Options\chrome.exe
CWDIllegalInDllSearch : 2

PSPath                : Microsoft.PowerShell.Core\Registry::HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVer
                        sion\Image File Execution Options\firefox.exe
CWDIllegalInDllSearch : 2




%d bloggers like this: