A recently disclosed User Account Control (UAC) bypass that leverages App Paths can be used for fileless attacks as well, security researcher Matt Nelson now says.
Last week the researcher revealed that App Paths and the Backup and Restore tool (sdclt.exe) in Windows 10 can be abused to bypass the UAC because sdclt.exe auto-elevates due to its manifest. Nelson published a proof-of-concept (PoC) script to demonstrate the attack, but warned that, because parameters weren’t supported, the payload had to be saved on the disk.
Nelson, who has a history of revealing UAC bypass techniques (such as last year’s Event Viewer and Disk Cleanup methods), now reveals that fileless attacks abusing the App Paths UAC bypass are possible as well. As before, however, the attack is possible only on Windows 10, because sdclt.exe’s manifest in previous releases prevents it from auto-elevation when started from medium integrity.
The researcher explains that, while analyzing the sdclt.exe binary to look for command line arguments, he discovered that, if a specific argument was used, a parameter could be added to sdclt.exe, which would be executed with elevated privileges.
The researcher published a PoC on GitHub to demonstrate the bypass and explains that the script takes a full path to the payload and any parameters. Moreover, it automatically adds the necessary keys, starts ‘sdclt.exe /kickoffelev,’ and then erases traces of the attack.
The same as before, the attack can be prevented by setting the UAC level to ‘Always Notify’ or by removing the current user from the Local Administrators group. The researchers notes that looking for new registry entries in HKCU:\Software\Classes\exefile\shell\runas\command\isolatedCommand is a good method of monitoring the system for this type of attack.