The Steam windows client privilege escalation vulnerability allows an attacker with normal user privilege can run arbitrary code as an administrator.
The Zero-day vulnerability was discovered by Vasily Kravets and the vulnerability resides in the Steam Client Service which was installed by steam for some internal purpose.
While reviewing the SDDL (Security Descriptor Definition Language), the researcher noted that any user associated with the group “Users” are allowed to start and stop the programs. The SDDL is a string that defines user access rights in the text form.
Regardless of the user permission Steam sets explicit key permissions, full control for all the users under “Users” group, and the same permission inherited for all the subkeys and their subkeys.
To demonstrate this Vasily created a test key, “HKLM\Software\Wow6432Node\Valve\Steam\Apps\test and restarted the service (Procmon’s log is above) and checked registry key permissions. Here I found that HKLM\SOFTWARE\Wow6432Node\Valve\Steam has explicit “Full control” for “Users” group, and these permissions inherit for all subkeys and their subkeys.”
The problem is this contain any symlink, then the malicious user in the “Users” group can restart other programs as well. The symlink is termed as a soft link that points to another file.
Vasily further chose the key HKLM\SYSTEM\ControlSet001\Services\msiserver, the msiserver is the windows installer service which can be started and stopped by any user that belongs to the user group “Users” same as like Steam’ service, but the program should be executed as NT AUTHORITY\SYSTEM.
Put all things together and we get to exploit that allows running any program with the highest possible rights on any Windows computer with Steam installed, Vasily added.
The vulnerability was reported to Valve on June 15, but the vendor marked the vulnerability as marked as “N\A” and the threat has been closed. Steam provided updates on August 6, 2019, but the vulnerability was not yet addressed.
Another researcher Matt Nelson also found the permission issue with the registry keys.
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