An issue was discovered in Valve Steam Client 2.10.91.91. The installer allows local users to gain NT AUTHORITY\SYSTEM privileges because some parts of %PROGRAMFILES(X86)%\Steam and/or %COMMONPROGRAMFILES(X86)%\Steam have weak permissions during a critical time window. An attacker can make this time window arbitrarily long by using opportunistic locks.

Valve Steam 2.10.91.91 uses weak permissions (Users: read and write) for the Install folder, which allows local users to gain privileges via a Trojan horse steam.exe file.