Description

A newly disclosed Windows privilege-escalation vulnerability in the Steam Client Service, dubbed BrokenPipe, allows a standard local Windows user to execute code with NT AUTHORITY\SYSTEM privileges. The issue affects the privileged steamservice.exe component and can be exploited without administrator credentials, a UAC prompt, or launching a Steam game. A public proof-of-concept has demonstrated exploitation against Steam version 10.96.30.42 on Windows 10 and Windows 11 x64 systems. The vulnerability stems from a signature-coverage gap in Steam's installation-script workflow. The Steam Client Service operates with SYSTEM privileges and accepts a legitimate Valve-signed VDF installation script together with a caller-controlled installation-root path. Although the VDF signature remains valid, the supplied installation path is not adequately covered by that signature. An unprivileged process can communicate with the Steam service through its IPC interface and manipulate this trusted installation workflow. The service can subsequently whitelist and execute a launcher from the attacker-controlled location with SYSTEM privileges. This allows a local attacker who already has code execution on the endpoint to escalate privileges and potentially gain complete control of the Windows system.