BrokenPipe PoC Exploits Steam Client Service for Silent SYSTEM Privilege Escalation on Windows

A new proof of concept called BrokenPipe shows how a standard user on Windows can elevate privileges to NT AUTHORITY\SYSTEM through the Steam Client Service without an administrator password or any UAC prompt.

The demonstration describes a path that lets any non-administrator user execute code as NT AUTHORITY\SYSTEM. Tests reproduced the behavior on Windows 10 and Windows 11 with Steam 10.96.30.42. The affected component is steamservice.exe, the executable behind the Steam Client Service.

This Windows service runs with maximum privileges and exposes an interface for maintenance and deployment tasks. The design, common in software that needs to update or install dependencies, becomes an attack surface when validation logic fails to close all vectors.

The technical explanation points to insufficient signature validation in VDF installation scripts. The service does not fully control the path it accepts and executes; part of that path remains outside signature coverage, allowing an attacker to dictate where the script is loaded from.

The exploit uses the service interface to add a malicious script to an allowlist and force its execution, resulting in the payload running with SYSTEM privileges. The flaw is not remotely exploitable; the attacker must first execute code locally with a standard account.

This scenario fits shared computers, library workstations, cybercafés, or corporate environments where some users should not be able to elevate privileges. A successful LPE can turn a limited intrusion into full machine control.

No public CVE is associated with the issue, and there is no confirmation of active exploitation in real campaigns. Valve received notification months before public disclosure, with March cited as the notification date.

Administrators should inventory Windows systems running Steam, evaluate whether the Steam Client Service is necessary on sensitive or shared endpoints, and apply compensating controls such as application allowlisting and script execution policies. Monitoring for SYSTEM-context processes tied to steamservice.exe and anomalous launches of cmd.exe from Steam paths is also recommended.