CVE-2020-15530: Steam Client Privilege Escalation Flaw

CVE-2020-15530 Overview

CVE-2020-15530 is a local privilege escalation vulnerability discovered in Valve Steam Client version 2.10.91.91. The vulnerability exists within the installer component, which allows local users to gain NT AUTHORITY\SYSTEM privileges due to weak file permissions applied to critical directories during installation. Specifically, certain parts of %PROGRAMFILES(X86)%\Steam and/or %COMMONPROGRAMFILES(X86)%\Steam have insecure permissions during a critical time window. An attacker can extend this exploitation window indefinitely by utilizing opportunistic locks (oplocks), making reliable exploitation significantly easier.

Critical Impact

Local attackers can escalate privileges to NT AUTHORITY\SYSTEM, gaining complete control over the affected Windows system through a race condition during Steam Client installation.

Affected Products

• Valvesoftware Steam Client version 2.10.91.91

Discovery Timeline

• 2020-07-05 - CVE-2020-15530 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15530

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition), specifically a Time-of-Check Time-of-Use (TOCTOU) vulnerability. The flaw resides in the Steam Client installer's handling of file permissions during the installation process. When the installer creates or modifies files and directories within the Steam installation paths, there exists a window where these filesystem objects have overly permissive access controls.

The attack requires local access to the target system and leverages the inherent race condition between when the installer sets up directory structures and when proper security permissions are applied. What makes this vulnerability particularly dangerous is that attackers can use Windows opportunistic locks (oplocks) to artificially extend this vulnerable time window, transforming what might be a narrow exploitation window into a reliable attack vector.

Root Cause

The root cause of CVE-2020-15530 lies in the Steam Client installer's failure to properly secure file and directory permissions atomically during the installation process. The installer creates directories and files with weak default permissions before applying the intended restrictive permissions. This design flaw creates a TOCTOU race condition where an attacker can intercept and manipulate these resources during the vulnerable window.

The use of opportunistic locks allows attackers to pause filesystem operations at critical points, giving them ample time to exploit the weak permissions. This technique transforms a difficult-to-exploit race condition into a deterministic privilege escalation attack.

Attack Vector

The attack is executed locally and requires the attacker to have existing low-privileged access to the Windows system where the Steam Client is being installed or updated. The exploitation process involves:

1. Monitoring for Steam Client installation or update operations
2. Placing opportunistic locks on target files/directories to pause the installer at the vulnerable moment
3. Exploiting the weak permissions window to plant malicious files or hijack existing components
4. When the installer resumes and executes with SYSTEM privileges, the attacker's payload executes with elevated permissions

The vulnerability mechanism centers on the improper handling of filesystem permissions during installation. The Steam Client installer applies weak default ACLs to installation directories before properly securing them, creating a race condition. Attackers leverage Windows opportunistic locks (oplocks) to extend this window indefinitely, allowing them to place malicious executables or DLLs in the vulnerable directories. When the installer continues execution with SYSTEM privileges, these malicious files are loaded, resulting in privilege escalation. For detailed technical analysis, refer to Daniel's IT Blog Analysis.

Detection Methods for CVE-2020-15530

Indicators of Compromise

• Unusual file creation or modification events in %PROGRAMFILES(X86)%\Steam during installation periods
• Presence of opportunistic lock operations targeting Steam installation directories
• Unexpected processes spawning from Steam installation paths with SYSTEM privileges
• Anomalous DLL or executable files appearing in Steam directories with recent timestamps

Detection Strategies

• Monitor Windows Security Event Logs for privilege escalation attempts originating from Steam-related paths
• Implement file integrity monitoring (FIM) on Steam installation directories to detect unauthorized modifications
• Track process creation events where parent processes are Steam-related and child processes have elevated privileges
• Utilize Sysmon logging to capture file creation and modification events in Program Files directories

Monitoring Recommendations

• Enable enhanced auditing on %PROGRAMFILES(X86)%\Steam and %COMMONPROGRAMFILES(X86)%\Steam directories
• Configure endpoint detection solutions to alert on oplock usage patterns targeting installation directories
• Monitor for unusual service installations or scheduled task creations following Steam installation activities
• Review system logs for unexpected SYSTEM-level process spawning during software installation windows

How to Mitigate CVE-2020-15530

Immediate Actions Required

• Update Valve Steam Client to the latest available version to ensure any patches are applied
• Restrict local user access on systems where Steam Client is installed in sensitive environments
• Consider temporarily disabling Steam Client auto-updates until the system can be secured
• Implement application whitelisting to prevent unauthorized executables from running in Steam directories

Patch Information

Valve Software has addressed this vulnerability in subsequent releases of the Steam Client. Users should ensure they are running the latest version of the Steam Client, which can be obtained through the Steam application's built-in update mechanism or by downloading the latest installer from the official Steam website. Organizations should verify that Steam Client installations are current and consider implementing automated patch management for gaming software in corporate environments.

Workarounds

• Restrict write permissions on Steam installation directories for standard users during and after installation
• Avoid installing or updating Steam Client on systems with untrusted local users
• Use dedicated, isolated systems for gaming applications where privilege escalation risks are acceptable
• Implement security monitoring to detect exploitation attempts while awaiting patches

# Configuration example - Restrict ACLs on Steam directories
icacls "%PROGRAMFILES(X86)%\Steam" /inheritance:r /grant:r "Administrators:(OI)(CI)F" /grant:r "SYSTEM:(OI)(CI)F" /grant:r "Users:(OI)(CI)RX"
icacls "%COMMONPROGRAMFILES(X86)%\Steam" /inheritance:r /grant:r "Administrators:(OI)(CI)F" /grant:r "SYSTEM:(OI)(CI)F" /grant:r "Users:(OI)(CI)RX"