Patch diff to SYSTEM

Key Takeaways

• LLMs (like Opus 4.5/4.6) significantly accelerate patch diffing and exploit development timelines, reducing the time from vulnerability discovery to functional exploit from weeks to hours.
• A Use-After-Free (UAF) vulnerability in Windows Desktop Window Manager (dwmcore.dll) allows for Local Privilege Escalation (LPE) to SYSTEM.
• The exploit leverages the DirectComposition API and a novel 'GetRECT' heap spray technique using CRegionGeometry buffers to reclaim freed memory.
• A Control Flow Guard (CFG) bypass gadget chain utilizing __fnINSTRING (user32.dll) and CStdAsyncStubBuffer2_Disconnect (combase.dll) was used to achieve arbitrary code execution.

Affected Systems

• Windows Desktop Window Manager (DWM)
• dwmcore.dll version 10.0.26100.7309

Vulnerabilities (CVEs)

• CVE-2026-20805

Attack Chain

The attacker initializes a DirectComposition device and injects batch buffer commands to create a CSynchronousSuperWetInk object. By manipulating the LookupMode property and releasing the ink trail, a Use-After-Free condition is triggered, leaving a dangling pointer in the manager's internal vector. The attacker then uses a 'GetRECT' heap spray with CRegionGeometry to reclaim the freed memory with a fake vtable. Finally, a CFG-bypass gadget chain using __fnINSTRING and CStdAsyncStubBuffer2_Disconnect alters memory protections and executes inline shellcode to spawn a SYSTEM shell.

Detection Availability

• YARA Rules: No
• Sigma Rules: No
• Snort/Suricata Rules: No
• KQL Queries: No
• Splunk SPL Queries: No
• EQL Queries: No
• Other Detection Logic: No

The article focuses on vulnerability research and exploit development; no specific detection rules or queries are provided.

Detection Engineering Assessment

EDR Visibility: Medium — EDRs can monitor for unusual child processes spawned by dwm.exe (like cmd.exe), but the in-memory exploitation, heap spraying, and CFG bypasses within DWM might evade standard API hooking. Network Visibility: None — This is a local privilege escalation exploit with no network component described. Detection Difficulty: Hard — The exploit operates entirely in memory using legitimate Windows APIs (DirectComposition) and system DLL gadgets, making it difficult to distinguish from normal DWM behavior until the final payload execution.

Required Log Sources

• Process Creation Logs (Event ID 4688 / Sysmon Event ID 1)

Hunting Hypotheses

Control Gaps

• In-memory CFG bypasses using legitimate system DLL gadgets (__fnINSTRING and CStdAsyncStubBuffer2_Disconnect)

Key Behavioral Indicators

• dwm.exe spawning cmd.exe or other suspicious child processes

False Positive Assessment

• Low

Recommendations

Immediate Mitigation

• Apply the relevant Windows security updates patching the DWM vulnerabilities (e.g., January 2026 Patch Tuesday updates).

Infrastructure Hardening

• Ensure Control Flow Guard (CFG) and other memory protections are enabled globally.

User Protection

• Implement endpoint security controls to monitor and block suspicious child processes originating from system processes like dwm.exe.

Security Awareness

• Educate security teams on the accelerating pace of exploit development facilitated by LLMs, emphasizing the need for rapid patch deployment.

MITRE ATT&CK Mapping

• T1068 - Exploitation for Privilege Escalation
• T1106 - Native API

Additional IOCs

• File Paths:
  • dwmcore.dll - Vulnerable Windows Desktop Window Manager component containing the UAF
  • user32.dll - System DLL used for the __fnINSTRING CFG bypass gadget
  • combase.dll - System DLL used for the CStdAsyncStubBuffer2_Disconnect CFG bypass gadget
• Command Lines:
  • Purpose: Inline shellcode execution payload to spawn a command prompt | Tools: cmd.exe, WinExec | Stage: Execution | cmd.exe