CVE-2020-5956
📋 TL;DR
This vulnerability in Insyde InsydeH2O firmware's SdLegacySmm SMI handler allows attackers to execute arbitrary code with System Management Mode (SMM) privileges by exploiting insufficient input validation of the CommBuffer. It affects systems running InsydeH2O firmware with specific kernel versions, potentially enabling firmware-level compromise.
💻 Affected Systems
- Insyde InsydeH2O firmware
📦 What is this software?
Insydeh2o by Insyde
Insydeh2o by Insyde
Insydeh2o by Insyde
Insydeh2o by Insyde
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SMM-level privileges, allowing persistent firmware-level malware installation that survives OS reinstallation and disk replacement.
Likely Case
Local privilege escalation to SMM level, enabling firmware persistence, bypassing security controls, and potentially accessing protected memory regions.
If Mitigated
Limited impact if SMM access is restricted through hardware security features like Intel Boot Guard or AMD Hardware Validated Boot.
🎯 Exploit Status
Requires local code execution capability to trigger SMI. SMM exploitation requires specialized knowledge of firmware internals.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Kernel 5.1 05.15.11, 5.2 05.25.11, 5.3 05.34.11, 5.4 05.42.11 or later
Vendor Advisory: https://www.insyde.com/security-pledge
Restart Required: Yes
Instructions:
1. Contact your hardware/OEM vendor for firmware updates. 2. Download appropriate firmware update from vendor. 3. Apply firmware update following vendor instructions. 4. Reboot system to activate new firmware.
🔧 Temporary Workarounds
Restrict SMM Access
allConfigure hardware security features to restrict SMM access if supported by platform
🧯 If You Can't Patch
- Implement strict access controls to prevent unauthorized local code execution
- Monitor for suspicious SMI/SMM activity using firmware security monitoring tools
🔍 How to Verify
Check if Vulnerable:
Check firmware version in BIOS/UEFI settings or using manufacturer-specific tools. Compare against affected version ranges.Check Version:
Manufacturer-specific commands vary. Common methods: wmic bios get smbiosbiosversion (Windows), dmidecode -t bios (Linux), or check BIOS/UEFI setup.Verify Fix Applied:
Verify firmware version after update shows patched version (05.15.11, 05.25.11, 05.34.11, or 05.42.11+ depending on kernel).📡 Detection & Monitoring
Log Indicators:
- Unusual SMI/SMM activity in firmware logs
- Unexpected firmware modification attempts
Network Indicators:
- Not network exploitable - local vulnerability
SIEM Query:
Search for firmware update events followed by unexpected system behavior or privilege escalation attempts