CVE-2021-42554: Insyde InsydeH2O Privilege Escalation Flaw

CVE-2021-42554 Overview

CVE-2021-42554 is a System Management Mode (SMM) memory corruption vulnerability in Insyde InsydeH2O UEFI firmware. The flaw resides in the FvbServicesRuntimeDxe driver and allows a local attacker with high privileges to write fixed or predictable data into System Management RAM (SMRAM). Successful exploitation enables privilege escalation to SMM, the most privileged execution context on x86 platforms. The vulnerability affects InsydeH2O Kernel versions 5.0, 5.1, 5.2, 5.3, 5.4, and 5.5 prior to the fixed builds, and is downstream-exposed in Siemens SIMATIC industrial PCs and RUGGEDCOM APE1808 devices that ship with affected firmware.

Critical Impact

Successful exploitation grants ring -2 SMM execution, bypassing OS, hypervisor, and kernel-mode protections and undermining Secure Boot trust on affected industrial and enterprise hardware.

Affected Products

• Insyde InsydeH2O Kernel 5.0 before 05.08.42, Kernel 5.1 before 05.16.42, Kernel 5.2 before 05.26.42, Kernel 5.3 before 05.35.42, Kernel 5.4 before 05.42.51, and Kernel 5.5 before 05.50.51
• Siemens SIMATIC Field PG M5/M6 and SIMATIC IPC127E, IPC227G, IPC277G, IPC327G, IPC377G, IPC427E, IPC477E, IPC627E, IPC647E, IPC677E, IPC847E, ITP1000
• Siemens RUGGEDCOM APE1808 family

Discovery Timeline

• 2022-02-03 - CVE-2021-42554 published to the National Vulnerability Database (NVD)
• 2025-11-04 - Last updated in the NVD database

Technical Details for CVE-2021-42554

Vulnerability Analysis

The vulnerability lives in FvbServicesRuntimeDxe, a DXE driver that provides Firmware Volume Block (FVB) services at runtime and exposes an SMM interface for non-volatile variable storage. SMM is the highest privilege execution mode on x86 processors, running below the operating system and hypervisor with full access to physical memory and platform hardware.

An attacker who already holds high local privileges can invoke the affected SMM handler and cause it to write attacker-influenced data to addresses inside SMRAM. While the write content is constrained to fixed or predictable values, an attacker can still corrupt SMM data structures, function pointers, or saved-state regions to redirect SMM execution.

The weakness is classified as CWE-787 (Out-of-bounds Write). Once SMM code execution is achieved, an attacker can persist below the operating system, disable firmware-based security features, and tamper with measurements used for Secure Boot and remote attestation.

Root Cause

The root cause is insufficient validation of input parameters supplied to the SMI (System Management Interrupt) handler exposed by FvbServicesRuntimeDxe. The handler trusts caller-supplied pointers or offsets without enforcing that target addresses fall outside SMRAM, enabling out-of-bounds writes into the protected SMM region.

Attack Vector

Exploitation requires local access and elevated privileges on the host operating system. An attacker triggers a software-generated SMI, marshalling crafted parameters into the communication buffer consumed by the vulnerable handler. The handler then performs the out-of-bounds write into SMRAM, providing a foothold for SMM privilege escalation. No user interaction is required, and the scope is changed because compromise of SMM impacts components beyond the originating security domain.

No public proof-of-concept code is currently associated with this CVE. Technical details are described in the Insyde Security Advisory SA-2022012 and the CERT Vulnerability Report ID 796611.

Detection Methods for CVE-2021-42554

Indicators of Compromise

• Unexpected SMI activity correlated with administrative or driver-loading events on systems running vulnerable InsydeH2O builds
• Firmware version strings reporting InsydeH2O Kernel 5.x builds below the fixed versions listed in the Insyde advisory
• Unsigned or unexpected modifications to UEFI variables managed by FvbServicesRuntimeDxe
• Failed Secure Boot or TPM measurement attestations on affected Siemens SIMATIC or RUGGEDCOM APE1808 hardware

Detection Strategies

• Enumerate firmware versions across the fleet and compare against fixed builds published in Insyde Security Advisory SA-2022012 and Siemens Security Advisory SSA-306654
• Use UEFI firmware integrity tools such as CHIPSEC to inspect SMRAM locking, SMI handler integrity, and protected range registers
• Monitor privileged process creation that loads kernel drivers capable of issuing SMIs, since exploitation requires local high-privilege code execution

Monitoring Recommendations

• Collect platform firmware telemetry, including SMBIOS firmware version and BIOS update events, into a central log store for version-drift analysis
• Enable TPM event log and Secure Boot measurement collection to identify firmware tampering attempts post-exploitation
• Alert on installation of low-level utilities or signed drivers historically abused to issue arbitrary SMIs from user mode

How to Mitigate CVE-2021-42554

Immediate Actions Required

• Inventory all systems using InsydeH2O firmware and Siemens SIMATIC or RUGGEDCOM APE1808 platforms, and identify those running InsydeH2O Kernel builds below the fixed versions
• Apply the firmware updates supplied by the original equipment manufacturer (OEM) that integrate the patched InsydeH2O Kernel build
• Restrict local administrator and kernel-driver loading privileges to limit which accounts can reach the vulnerable SMM handler
• Validate Secure Boot, BIOS write protection, and SMM locking are enabled on every affected endpoint

Patch Information

Insyde published fixed Kernel builds: 5.0 at 05.08.42, 5.1 at 05.16.42, 5.2 at 05.26.42, 5.3 at 05.35.42, 5.4 at 05.42.51, and 5.5 at 05.50.51. Hardware vendors must integrate these updates into platform firmware. Siemens delivers fixed firmware through the advisories referenced in Siemens Security Advisory SSA-306654, and NetApp tracks downstream exposure in NetApp Security Advisory NTAP-20220216-0007.

Workarounds

• No code-level workaround removes the vulnerability; firmware update is the only complete remediation
• Enforce least privilege so that non-administrative users cannot load drivers or invoke privileged firmware interfaces
• Disable unused OEM management agents that expose SMI-triggering interfaces to user-mode software
• Use platform configuration to ensure BIOS administrator passwords and firmware update authentication are enabled to deter offline tampering

# Configuration example: verify InsydeH2O firmware version on Linux
sudo dmidecode -s bios-vendor
sudo dmidecode -s bios-version
sudo dmidecode -s bios-release-date

# Confirm Secure Boot status
mokutil --sb-state