CVE-2021-42060

Executive Summary

CVE-2021-42060 is a System Management Mode (SMM) callout vulnerability in Insyde InsydeH2O firmware affecting multiple kernel versions across a wide range of platforms. The vulnerability resides in the Int15ServiceSmm component and allows an attacker with high privileges to hijack execution flow of code running in SMM, leading to complete compromise of the System Management Interrupt (SMI) handler environment. Successful exploitation results in privilege escalation to the most privileged execution mode on the platform (SMM), bypassing all operating system-level security controls and enabling persistent firmware-level rootkits. This vulnerability affects a broad ecosystem of consumer and enterprise devices leveraging InsydeH2O firmware.

Technical Analysis

The vulnerability is classified as an SMM callout vulnerability in the Int15ServiceSmm component of the InsydeH2O Kernel. SMM is the most privileged operating mode on x86/x64 platforms, running in a separate address space that is invisible to the operating system and hypervisor. Code executing in SMM is triggered via System Management Interrupts (SMIs), typically through legacy INT 15h BIOS services that are intercepted and serviced by firmware handlers.

The flaw involves a callout — a term used when SMM code invokes functions outside of SMM's protected memory context. This violates the fundamental secure coding practice for SMM code, which mandates that all operations remain within SMM's private memory space, using only communication buffers validated against untrusted system memory. When SMM code performs a callout into non-SMM code or calls functions that operate on pointers supplied from outside SMM, an attacker who controls the relevant memory contents can redirect the execution flow.

The attack surface is the Interface 15H (INT 15h) BIOS interrupt handler path. This legacy interrupt is used for various system services — memory map queries, E820 map retrieval, and system configuration access. On modern UEFI systems, these services are emulated or redirected through SMM handlers. The Int15ServiceSmm module specifically handles these requests within SMM context.

The vulnerability mechanism involves an SMM handler that performs a callout while processing an INT 15h request. The callout pattern typically involves passing pointers to memory in the victim's address space (outside SMM) and then calling functions that dereference those pointers. If the attacker can pre-position malicious data at the expected memory locations, the handler executes attacker-controlled code in SMM context.

The affected versions span four Kernel major releases:

• Kernel 5.0 through 05.08.41
• Kernel 5.1 through 05.16.41
• Kernel 5.2 before 05.23.22
• Kernel 5.3 before 05.32.22

This broad version range indicates the vulnerability existed in the codebase for an extended period, potentially affecting an enormous number of devices across multiple manufacturers that license InsydeH2O firmware, including major PC OEMs and embedded system vendors.

Exploitation Scenarios

Based strictly on the information provided in the NVD description, exploitation requires the attacker to already possess high privileges (CVSS PR:H). This indicates the attacker has already achieved kernel-level code execution on the target system. The following attack chain is described at the vulnerability-class level:

Step 1: Target Identification & Reconnaissance The attacker identifies a system running affected InsydeH2O firmware. This requires knowledge of the system's firmware vendor and version. Tools such as fwts (Firmware Test Suite), chipsec, or direct querying of SMBIOS tables can reveal the firmware vendor and version. Enterprises with heterogeneous device fleets are particularly attractive targets as they typically contain multiple models with similar firmware versions.

Step 2: Gaining Initial Privilege The attacker must first achieve arbitrary code execution in kernel mode (Ring 0). This can be accomplished through:

• Exploiting a separate kernel driver vulnerability
• Loading an unsigned driver if Secure Boot is misconfigured or disabled
• Using already-known kernel exploits relevant to the specific OS version

Step 3: Triggering the SMM Callout Vulnerability With kernel-level access, the attacker prepares the necessary memory structures and invokes an INT 15h call designed to reach the vulnerable SMM handler. The trigger mechanism involves sending an SMI request through the kernel but over the communication channel used by the vulnerable SMM handler — since the handler is responsible for servicing a system-management mode interrupt generated by INT 15h operations. Exact trigger details are not available in the public description.

Step 4: Hijacking Execution Flow in SMM Once the SMM handler is invoked, the attacker's pre-positioned data causes the handler to perform a callout to attacker-controlled memory. Because SMM code executes with the highest privilege level and is invisible to the operating system, any change made in SMM persists beyond the SMI handler's lifetime.

Step 5: Post-Exploitation — Persistent Firmware-Level Compromise After successful exploitation, the attacker achieves full control of the platform's system management mode. Capabilities include:

• Reading and modifying SMRAM contents (including the SMI handler code itself)
• Installing a persistent SMM rootkit that survives OS reinstalls and disk replacement
• Bypassing virtualization-based security (VBS), Credential Guard, and Device Guard
• Intercepting and manipulating all system memory transactions
• Disabling or subverting firmware-level security features

No public PoC or exploit code is currently known; the attack chain is inferred from the vulnerability class and CVSS vector.

Impact Assessment

The CVSS score of 8.2 (High) reflects the severe consequences of this vulnerability. The vector — AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H — indicates:

• Attack Vector (Local): Requires physical or local access to the target device — the attacker must already be operating within the target environment
• Attack Complexity (Low): No special conditions or race conditions needed; the exploitation path is straightforward
• Privileges Required (High): Kernel-level privileges are prerequisite — this is a privilege escalation, not a remote code execution
• User Interaction (None): No user interaction required — automatic exploitation possible once prerequisites are met
• Scope (Changed): The vulnerability allows crossing a security boundary beyond the vulnerable component — from kernel mode to SMM, a distinct trust domain
• CIA Impact (High/High/High): Complete loss of confidentiality, integrity, and availability

The business impact is significant:

Affected Ecosystem: InsydeH2O firmware is used by dozens of OEMs including major laptop manufacturers, desktop vendors, and embedded systems. A vulnerability in the kernel affects all platforms using the affected versions, creating a supply chain blast radius spanning multiple manufacturer product lines.

Trust Boundary Violation: The SMM → OS boundary is one of the last remaining trust boundaries on modern systems. Compromise of this boundary undermines all security mechanisms built on top — Secure Boot, TPM attestation, hypervisor protections, and OS security features.

Persistent Threat: Unlike memory corruption in OS kernel components, SMM compromise persists across reboots, OS reinstalls, and even firmware updates (if the update mechanism is subverted). The attacker gains a stealthy, persistent foothold that is exceptionally difficult to detect or remove.

Impact on Secure Boot/Attestation: Enterprise environments relying on hardware-backed attestation (e.g., Windows Defender System Guard) are directly undermined if SMM is compromised.

Detection Guidance

Sigma Rule

title: Suspicious INT 15h SMI Trigger Activities
id: 4a4a1c45-3e2c-4a69-b7dd-7a19dc37a123
status: experimental
description: Detects potential attempts to trigger SMM callout vulnerability in InsydeH2O via unusual INT 15h activity from kernel drivers
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2021-42060
author: Valtersit Threat Intelligence
date: 2024/01/15
logsource:
    category: driver_load
    product: windows
    definition: Requires Sysmon or ETW-based driver load logging
detection:
    selection_driver:
        EventID: 6
        Image|endswith: '.sys'
        Hashes|contains: 'unknown'
    selection_int15:
        EventID:
            - 1  # Process creation (if triggered from user mode)
            - 11  # File create (for rootkit persistence)
    filter:
        Image|startswith:
            - 'C:\\Windows\\System32\\'
            - 'C:\\Windows\\SysWOW64\\'
    condition: selection_driver and not filter
    timeframe: 10m
level: high
falsepositives:
    - Legitimate firmware update utilities making INT 15h calls
    - Compatibility layers (DOS emulation)
tags:
    - attack.privilege_escalation
    - attack.t1068

YARA Rule

rule InsydeH2O_SMM_Callout_Binary_Signature
{
    meta:
        author = "Valtersit Threat Intelligence"
        description = "Detects binary artifacts associated with exploitation of CVE-2021-42060 SMM callout"
        cve = "CVE-2021-42060"
        date = "2024-01-15"
        severity = "high"

    strings:
        $smm_string = "SMM" ascii wide
        $int15_string = "Int15ServiceSmm" ascii wide
        $memory_map = "E820" ascii wide
        $smi_port = { B2 00 }
        $smram_desc = "SMRAM" ascii wide
        $prm_string = "PRM" ascii wide

    condition:
        uint16(0) == 0x5A4D and  // MZ header
        (7 of them) or
        ($int15_string and ($smm_string or $smram_desc))
}

Suricata IDS Rule

# Network-based detection is not applicable for this local vulnerability.
# The following rule is illustrative for detecting SMI-related traffic on management interfaces
# or remote management tools that might be used to stage attacks.
alert ip any any -> any any (msg:"Potential SMM exploitation - POSSIBLE MANAGEMENT TRAFFIC ANOMALY"; 
    content:"|5c 00 73 00 79 00 73 00 74 00 65 00 6d 00 5c 00|"; 
    nocase; 
    sid:4206001; 
    rev:1; 
    classtype:attempted-administrator;
    metadata:cve CVE-2021-42060;
    reference:url,nvd.nist.gov/vuln/detail/CVE-2021-42060;)

Elastic Detection

// EQL rule for detecting unusual behavior in firmware update processes or kernel driver activity
sequence by process.entity_id with maxspan=5m
  [process where event.type == "start" and 
   (process.name : "*update*" or process.name : "*flash*" or process.name : "*bios*") and
   process.code_signature.trusted == false]
  [file where event.type == "creation" and 
   file.extension == "efi" and 
   file.path : "*Temp*"]
  [process where event.action == "already_running" and 
   process.thread.id != 0 and
   not process.name : "System"]

Splunk SPL Query

index=windows EventCode=6 OR EventCode=1
| where Image LIKE "%driver%" OR Image LIKE "%.sys"
| search "SMI" OR "INT15" OR "SMM" OR "firmware"
| eval CVE="CVE-2021-42060"
| stats count by host, Image, CommandLine, EventCode
| where count > 5
| sort - count
| table _time, host, Image, CommandLine, EventCode, count

Wazuh OSSEC Rule

<group name="cve-2021-42060,">
    <rule id="420601" level="12">
        <decoded_as>syscheck</decoded_as>
        <field name="file" type="pcre2">(?i)(\.efi|\.sys)$</field>
        <description>CVE-2021-42060 - Unusual firmware or kernel driver modification detected</description>
        <group>cve-2021-42060,</group>
    </rule>
    <rule id="420602" level="10">
        <if_sid>420601</if_sid>
        <match>added|modified</match>
        <description>CVE-2021-42060 - New or modified EFI/SYS file present - possible SMM exploitation artifact</description>
        <group>cve-2021-42060,</group>
    </rule>
    <rule id="420603" level="14">
        <if_sid>420601</if_sid>
        <match>deleted</match>
        <description>CVE-2021-42060 - Security critical file removed - possible firmware compromise</description>
        <group>cve-2021-42060,</group>
    </rule>
    <rule id="420604" level="8">
        <if_sid>4402</if_sid>
        <match>SMM|SMI|INT15</match>
        <description>CVE-2021-42060 - Potential SMM related event detected in system log</description>
        <group>cve-2021-42060,</group>
    </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 2/4 (CLAIMED) — A single source (NVD) indicates patched versions exist (Kernel 5.2 ≥ 05.23.22, Kernel 5.3 ≥ 05.32.22). This is not independently confirmed by vendor advisories, multiple sources, or empirical testing.

Immediate Mitigations (no patch applied yet):

1. Restrict Local Access: Enforce strict physical and remote access control to affected systems. This vulnerability requires local access and prior high privileges, so limiting who can log in directly reduces attack surface
2. Enable/Enforce Secure Boot: Ensure Secure Boot is enabled to prevent unauthorized code loading at boot, which increases the difficulty of initial compromise
3. Enable Virtualization-Based Security (VBS): If available, enable HVCI (Hypervisor-protected Code Integrity) and Credential Guard to raise the bar for kernel code execution
4. Driver Signing Enforcement: Ensure all kernel drivers are signed and enforce driver signature validation
5. Monitor and Audit Kernel-Level Activity: Deploy EDR solutions that monitor for kernel driver loading anomalies and unusual SMI/SMM activity

Patch/Upgrade Steps:

1. Contact your device OEM (original equipment manufacturer) — do NOT attempt to flash firmware directly from Insyde, as firmware is customized per OEM
2. Check OEM firmware update portal/CVEs feed for firmware releases that incorporate Insyde Kernel 5.2 ≥ 05.23.22 or Kernel 5.3 ≥ 05.32.22
3. Before deployment in enterprise environments, test firmware updates in a staging environment with representative hardware
4. Apply firmware updates through the OEM's recommended update mechanism (e.g., Windows Update, vendor update utility, BIOS flash from within OS)
5. After update, verify firmware version using wmic bios get smbiosbiosversion (Windows) or dmidecode -t bios (Linux)
6. Re-enable Secure Boot if the firmware update process disabled it

Long-Term Hardening:

1. Establish a firmware asset management program to track BIOS/UEFI versions across the enterprise fleet
2. Ensure firmware updates are included in the regular patch management cycle, not treated as optional
3. Consider hardware attestation solutions that verify SMM integrity at boot (e.g., Boot Guard, DRTM-based attestation)
4. For high-security environments, evaluate the use of hardware with verified SMM lockdown features
5. Monitor vendor security advisories for Insyde and all other firmware vendors
6. Require firmware integrity verification in your security compliance programs

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2021-42060
2. Insyde Software Security Advisory: https://www.insyde.com/security-pledge
3. MITRE CVE Record: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-42060
4. FIRST EPSS (Exploit Prediction Scoring System): https://www.first.org/epss/

Threat Actor Campaigns & Exploit-in-the-Wild Evidence: As of this report's date, there is no evidence of exploitation in the wild for CVE-2021-42060. The EPSS score is 0.00351 (27.9th percentile), indicating a very low predicted probability of exploitation in the next 30 days. The CVE has NOT been added to CISA's Known Exploited Vulnerabilities (KEV) catalog. However, the vulnerability class (SMM callout) has been exploited in past real-world campaigns — notably by sophisticated actors targeting managed enterprise endpoints for persistent firmware-level access. The historical precedent of such exploitation reinforces the need for prompt remediation despite the lack of current publicly known exploitation.