THREAT INTELLIGENCE REPORT — CVE-2021-42059

Classification: Insyde InsydeH2O UEFI Kernel — Stack-Based Buffer Overflow leading to Arbitrary Code Execution in DXE Driver

Executive Summary

CVE-2021-42059 is a stack-based buffer overflow vulnerability (CWE-787) in the Insyde InsydeH2O UEFI Kernel, affecting the DisplayTypeDxe DXE driver component. The flaw exists across Kernel branches 5.0 through 5.4 and allows a local attacker with high privileges to achieve arbitrary code execution within the UEFI firmware environment, potentially before the operating system loads.

Successful exploitation grants the attacker the ability to execute code at the firmware/privileged level, which can be used to persist malware across operating system reinstalls, bypass security controls (including Secure Boot and endpoint detection), and gain a foothold well below the OS kernel. The vulnerability carries a CVSS score of 6.7 (High) reflecting the high privileges required and the high confidentiality, integrity, and availability impact on successful exploitation. No public exploit code or PoC has been identified, and the CVE is not listed on CISA's Known Exploited Vulnerabilities (KEV) catalog.

Technical Analysis

The vulnerability exists within the Insyde InsydeH2O UEFI Kernel in the DisplayTypeDxe DXE driver. DXE (Driver Execution Environment) is a phase in the UEFI boot process where device drivers, bus drivers, and system services are initialized before the boot loader is invoked. This phase runs with the highest privilege level of any firmware component before the OS takes control.

The specific flaw is a stack-based buffer overflow (CWE-787 — Out-of-bounds Write) in the DisplayTypeDxe driver. When a buffer is allocated on the stack and data is written to it without proper bounds-checking, adjacent stack memory can be overwritten. In a UEFI DXE driver context, this can corrupt function control data such as return addresses or function pointers, allowing an attacker to redirect execution flow to attacker-controlled code.

The description indicates the overflow "leads to arbitrary code execution," meaning an attacker can fully control the program counter. The "DisplayTypeDxe" naming suggests this driver handles display type detection and configuration — parsing display-related data structures such as EDID (Extended Display Identification Data), display device enumerations, or mode-set information. A stack overflow in this context can be triggered by supplying crafted input that is processed by the driver during the UEFI boot phase.

Affected versions span four kernel branches:

• Kernel 5.0 before build 05.08.41
• Kernel 5.1 before build 05.16.41
• Kernel 5.2 before build 05.26.41
• Kernel 5.3 before build 05.35.41
• Kernel 5.4 before build 05.42.20

The National Vulnerability Database does not provide additional implementation-level details (specific function names, call chains, or input vectors beyond the driver name).

Attack surface: The attack surface is the UEFI firmware environment itself. Due to the high privilege requirement (PR:H) in the CVSS vector, an attacker must already have kernel-level or system-level access on the OS, or have the ability to modify firmware variables or configuration storage. Modifying the display configuration data that is consumed during DXE driver initialization can lead to the overflow being triggered on the next boot.

Exploitation Scenarios

Important note: No public PoC, exploit code, or exploit framework integration exists for this vulnerability as of the writing of this report. The attack chain below is inferred from the vulnerability class and the components named in the NVD description.

Step 1 — Reconnaissance and Target Identification

An attacker must first target a device that uses Insyde InsydeH2O UEFI firmware with an affected kernel version (5.0.x, 5.1.x, 5.2.x, 5.3.x, or 5.4.x below the fixed build numbers). This can be determined via:

• fwupdmgr (Linux) or Get-FirmwareType (Windows PowerShell) to identify firmware vendor.
• dmidecode output on Linux to identify the BIOS vendor and version.
• Physical inspection of the firmware settings screen ("About" section) of the OEM device.

Step 2 — Triggering the Flaw

The attacker must achieve local access with high privilege (as reflected by PR:H in the CVSS vector) to the target system. With those privileges, the attacker would modify the display-related configuration data that the DisplayTypeDxe driver consumes during the DXE phase. When the system reboots and the firmware initializes, the driver reads this crafted data into a stack buffer without proper bounds checking, leading to a stack overflow.

The malformed data can be delivered by overwriting firmware variables (e.g., via a compromised OS kernel driver) or by writing to the SPI flash containing the UEFI variable storage, if that level of access is available.

Step 3 — Payload and Execution

In a classic stack-based buffer overflow scenario, the attacker would craft the malicious data such that it overwrites the saved return address (or a function pointer) on the stack with the address of a shellcode payload. When the vulnerable function in DisplayTypeDxe returns, the program counter is redirected to the attacker's code in memory. Because the DXE phase runs at a highly privileged firmware level (ring -2 / ring -3), the executed code has complete control over the platform.

Step 4 — Post-Exploitation Impact

Once arbitrary code execution is achieved in the DXE firmware environment:

• The attacker can install persistent firmware malware (implants) that survives operating system reinstalls and even full disk encryption.
• Secure Boot bypass: Code executing from DXE can modify the Secure Boot variable store, disabling signature enforcement.
• Kernel pacification: The attacker can read and modify the OS kernel image in memory before it executes, allowing the deployment of a kernel rootkit.
• Lateral movement / supply chain compromise: If the firmware update pipeline on the device is affected, the payload could be carried over to all future firmware updates.

Impact Assessment

Business Impact: High. UEFI firmware vulnerabilities are widely considered one of the most serious classes of platform privilege escalation flaws because successful exploitation results in a maturity level of trust compromise that is below the OS kernel and below most hypervisors.

Affected Systems: Any enterprise laptop, desktop, server, embedded device, or workstation that ships with Insyde InsydeH2O firmware in the affected version ranges. OEMs that utilize this firmware include Lenovo, Dell, Acer, ASUS, and others.

Blast Radius: The CVSS vector indicates:

• Attack Vector: Local (AV:L) — Physical access or an already-compromised high-privilege session is required.
• Attack Complexity: Low (AC:L) — No special conditions beyond the crafted input are required.
• Privileges Required: High (PR:H) — The attacker must already possess elevated system access (kernel or root).
• User Interaction: None (UI:N) — No user action required.
• Confidentiality / Integrity / Availability Impact: High (C:H/I:H/A:H) — Complete compromise of data confidentiality, system integrity (including against reinstallation), and availability.

CVSS Score: 6.7 (High). While the high privilege requirement reduces the overall score, the fact that the impact subscores are all High indicates that once exploited, the potential for data theft, system compromise, and denial-of-service is maximal.

Detection Guidance

Detection properties: This vulnerability impacts the firmware realm, which operates below the OS, making traditional EDR/AV-era detection difficult. Detection focuses on (1) abnormal access to firmware and SPI flash, (2) boot-time telemetry, and (3) attempts to manipulate UEFI variable space.

Sigma Rule

title: Suspicious UEFI Variable Modification Targeting DisplayTypeDxe
id: 6f9f2c7a-2c41-4a3b-9d3f-3c8c9b9a6e12
status: experimental
description: Detects attempts to modify UEFI firmware variables related to display configuration, which may indicate preparation for exploiting CVE-2021-42059.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2021-42059
author: Valters IT Threat Intelligence
date: 2024-05-01
tags:
    - attack.privilege-escalation
    - attack.t1068
    - cve.2021-42059
logsource:
    category: process_creation
    product: windows
detection:
    selection_uefi_tools:
        Image|endswith:
            - '\fwupd.exe'
            - '\UEFITool.exe'
            - '\python.exe'
        CommandLine|contains:
            - 'setvar'
            - 'UEFI'
            - 'DisplayType'
            - 'firmware'
    selection_kernel_compromise:
        CommandLine|contains:
            - 'RTC'
            - 'NVRAM'
            - 'SPI'
            - 'flash'
    condition: selection_uefi_tools or selection_kernel_compromise
level: high
falsepositives:
    - Legitimate OEM firmware update tools
    - Administrator using firmwware management scripts

YARA Rule

rule CVE_2021_42059_DisplayTypeDxe_Overflow {
    meta:
        author = "Valters IT Threat Intelligence"
        description = "Detects strings and patterns that may appear in crafted input targeting Insyde DisplayTypeDxe driver."
        cve = "CVE-2021-42059"
        date = "2024-05-01"
    strings:
        $s1 = "DisplayType" ascii wide nocase
        $s2 = "InsydeH2O" ascii wide nocase
        $s3 = "DXE" ascii wide nocase
        $s4 = { A5 5A 5A A5 } // Common guard pattern in firmware memory
    condition:
        filesize < 1MB and 2 of them
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"CVE-2021-42059 - Potential UEFI DisplayTypeDxe Firmware Patching Attempt"; flow:established; content:"DisplayType"; depth:50; content:"Insyde"; distance:0; within:100; content:"DXE"; distance:0; within:100; sid:20214059; rev:1; metadata:cve CVE-2021-42059;)

Note: In practice, this vulnerability is exploited locally and does not generate meaningful network fingerprints. This rule would only fire if an attacker exfiltrates firmware images or patches over the network for staging.

Elastic Detection

sequence by host.id with maxspan=15s
  [process where event.type == "start" and
     process.name in ("fwupd.exe", "UEFITool.exe", "python.exe") and
     process.args: ("*setvar*", "*NVRAM*", "*SPI*")]
  [process where event.type == "start" and
     process.args: ("*DisplayType*", "*firmware*")]

Splunk SPL Query

index=windows ProcessStart
| search Image IN ("*fwupd.exe", "*UEFITool.exe", "*python.exe")
    AND (CommandLine="*setvar*" OR CommandLine="*NVRAM*" OR CommandLine="*SPI*")
    OR CommandLine="*DisplayType*"
| stats count by host, user, Image, CommandLine
| where count > 1
| table _time, host, user, Image, CommandLine, count

Wazuh OSSEC Rule

<group name="uefi,exploitation,cve_2021_42059,">
    <rule id="120059" level="12">
        <if_sid>5715</if_sid>
        <field name="win.eventdata.commandLine">|(?i)(DisplayType|InsydeH2O|UEFI)</field>
        <field name="win.eventdata.image">|(?i)(fwupd|UEFITool|python)</field>
        <description>CVE-2021-42059 - Potential UEFI firmware exploitation attempt against DisplayTypeDxe driver.</description>
        <group>uefi,exploitation,cve_2021_42059,</group>
    </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 2/4 (CLAIMED) — the patch is listed in NVD but not independently confirmed.

1 — Immediate Mitigations

• Defer if not critical: Since the vulnerability requires High Privileges (PR:H) — typically kernel or root-equivalent — the immediate risk to organizations is limited if OS-level integrity is already maintained.
• Restrict administrative access: Severely limit who can log in with root or administrator-level privileges on endpoints with Insyde H2O firmware. Implement Just-In-Time (JIT) admin elevation where possible.
• Enable Secure Boot: Though the DXE phase executes before Secure Boot enforcement for OS loading, enabling it will prevent the UEFI-implanted payloads from loading unsigned OS components.
• Monitor SPI flash writes: Deploy OS-level agent monitoring to alert on SPI flash programming or physical memory region writes that target firmware-reserved regions.

2 — Patch / Upgrade Steps

• Apply firmware update: Obtain and deploy the fixed firmware from the OEM that delivers:
  • Kernel 5.0 build 05.08.41
  • Kernel 5.1 build 05.16.41
  • Kernel 5.2 build 05.26.41
  • Kernel 5.3 build 05.35.41
  • Kernel 5.4 build 05.42.20
• Contact the OEM or vendor's hardware support channel, as the firmware binaries are distributed per-OEM. The Insyde advisory should be referenced by the OEM when they release an update.
• After patching, verify the firmware version via the UEFI setup screen or fwupdmgr get-devices on Linux.

3 — Long-Term Hardening

• Hardware Root of Trust: Deploy devices with a TPM 2.0 and enable measured boot (UEFI Secure Boot, TPM PCR measurement chain) to detect the presence of UEFI implants.
• Firmware inventory: Maintain an asset inventory of all UEFI firmware versions in use; track against the affected list and monitor OEM dashboards for security advisories.
• Zero trust for BIOS/firmware: Treat firmware as part of the trusted computing base; apply firmware updates through the same change-management process as OS patches.
• Workstation rotation: For sensitive data, consider hardware rotation to newer devices if OEM patches are not available in a timely manner.
• Physical security: Since local access is required (AV:L), ensure physical access to devices with sensitive workloads is restricted (locks, keycard access).

References

1. NVD — CVE-2021-42059: https://nvd.nist.gov/vuln/detail/CVE-2021-42059
2. Insyde Software (vendor security advisories): https://www.insyde.com/security-pledge
3. MITRE CWE-787: https://cwe.mitre.org/data/definitions/787.html
4. CISA KEV Catalog (CVE-2021-42059 NOT listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
5. EPSS Score Overview: https://www.first.org/epss/model

Threat Actor Activity: No known threat actors have been identified leveraging this specific CVE. The CVE is not present in CISA's KEV list, no associated exploit-in-the-wild reports were found, and the EPSS score (0.00346 — 27.5th percentile) suggests a low probability of current exploitation. The primary risk vector is future weaponization — a local privilege escalation path from kernel-level access to persistent firmware-level deep compromise.

Report compiled by: Valters IT — Threat Intelligence Division
Date: 2024-05-01
Classification: UNCLASSIFIED / TLP:CLEAR (see CISA definition)