Insyde UEFI Flaw Enables Digital Certificate Injection via NVRAM Variable

A critical vulnerability (CVE-2025-4275) in Insyde H2O UEFI firmware allows attackers to bypass Secure Boot protections by injecting malicious digital certificates via an unprotected NVRAM variable.

Dubbed Hydroph0bia, this flaw enables pre-boot execution of unsigned code, posing severe risks to enterprise and consumer devices.

Insecure NVRAM Variable Handling

The vulnerability stems from the improper use of the SecureFlashCertData NVRAM variable, which stores public keys used in UEFI trust validation.

Insyde H2O firmware relies on this variable to exchange certificates between DXE drivers and verification libraries, like LoadImage and StartImage.

However, the variable lacks write-protection, allowing attackers with administrative privileges to overwrite it with rogue certificates.

Key technical factors:

• UEFI Secure Boot Bypass: Injected certificates are trusted during early boot, enabling execution of malicious UEFI applications.
• Library Function Flaw: Common functions LibGetVariable fail to validate whether the variable is volatile or firmware-set, permitting OS-level tampering.
• Persistence Mechanism: Compromised systems may retain malware across reboots and OS reinstalls due to firmware-level persistence.

From Certificate Injection to Rootkit Deployment

Attackers exploit this flaw in three stages:

1. Certificate Injection:
   • On Windows: Use PowerShell or SetFirmwareEnvironmentVariable to write a malicious certificate to SecureFlashCertData.
   • On Linux: Modify the variable via efivarfs: bashecho -n "malicious_cert_data" > /sys/firmware/efi/efivars/SecureFlashCertData-$(uuidgen)
2. Malicious Payload Execution:
   During boot, the firmware validates attacker-signed UEFI binaries (e.g., cloak.dat), enabling bootkits like BlackLotus.
3. Evasion:
   Malware operates below the OS layer, avoiding detection by EDR tools and surviving disk wipes.

Mitigation Strategies and Firmware Updates

To address CVE-2025-4275:

Action	Implementation
Firmware Updates	Apply vendor-provided patches locking SecureFlashCertData.
UEFI Variable Locking	Use VariablePolicyLib to enforce write restrictions on critical NVRAM variables.
Runtime Monitoring	Deploy firmware integrity tools (e.g., Binarly efiXplorer) to detect tampering.

Risk Factor Analysis

Factor	Details	CVSS Score
Likelihood	High (requires local admin access; exploit tools publicly available)
Severity	Critical (full system compromise, persistence across OS reinstalls)	8.2 (High)
Impact	Loss of confidentiality, integrity, and availability at firmware level

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Vendor Response and Industry Implications

Insyde has released patches (INSYDE-SA-2025002) and notified OEM partners, but widespread remediation depends on device manufacturers issuing firmware updates.

This vulnerability underscores systemic risks in UEFI supply chains, where shared codebases propagate flaws across vendors like HP, Dell, and Lenovo.

Security teams should prioritize firmware updates and adopt UEFI-aware threat detection frameworks to mitigate pre-boot attack surfaces.

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