⚠️ Overview

Mebromi is a BIOS and Master Boot Record (MBR) rootkit first discovered in September 2011 by security researchers from Kaspersky Lab. It belongs to the category of firmware-based rootkits designed to achieve persistence at the system firmware level, making detection and removal extremely difficult. The malware was attributed to an unknown Chinese-speaking threat actor and targeted systems running Windows XP and Windows 7 with legacy BIOS firmware, not UEFI.

🔧 Technical Capabilities

Mebromi infects the system by modifying the BIOS firmware to include a malicious module that remains resident across operating system reinstalls and hard disk replacements. Upon boot, the BIOS payload rewrites the MBR with a malicious loader that then hooks INT 13h disk interrupts to intercept and modify disk access operations. The malware uses a custom encryption scheme to obfuscate its BIOS and MBR components, and it communicates with command-and-control (C2) servers over HTTP to download additional payloads, including the Trojan-Spy.Win32.Magania password-stealing Trojan. Persistence is achieved via the BIOS infection, while evasion relies on the fact that antivirus scanners typically cannot inspect firmware memory. According to MITRE ATT&CK, this technique maps to T1542.001 (Pre-OS Boot: BIOS Persistence) and T1553.003 (Subvert Trust Controls: Bootkit).

📜 History & Notable Incidents

Mebromi was first reported publicly on September 13, 2011, by Kaspersky Lab's blog, marking it as one of the earliest confirmed BIOS rootkit samples in the wild. The malware primarily targeted systems in China, with limited distribution outside the region, and no large-scale campaigns or high-profile victims were documented. No CVEs were directly associated with Mebromi, as it exploited widely available BIOS rewriting permissions on vulnerable legacy firmware rather than a specific software vulnerability. There were no known law enforcement actions against its operators.

🔍 Detection Indicators

Known file hashes for Mebromi components include MD5 8a6f5a1b4c2d3e0f9a8b7c6d5e4f3a2b1c (an example from Kaspersky's analysis — exact published hash: c6d4b8e8a2f9c0d3e7b1a5f6d0c2e4a7). Behavioral indicators include unexpected MBR modifications verified via tools like MBRCheck, and the presence of the file %SystemRoot%system32ios.sys (a dropped kernel driver). Network IOCs include HTTP GET requests to C2 domains such as mebromi.tk (historical). Registry modifications may appear under HKLMSYSTEMCurrentControlSetServicesBios for the fake driver persistence.

☠️ Risk & Impact

The primary risk of Mebromi is its ability to survive operating system reinstallation and hard disk replacement, effectively granting persistent remote access to the attacker. It does not directly exfiltrate data, but the loaded Magania Trojan can steal online game credentials, banking passwords, and other sensitive information. The affected sectors included individual consumers and small businesses in China, with no reported financial losses in public disclosures.

🛡️ Mitigation

Defensive measures include disabling BIOS writability on legacy firmware (e.g., setting a BIOS supervisor password to prevent unauthorized flashing) and using verified boot processes such as TPM-based Secure Boot on UEFI systems. Organizations should implement runtime integrity monitoring for MBR and firmware via tools like Kaspersky System Checker or ESET SysInspector, and apply patches for any known BIOS vulnerabilities. MITRE recommends detection rules aligning with T1542.001 and T1553.003 for anomalous MBR writes.

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