Lazarus Group Uses New Tactic to Evade Detection

Attackers conceal malicious code within a BMP file to slip past security tools designed to detect embedded objects within images.



Security researchers with Malwarebytes have observed North Korea-affiliated advanced persistent threat actor Lazarus Group employing a new technique to deliver malware while evading security tools.

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Lazarus Group, an active and sophisticated group known for attacking targets around the world, recently expanded its primary mission beyond monetary theft to include stealing defense secrets. The group is known for developing custom malware families and using novel tactics.

One of its newest methods involves embedding a malicious HTML Application (HTA) file within a compressed zlib file, within a PNG file. During run time, the PNG file is converted into a BMP file format. Because the BMP file is uncompressed, converting from PNG to BMP automatically decompresses the malicious zlib object. Researchers call this a clever way to evade detection. Because the malicious object is compressed within the PNG image, it bypasses static detection.

This attack likely started with a phishing campaign in which emails arrives with a malicious file attached. When opened, the file prompts its viewer to enable macros. Doing this will lead to a message box; clicking this will load the final phishing lure — a participation form for a fair in a South Korean city. The document is weaponized with a macro that executes when it's opened.

While attribution is consistently a challenge in cyberattacks, the team found several signs that connect this activity with Lazarus Group, as outlined in a blog post on their findings.

"There are several similarities between this attack and past Lazarus operations and we believe these are strong indicators to attribute this attack to the Lazarus threat actor," writes Hossein Jazi, senior threat intelligence analyst.

Read the full blog post for more information.

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