Hackers make drive-by download attacks stealthier with fileless infections

New attacks with the Angler exploit kit inject code directly in browser processes without leaving files on disk, a researcher found

Cybercriminals are increasingly infecting computers with malware that resides only in memory in order to make their attacks harder to detect.

Recent attacks launched with the Angler exploit kit -- a Web-based attack tool -- injected malicious code directly into other processes and did not create malicious files on affected computers, an independent malware researcher known online as Kafeine said Sunday in a blog post.

Fileless malware threats are not new, but their use is rare, especially in large scale attacks, because they don't persist across system reboots when random access memory (RAM) is cleared.

In a typical drive-by download attack the victims visit a compromised website that redirects their browsers to an attack page -- usually an exploit kit's landing page. The exploit kit scans browsers for outdated versions of Flash Player, Adobe Reader, Java or Microsoft Silverlight and tries to exploit known vulnerabilities in those plug-ins to install malware.

The payload is usually a program called a dropper whose purpose is to download and install one or more malware programs.

The recent Angler exploits seen by Kafeine had a different final stage. Instead of installing a malware program on disk, they injected malicious code directly in the browser process, making it much harder for security software to detect the attack.

Kafeine said that his usual tools were not able to capture the payload and that it even bypassed a host-based intrusion prevention system (HIPS) he was using.

The fileless infection technique opens a wide range of possibilities for attackers as it provides a powerful way to bypass antivirus detection, it's ideal for running a one-time information stealing program and it allows them to gather information about a compromised computer before deploying a more persistent threat that defeats its defenses, he said.

"The introduction of memory-based malware is definitely a step up for cyber-criminals," said Bogdan Botezatu, a senior e-threat analyst at Bitdefender, Tuesday via email. "I didn't expect to see this technique included in a commercially-available exploit kit though, as money-driven cyber-criminals would rather trade stealth for persistence."

Malware that resides only in memory is more typical of high-profile and state-sponsored attacks, because it allows attackers to infect the target, exfiltrate information and leave no trace on disk for forensic analysis, Botezatu said.

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Lucian Constantin

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