A high-severity zero-day in the widely used WinRAR file compressor is under active exploitation by two Russian cybercrime groups. The attacks backdoor computers that open malicious archives attached to phishing messages, some of which are personalized.
Security firm ESET said Monday that it first detected the attacks on July 18, when its telemetry spotted a file in an unusual directory path. By July 24, ESET determined that the behavior was linked to the exploitation of an unknown vulnerability in WinRAR, a utility for compressing files and has an installed base of about 500 million. ESET notified WinRAR developers the same day, and a fix was released six days later.
Serious effort and resources
The vulnerability seemed to have super Windows powers. It abused alternate data streams, a Windows feature that allows different ways of representing the same file path. The exploit abused that feature to trigger a previously unknown path traversal flaw that caused WinRAR to plant malicious executables in attacker-chosen files paths %TEMP% and %LOCALAPPDATA%, which Windows normally makes off-limits because of their ability to execute code.
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