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Emotet, a notorious email-based malware behind several botnet-driven spam campaigns and ransomware attacks, contained a flaw that allowed cybersecurity researchers to activate a kill-switch and prevent the malware from infecting systems for six months.

“Most of the vulnerabilities and exploits that you read about are good news for attackers and bad news for the rest of us,” Binary Defense’s James Quinn said.

“However, it’s important to keep in mind that malware is software that can also have flaws. Just as attackers can exploit flaws in legitimate software to cause harm, defenders can also reverse-engineer malware to discover its vulnerabilities and then exploit those to defeat the malware.”

The kill-switch was alive between February 6, 2020, to August 6, 2020, for 182 days, before the malware authors patched their malware and closed the vulnerability.

Since its first identification in 2014, Emotet has evolved from its initial roots as a banking malware to a “Swiss Army knife” that can serve as a downloader, information stealer, and spambot depending on how it’s deployed.

Early this February, it developed a new feature to leverage already infected devices to identify and compromise fresh victims connected to nearby Wi-Fi networks.

Along with this feature update came a new persistence mechanism, according to Binary Defense, which “generated a filename to save the malware on each victim system, using a randomly chosen exe or dll system filename from the system32 directory.”

The change in itself was straight-forward: it encrypted the filename with an XOR key that was then saved to the Windows registry value set to the victim’s volume serial number.

The first version of the kill-switch developed by Binary Defense, which went live about 37 hours after Emotet unveiled the above changes, employed a PowerShell script that would generate the registry key value for each victim and set the data for each value to null.

images from Hacker News