Researchers have uncovered what they say is the very first malware to achieve persistence in Microsoft Exchange email servers, which allows attackers to secretly execute commands via malicious emails featuring attachments with hidden code.

Dubbed LightNeuron, the furtive backdoor has been targeting Exchange servers since at least 2014, according to a blog post from ESET, whose researchers have provisionally linked the threat to the Russian cyber espionage group Turla. ESET discovered the backdoor on three victims: an unidentified Brazilian organization, a Ministry of Foreign Affairs in Eastern Europe and a regional diplomatic organization in the Middle East.

In addition to the confirmed Windows-based version, ESET believes there may be a Linux variant in use as well, based on artifacts turned up during its investigation.

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