An anonymous reader quotes a report from Bleeping Computer: Today's massive ransomware outbreak was caused by a malicious software update for M.E.Doc, a popular accounting software used by Ukrainian companies. According to several researchers, such as Cisco Talos, ESET, MalwareHunter, Kaspersky Lab, and others, an unknown attacker was able to compromise the software update mechanism for M.E.Doc's servers, and deliver a malicious update to customers. When the update reached M.E.Doc's customers, the tainted software packaged delivered the Petya ransomware -- also referenced online as NotPetya, or Petna. The Ukrainian software vendor appears to have inadvertently confirmed that something was wrong when, this morning, issued a security advisory. Hours later, as the ransomware outbreak spread all over Ukraine and other countries across the globe causing huge damages, M.E.Doc denied on Facebook its servers ever served any malware. According to security researcher MalwareHunter, this is not the first time M.E.Doc has carried a malicious software update that delivered ransomware. Back in May, the company's software update mechanism also helped spread the XData ransomware.