Photo from NBU

"The nature of this malicious code, its mass distribution, and the fact that at the time of its distribution it was not detected by any anti-virus software, suggest that this attack is preparation for a mass cyber attack on the corporate networks of Ukrainian businesses," the central bank warned financial institutions earlier this month, in a letter seen by Reuters, according to BankInfoSecurity.

It added that the attacks have been spreading via malicious Microsoft Word documents attached to emails.

"In order to prevent cyber attacks, the National Bank of Ukraine consistently cooperates with banking sector participants, the State Service of Special Communication and Information Protection of Ukraine (SSCIPU), as well as relevant units of the Security Service of Ukraine and the National Police of Ukraine," a spokesman for the National Bank of Ukraine tells ISMG.

Read alsoUkrainian businesses facing threat of another cyberattack - SBU"On August 11, the NBU promptly informed banks about new malicious code, its characteristics, indicators of compromise and the need to take preventive measures to prevent the networks from being attacked by malicious codes."

The bank is also spearheading the creation of a new group that would facilitate more real-time sharing of threat intelligence across the financial services sector.

"The NBU is involved in efforts to establish the NBU Computer Security Incident Response Team (CSIRT-NBU) to respond promptly to cyber incidents and share information in real time with all the banking sector participants and law enforcement agencies," the NBU spokesman says.

Malware and ransomware have long been distributed via malicious files attached to spam emails, designed to trick recipients into executing the attachment or otherwise aid the attack (see Hello! Can You Please Enable Macros?). If such attachments do get opened, they typically function as a "dropper," downloading additional malware from an attacker-controlled server onto the by now infected, or "zombie," endpoint.

Earlier this month, Ukraine's national computer emergency response team, CERT-UA, warned that there is an elevated risk of attacks from August 20 to 25 as Ukraine celebrates its 1991 independence from the USSR.

Accordingly, CERT-UA advised organizations in Ukraine to take precautions to defend themselves against a potential reprise of the NotPetya - aka Petya-A, SortaPetya, Petna, ExPetr, GoldenEye, Nyetya, Diskcoder.C - campaign launched on June 26. Cyber police in Ukraine, as well as such security firms as Cisco Talos, ESET, Microsoft and Symantec, have said the attacks were facilitated by a "cunning backdoor" that attackers added to widely used accounting software called M.E. Doc (see NotPetya Patient Zero: Ukrainian Accounting Software Vendor).

Read alsoUkraine police make arrest in Petya ransomware case – mediaUkrainian officials have blamed Russia for launching NotPetya and other attacks. The Russian government has denied those accusations.

NATO, meanwhile, has said the attack "can most likely be attributed to a state actor."

While the timing could be coincidence, the attack was launched on the eve of Ukraine's Constitution Day, commemorating the signing of the country's constitution in 1996, following the country's 1991 independence.

NotPetya was just the latest in a series of attacks that have used malware designed to look like previously seen strains of malware. The four strains are called XData, PSCrypt, NotPetya, as well as a WannaCry lookalike (see Ukraine Power Supplier Hit by WannaCry Lookalike).

There's evidence that the same group of attackers may be behind more than one of those malware campaigns. The anti-malware researchers behind MalwareHunter Team say that backdoored M.E. Doc software was used to distribute not just NotPetya, but also the XData malware, which appeared in mid-May.