By Jack Stubbs

MOSCOW (Reuters) – Technical indicators suggest a cyber attack which hit Russia and other countries this week was carried out by hackers behind a similar but bigger assault on Ukraine in June, security researchers who analyzed the two campaigns said on Wednesday.

Russia-based cyber firm Group-IB said the BadRabbit virus used in this week’s attack shared a key piece of code with the NotPetya malware that crippled businesses in Ukraine and worldwide earlier this year, suggesting the same group was responsible.

The BadRabbit attack hit Russia, Ukraine and other countries on Tuesday, taking down Russia’s Interfax news agency and delaying flights at Ukraine’s Odessa airport.

Multiple cyber security investigators have linked the two attacks, citing similarities in the malware coding and hacking methods, but stopped short of direct attribution.

Still, experts caution that attributing cyber attacks is notoriously difficult, as hackers regularly use techniques to cover their tracks and sometimes deliberately mislead investigators about their identity.

Security researchers at Cisco’s Talos unit said BadRabbit bore some similarities with NotPetya as they were both based on the same malware, but large parts of code had been rewritten and the new virus distribution method was less sophisticated.

They confirmed BadRabbit used a hacking tool called Eternal Romance, believed to have been developed by the U.S. National Security Agency (NSA) before being stolen and leaked online in April.

NotPetya also employed Eternal Romance, as well as another NSA tool called Eternal Blue. But Talos said they were used in a different way and there was no evidence Bad Rabbit contained Eternal Blue.

“It is highly likely that the same group of hackers was behind (the) BadRabbit ransomware attack on Oct. 25, 2017 and the epidemic of the NotPetya virus, which attacked the energy, telecommunications and financial sectors in Ukraine in June 2017,” Group-IB said in a technical report.

Matthieu Suiche, a French hacker and founder of the United Arab Emirates-based cyber security firm Comae Technologies, said he agreed with the Group-IB assessment that there was “serious reason to consider” that BadRabbit and NotPetya were created by the same people.

But some experts have said the conclusion is surprising as the NotPetya attack is widely thought to have been carried out by Russia, an allegation Moscow denies.

Ukrainian officials have said the NotPetya attack directly targeted Ukraine and was carried about by a hacking group widely known as Black Energy, which some cyber experts say works in favor of Russian government interests. Moscow has repeatedly denied carrying out cyber attacks against Ukraine.

The majority of BadRabbit’s victims were in Russia, with only a few in other countries such Ukraine, Bulgaria, Turkey and Japan.

Group-IB said some parts of the BadRabbit virus dated from mid-2014, however, suggesting the hackers used old tools from previous attacks. “This corresponds with BlackEnergy timeframes, as the group started its notable activity in 2014,” it said.

(Additional reporting by Eric Auchard; Editing by Jim Finkle/Mark Heinrich)