Ukrainian police on Tuesday seized the servers of an accounting software firm suspected of spreading a malware virus which crippled computer systems at major companies around the world last week, a senior police official said.
Ukraine's Cyber Police head Serhiy Demedyuk said the servers of M.E.Doc – Ukraine's most popular accounting software – had been seized as part of an investigation into the attack.
Though they are still trying to establish who was behind last week's attack, Ukrainian intelligence officials and security firms have said some of the initial infections were spread via a malicious update issued by M.E.Doc, charges the company's owners deny.
The owners were not immediately available for comment on Tuesday.
Premium Service, which says it is an official dealer of M.E.Doc's software, wrote a post on M.E.Doc's Facebook page saying masked men were searching M.E.Doc's offices and that the software firm's servers and services were down.
Premium Service could not be reached for further comment.
Cyber Police spokeswoman Yulia Kvitko said investigative actions were continuing at M.E.Doc's offices, adding that further comment would be made on Wednesday.
Planned months in advance
The police move came after cybersecurity investigators unearthed further evidence on Tuesday that the attack had been planned months in advance by highly-skilled hackers, who they said had inserted a vulnerability into the M.E.Doc progamme.
Researchers at Slovakian security software firm ESET said they had found a "backdoor" written into some of M.E.Doc's software updates, likely with access to the company's source code, which allowed hackers to enter companies' systems undetected.
"We identified a very stealthy and cunning backdoor that was injected by attackers into one of M.E.Doc's legitimate modules," ESET senior malware researcher Anton Cherepanov said in a technical note. "It seems very unlikely that attackers could do this without access to M.E.Doc's source code."
"This was a thoroughly well-planned and well-executed operation," he said.
ESET said at least three M.E.Doc updates had been issued with the "backdoor vulnerability", and the first one was sent to clients on April 14, more than two months before the attack.
Little known outside Ukrainian accounting circles, M.E.Doc is used by around 80 percent of companies in Ukraine. The software allows its 400,000 clients to send and collaborate on financial documents between internal departments, as well as file them with the Ukrainian state tax service.
Ukraine also took steps on Tuesday to extend its state tax deadline by one month to help businesses hit by the malware assault.On Saturday Ukrainian intelligence officials accused Russian security services of being behind the attack, and cyber security researchers linked it to a suspected Russian group who attacked the Ukrainian power grid in December 2016.
A Kremlin spokesman dismissed charges of Russian involvement as "unfounded blanket accusations".