The Main Intelligence Directorate (GUR) of Ukraine’s Ministry of Defense claims it hacked the Russian aerospace and defense company Tupolev, which develops Russia’s supersonic strategic bombers.

According to Ukrainian news outlets, a source within GUR said the military intelligence hackers breached Tupolev’s systems and stole 4.4 gigabytes of classified information.

This stolen data includes personal data of Tupolev personnel, internal communications (including messages exchanged by the company’s management), procurement documents, resumes of engineers and designers, and minutes of closed meetings.

While the source didn’t disclose when Tupolev’s systems were breached, it said the HUR hackers were inside Tupolev’s network for a long time, sufficient to collect other information that could be used in future operations to compromise other organizations in Russia’s defense sector.

“The value of the data obtained is hard to overstate. There is now virtually nothing secret left in Tupolev’s operations as far as Ukrainian intelligence is concerned,” the anonymous source told Kyiv Post.

“We now have comprehensive information on individuals directly involved in maintaining Russia’s strategic aviation. The effects of this operation will be felt both on the ground and in the skies.”

They also defaced Tupolev’s official website and added an image of an owl with an airplane in its claws. At the moment, the website redirects to the site of United Aircraft Corporation (UAC), a state-owned company created in 2006 to merge aircraft manufacturers Tupolev, Mikoyan, Ilyushin, Irkut, Sukhoi, and Yakovlev under a single brand.

A United Aircraft Corporation spokesperson was not immediately available for comment when contacted by BleepingComputer earlier today.

​This follows an operation by Ukraine’s Security Service (SBU) that used first-person-view (FPV) drones to hit 41 warplanes at four Russian airfields, including A-50 surveillance planes and Tu-160, Tu-22, and Tu-95s strategic bombers.

Previously, GUR claimed it breached the servers of the Russian Ministry of Defense (Minoborony) and stole sensitive documents containing secret service information.

Ukraine’s military intelligence has also claimed unconfirmed breaches into the Russian Federal Air Transport Agency(Rosaviatsia), the Russian Center for Space Hydrometeorology, and the Russian Federal Taxation Service(FNS). In two of these attacks, GUR said its operatives also destroyed databases and wiped servers storing data backups for additional operational disruption.

Ukrainian hacktivists have also been targeting Russian organizations since Russia invaded Ukraine in February 2014. For instance, in January, the Ukrainian Cyber Alliance (UCA) group hacked the Russian internet service provider Nodex and wiped compromised systems and backups.

UCA cyber activists have also claimed other breaches impacting various Russian entities, including Vladimir Putin’s political adviser Vladislav Surkov, the Russian Ministry of Defense​​​, the Donetsk People’s Republic’s Ministry of Coal and Energy, the Commonwealth of Independent States Institute (financed by the Russian state company Gazprom), and multiple Russian military officers and media outlets.