What happened? Telephone notifications were sent to millions of people on Sunday in Ukraine, alerting them of a "nuclear" or "chemical threat", or even "fighting" in the streets of kyiv. One of these messages was written in Russian and included a link to a Telegram channel called "Hitcore", whose logo is a metal eagle plastered on the Russian flag. A few hours later, the Russian Minister of Emergency Situations declared that a "massive cyberattack" had hit his own application, leading to "false information" being sent to the public. Several regional accounts run by the ministry posted messages warning of an imminent nuclear attack on Russia. For example, a Telegram message was sent by authorities in the border region of Belgorod, warning that "nuclear warheads […] have been launched on all regions of Russia" and urging the population to "urgently stock up on food for three weeks." Which applications were affected? The Ukrainian air alert application is widely used to warn of Russian attacks by missiles and drones. It has been downloaded more than 46 million times and has more than 6.7 million daily users, according to its developer, Ajax Systems. Russia does not have a national air warning system and, outside of border regions, does not systematically send alerts to citizens about impending Ukrainian attacks. Few Russians use the relevant application, managed by the Ministry of Emergency Situations. An act of war? It is still unclear whether these attacks were part of the war, whether they were carried out by one side or the other, or whether they were launched by independent actors or groups. The Ukrainian Center for Strategic Communications, a government-backed project set up to counter Russian propaganda and online threats, said "Russians hacked" the app. A pro-Russian group calling itself Hitcore has claimed responsibility for hacking Ukraine's alert system. So far, no one has claimed responsibility for the hack.