NEW YORK (NYTIMES) - A quickly spreading ransomware attack is hitting countries across the world including Ukraine, Russia, Spain, France and the United States, just weeks after a ransomware attack known as "WannaCry."

Several private companies have confirmed that they have been hit by the attack, including the US pharmaceutical giant Merck, the Danish shipping company AP Moller-Maersk, the British advertising firm WPP, Saint-Gobain of France, and the Russian steel, mining and oil firms Evraz and Rosneft.

WHAT WE KNOW

Ukraine's deputy prime minister, Pavlo Rozenko, tweeted a picture of a computer screen, along with a message saying the government's entire computer system had been shut down.

Cybersecurity researchers first said that the new ransomware appeared to be a variation of a well-known ransomware strain called Petya. One researcher from the Moscow-based cybersecurity firm Kaspersky Lab reported the new ransomware was a strain of Petya first identified in March 2016.

Kaspersky found evidence that the latest strain had been created June 18, suggesting it has been hitting victims for more than a week. But Kaspersky also said that it was still investigating the attack and that it could be a new type of ransomware that has never been seen before.

Kaspersky reported that approximately 2,000 computer systems had been affected by the new ransomware so far.

Symantec, a Silicon Valley cybersecurity firm, confirmed that the ransomware was infecting computers through at least one exploit, or vulnerability to computer systems, known as Eternal Blue.

Eternal Blue was leaked online in April by a mysterious group of hackers known as the Shadow Brokers, who have previously released hacking tools used by the National Security Agency. The same vulnerability was used in May to spread the WannaCry ransomware, in which hundreds of thousands of computers in more than 150 countries were affected.

Photographs and videos of computers affected by the attack show a message of red text over a black screen. The message read: "Oops, your important files have been encrypted. If you see this text then your files are no longer accessible because they have been encrypted. Perhaps you are busy looking to recover your files but don't waste your time."

Several cybersecurity researchers have identified a Bitcoin address to which the attackers are demanding a payment of US$300 (S$415) from their victims. At least some of the victims appear to be paying the ransom.

WHAT WE DON'T KNOW

Who is behind the ransomware attack. The original Petya ransomware was developed and used by cybercriminals, and variations have been sold through dark web trading sites, which are accessible only by using browsers that mask a user's identity, making it difficult for cybersecurity researchers to track.

Why it is spreading as quickly as it is. Cybersecurity researchers believe that like WannaCry, the ransomware infects computers using vulnerabilities in the central nerve of a computer, called a kernel, making it difficult for anti-virus firms to detect. It is not yet known if the new ransomware uses any new vulnerabilities, or variants of the vulnerabilities, made public by the group known as the Shadow Brokers.

It's unclear if systems protected against WannaCry can still be affected by the new ransomware attack.

WHAT IS RANSOMWARE?

Ransomware is one of the most popular forms of online attack today. It typically begins with attackers sending their victims an e-mail that includes a link, or a file, which appears innocuous, but which contains dangerous malware.

Once a victim clicks on the link or opens the attachment, the computer becomes infected. The program encrypts the computer, essentially locking the user out of files, folders and drives on that computer. In some cases, the entire network the computer is connected to can become infected.

The victim then receives a message demanding payment in exchange for attackers unlocking the system. The payment is usually requested in Bitcoin, a form of digital currency.