What is WannaCry ransomware?

The WannaCry ransomware consists of multiple components. It arrives on the infected computer in the form of a dropper, a self-contained program that extracts the other application components embedded within itself. Those components include: 
  • An application that encrypts and decrypts data
  • Files containing encryption keys
  • A copy of Tor 
The program code is not obfuscated and was relatively easy for security pros to analyze. Once launched, WannaCry tries to access a hard-coded URL (the so-called kill switch); if it can't, it proceeds to search for and encrypt files in a slew of important formats, ranging from Microsoft Office files to MP3s and MKVs, leaving them inaccessible to the user. It then displays a ransom notice, demanding $300 in Bitcoin to decrypt the files.

How does WannaCry infect PCs?

The attack vector for WannaCry is more interesting than the ransomware itself. The vulnerability WannaCry exploits lies in the Windows implementation of the Server Message Block (SMB) protocol. The SMB protocol helps various nodes on a network communicate, and Microsoft's implementation could be tricked by specially crafted packets into executing arbitrary code. 
It is believed that the U.S. National Security Agency discovered this vulnerability and, rather than reporting it to the infosec community, developed code to exploit it, called EternalBlue. This exploit was in turn stolen by a hacking group known as the Shadow Brokers, who released it obfuscated in a seemingly political Medium post on April 8, 2017. Microsoft itself had discovered the vulnerability a month prior and had released a patch, but many systems remained vulnerable, and WannaCry, which used EternalBlue to infect computers, began spreading rapidly on May 12. In the wake of the outbreak, Microsoft slammed the U.S. government for not having shared its knowledge of the vulnerability sooner.
Even if a PC has been successfully infected, WannaCry won't necessarily begin encrypting files. That's because, as noted above, it first tries to access a very long, gibberish URL before going to work. If it can access that domain, WannaCry shuts itself down. It's not entirely clear what the purpose of this functionality is. Some researchers believed this was supposed to be a means for the malware's creators to pull the plug on the attack. However, Marcus Hutchins, the British security researcher who discovered that WannaCry was attempting to contact this URL, believes it was meant to make analysis of the code more difficult. Many researchers will run malware in a "sandbox" environment, from within which any URL or IP address will appear reachable; by hard-coding into WannaCry an attempt to contact a nonsense URL that wasn't actually expected to exist, its creators hoped to ensure that the malware wouldn't go through its paces for researchers to watch.
Hutchins not only discovered the hard-coded URL but paid $10.96 to register the domain and set up a site there, thus helping blunt, though not stop, the spread of the malware. Shortly after being hailed as a hero for this, Hutchins was arrested for supposedly developing different malware in 2014. He has proclaimed his innocence.

WannaCry patch 

Ironically, the patch needed to prevent WannaCry infections was actually available before the attack began: Microsoft Security Bulletin MS17-010, released on March 14, 2017, updated the Windows implementation of the SMB protocol to prevent infection via EternalBlue. However, despite the fact that Microsoft had flagged the patch as critical, many systems were still unpatched as of May of 2017 when WannaCry began its rapid spread.
For those unpatched systems that are infected, there is little remedy beyond restoring files from a safe backup — so let that be a lesson that you should always back up your files. While those monitoring the bitcoin wallets identified in the extortion message say that some people are paying the ransom, there's little evidence that they're regaining access to their files. 

WannaCry and Windows 10

As noted, Microsoft released a patch for the SMB vulnerability that WannaCry exploits two months before the attack began. While unpatched Windows 10 systems were vulnerable, the automatic update feature built into the OS meant that almost all Windows 10 systems were protected by May of 2017.
The Microsoft SMB patch was initially only available for currently supported versions of Windows, which notably excluded Windows XP. There are still millions of internet-connected Windows XP systems out there — including at Britain's National Health Service, where many WannaCry attacks were reported — and Microsoft eventually made the SMB patch available for older versions of the OS as well. However, a later analysis found that the vast majority of WannaCry infections struck machines running Windows 7, an operating system Microsoft does still support.

Symantec fingers the Lazarus Group

After the initial dust settled, various security researchers began working to try to figure out the origins of WannaCry. Symantec had a provocative take: they believed that the code might have a North Korean origin. They laid out the evidence in a blog post, where they discussed a little-known fact: that WannaCry had actually been circulating for months before it exploded across the internet on May 12, 2017. This earlier version of the malware, dubbed Ransom.Wannacry, used stolen credentials to launch targeted attacks, and there were "substantial commonalities in the tools, techniques and infrastructure used by the attackers” between this version of WannaCry and those used by the Lazarus Group.
The Lazarus Group in turn is a hacking group that has been tied to North Korea. Beginning their run in 2009 with crude DDoS attacks on South Korean government computers, they've become increasingly sophisticated, hacking Sony and pulling off bank heists.
On the other hand, without an explicit claim of responsibility, it's impossible to know for sure that either the initial wave of WannaCry attacks or the later EternalBlue-driven explosion was directed by North Korea, since malware code is copied liberally by various groups.

WannaCry still active, but causing fewer tears of sorrow

Despite all the publicity—not to mention the patches and best practices to help prevent it—WannaCry is still infecting systems. In March 2018, Boeing was hitwith a suspected WannaCry attack. The company claimed it did little damage, however, affecting only a few production machines. Boeing was able to stop the attack and bring the affected systems back quickly.
A key reason why Boeing was able to recover so well was that patches for the vulnerabilities that WannaCry exploits were readily available. The fact that they weren’t already in place before the attack explains why WannaCry can still do damage more than a year later. Few organizations are effective at keeping up with patching.
The bigger danger today are from WannaCry variants, or more specifically, new malware based on the same EternalBlue code as Wannacry. In May 2018, ESET released research that showed detections of EternalBlue-based malware spiking past their highest level in 2017. Immediately after WannaCry, detections of EternalBlue-based attacks dropped to a few hundred a day, but steadily rose again until spiking in April.
All EternalBlue-based malware exploits the same Windows vulnerability, so the fact that these attacks are increasing suggests that plenty of unpatched Windows systems are still out there. It’s only a matter of time before an attacker finds them.