More than two years after the WannaCry outbreak made ransomware a household name, the critical Windows vulnerability exploited by the malware remains unpatched on many systems worldwide.
Security vendor Sophos Wednesday reported that in August 2019 alone, it had detected and blocked more than 4.3 million attempts by WannaCry-infected hosts to spread some version of the malware to systems running the company's endpoint security software. Most of the failed attacks have been targeted at systems based in the US.
A Sophos analysis of attack data gathered over a three-month period between October 2018 and December 2018 showed there were more than 5.1 million similar attempts.
WannaCry detections in fact appear to be at an all-time high, Sophos said in a report summarizing its analysis of the malware, but there's some good news here: virtually all of the more than 12,000 WannaCry variants in circulation currently are broken and cannot encrypt data on infected systems.
So while WannaCry variants continue to be very active, the malware's potential to damage systems is low—and probably explains why concerns over the persisting infections have remained low as well.
"The vulnerability that caused WannaCry to spread rapidly remains an ongoing threat," says Andrew Brandt, principal researcher at Sophos. "The enterprise tendency to defer patching for some kinds of critical updates may, in some limited cases, do more harm than what it attempts to prevent."
And while the thousands of variants in the wild currently may not be encrypting infected systems, impacted organizations should not overlook fact that their systems remain vulnerable in the first place adds Peter Mackenzie, global malware escalations manager at Sophos.
"All of the samples we analyzed had the capability to spread to new machines," he notes. "The fact that the final payload was corrupted didn’t change the fact that malware was taking up network bandwidth and copying unwanted files to your machines."
Significantly, the vulnerability that WannaCry exploited continues to be heavily exploited by other malware, such as the TrickBot worm, which then leads to a targeted Ryuk ransomware attacks, Mackenzie cautions.
WannaCry impacted some 200,000 computers in 150 countries in May 2017. The malware took advantage of a critical security flaw in Microsoft's SMB protocol that allows Windows users to share files. The malware spread in wormlike fashion from one vulnerable system to another using EternalBlue, a zero-day exploit belonging to the NSA that was publicly leaked in a massive data dump before the WannaCry outbreak.
Microsoft had issued a patch for the SMB flaw prior to WannaCry. But millions of systems were still unpatched when the malware began spreading rapidly, causing widespread concerns.
The outbreak ended as suddenly as it began when two UK-based security researchers discovered a hidden feature in WannaCry that caused the malware to stop spreading if it discovered a specific Web domain name was live. By simply registering the domain name, the researchers effectively shut down WannaCry.
In its report this week Sophos theorized one potential reason the "kill-switch" was included in the code was because the attackers—later identified as North Korea's Lazarus group—needed a way to stop the malware when they wanted.
A Clear & Persisting Threat
Since the original outbreak, security researchers have reported discovering numerous variants of WannaCry in the wild. Sophos' analysis showed that a vast majority of the current collection of over 12,000 WannaCry variants contain code altered from the original sample.
The alterations have allowed the new samples to bypass the kill switch mechanism and spread freely to systems that have not yet implemented the Microsoft patch. But the alterations also appear to have broken the encryption component in the malware - rendering it ineffective.
Mackenzie says many new variants are likely simply the result of data corruption or incomplete file transfer via EternalBlue. "Nobody’s creating them per se, but the act of copying back and forth so often may introduce errors in replication which then magnify over time."
Fortunately, unlike biological threats, there's no chance of a "superstrain" of WannaCry resulting from such mutation, he says. He adds it's hard to estimate how many systems still remain vulnerable to the EternalBlue exploit, though the number could well be in the millions.
The reason why threat actors are not going after those systems with a functional encryption component is likely because there are better ways of distributing ransomware these days, Mackenzie says, pointing to the so-called "automated active attack" model used by malware like SamSam.
Meanwhile, the motive for launching WannaCry may have been broader than ransomware, Brandt says. "WannaCry has been roundly attributed to the DPRK, who may not have actually cared very much for collecting ransom so much as sowing chaos and destruction across the West in retaliation for sanctions and perceived slights and threats."
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