ArsTechnica has a good story on the REvil ransomware attack of last weekend, with technical details:

This weekend’s attack was carried out with almost surgical precision. According to Cybereason, the REvil affiliates first gained access to targeted environments and then used the zero-day in the Kaseya Agent Monitor to gain administrative control over the target’s network. After writing a base-64-encoded payload to a file named agent.crt the dropper executed it.

[…]

The ransomware dropper Agent.exe is signed with a Windows-trusted certificate that uses the registrant name “PB03 TRANSPORT LTD.” By digitally signing their malware, attackers are able to suppress many security warnings that would otherwise appear when it’s being installed. Cybereason said that the certificate appears to have been used exclusively by REvil malware that was deployed during this attack.

To add stealth, the attackers used a technique called DLL Side-Loading, which places a spoofed malicious DLL file in a Windows’ WinSxS directory so that the operating system loads the spoof instead of the legitimate file. In the case here, Agent.exe drops an outdated version that is vulnerable to DLL Side-Loading of “msmpeng.exe,” which is the file for the Windows Defender executable.

Once executed, the malware changes the firewall settings to allow local windows systems to be discovered. Then, it starts to encrypt the files on the system….

REvil is demanding $70 million for a universal decryptor that will recover the data from the 1,500 affected Kaseya customers.

More news.

Note that this is yet another supply-chain attack. Instead of infecting those 1,500 networks directly, REvil infected a single managed service provider. And it leveraged a zero-day vulnerability in that provider.

EDITED TO ADD (7/13): Employees warned Kaseya’s management for years about critical security flaws, but they were ignored.