Attackers began scanning for servers vulnerable to the ProxyShell attack chain almost as soon as Orange Tsai’s presentation went live. Given the popularity of its predecessor, ProxyLogon, with attackers, we knew exploitation was coming. These vulnerabilities are likely popular because of the ubiquity of Microsoft Exchange — threat actors know they have a higher potential for successful attacks by targeting services like this. The former success of attacks leveraging ProxyLogon also draws attackers to ProxyShell, relying on attacks and tactics known to work.

ProxyShell is now being used to deploy the LockFile ransomware and I expect other actors will integrate it into their attacks. The threat is certainly real, as CISA warned organizations over the weekend of in-the-wild exploitation. To protect against attacks, organizations should ensure they’re applying the patches released in April and May for Microsoft Exchange Servers.