The Justice Department today announced a court-authorized operation to copy and remove malicious web shells from hundreds of vulnerable computers in the United States running on-premises versions of Microsoft Exchange Server software used to provide enterprise-level e-mail service.
Through January and February 2021, certain hacking groups exploited zero-day vulnerabilities in Microsoft Exchange Server software to access e-mail accounts and place web shells (which are pieces of code or scripts that enable remote administration) for continued access. Other hacking groups followed suit starting in early March after the vulnerability and patch were publicized. Although many infected system owners successfully removed the web shells from thousands of computers, others appeared unable to do so, and hundreds of such web shells persisted unmitigated.
Today’s operation removed one early hacking groups remaining web shells, which could have been used to maintain and escalate persistent, unauthorized access to U.S. networks. The FBI conducted the removal by issuing a command through the web shell to the server, which was designed to cause the server to delete only the web shell (identified by its unique file path). This is unrelated to Microsofts 13 April announcement. (Source: U.S. Department of Justice)