The U.S. National Security Agency and the Australian Signals Directorate issued a joint report warning of threat actors increasingly exploiting vulnerable web servers to deploy web shells. Web shells are malicious tools that hackers can deploy on a compromised internal or internet-exposed server to gain and maintain access, as well as remotely execute arbitrary commands. The 17-page long security advisory contains a wide range of information for security teams who want to detect hidden web shells, to manage the response and recovery processes after.
Source: https://www.bleepingcomputer.com/news/security/nsa-hackers-exploit-these-vulnerabilities-to-deploy-backdoors/