In SolarWinds Serv-U before 15.2.2 Hotfix 1, there is a directory containing user profile files (that include users' password hashes) that is world readable and writable. An unprivileged Windows user (having access to the server's filesystem) can add an FTP user by copying a valid profile file to this directory. For example, if this profile sets up a user with a C:\ home directory, then the attacker obtains access to read or replace arbitrary files with LocalSystem privileges.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-25276
• https://www.trustwave.com/en-us/resources/blogs/spiderlabs-blog/full-system-control-with-new-solarwinds-orion-based-and-serv-u-ftp-vulnerabilities/