Webmin through 1.991, when the Authentic theme is used, allows remote code execution when a user has been manually created (i.e., not created in Virtualmin or Cloudmin). This occurs because settings-editor_write.cgi does not properly restrict the file parameter.
https://webmin.com/changes.html
https://github.com/webmin/webmin/releases
https://github.com/webmin/webmin/issues/1635
https://github.com/webmin/webmin/commit/6a2334bf8b27d55c7edf0b2825cd14f3f8a69d4d