Proxmox Virtual Environment (PVE) and Proxmox Mail Gateway (PMG) are vulnerable to SSRF when proxying HTTP requests between pve(pmg)proxy and pve(pmg)daemon. An attacker with an unprivileged account can craft an HTTP request to achieve SSRF and file disclosure of any files on the server. Also, in Proxmox Mail Gateway, privilege escalation to the root@pam account is possible if the backup feature has ever been used, because backup files such as pmg-backup_YYYY_MM_DD_*.tgz have 0644 permissions and contain an authkey value. This is fixed in pve-http-server 4.1-3.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.