CVE-2021-31535

A missing validation flaw was found in libX11. This flaw allows an attacker to inject X11 protocol commands on X clients, and in some cases, also bypass, authenticate (via injection of control characters), or potentially execute arbitrary code with permissions of the application compiled with libX11. The highest threat from this vulnerability is to confidentiality, integrity, as well as system availability.

Mitigation

xterm should not be used to display less trusted data, e.g. from SSH connections to less trusted remote machines.
To avoid attacks via .Xdefaults on kiosk type machines, where graphical user has no permission to execute arbitrary operating system commands or sometimes not even to send hardware keyboard keystrokes, the .Xdefaults must not be modifiable by the user.