An issue was discovered in SDDM before 0.19.0. It incorrectly starts the X server in a way that - for a short time period - allows local unprivileged users to create a connection to the X server without providing proper authentication. A local attacker can thus access X server display contents and, for example, intercept keystrokes or access the clipboard. This is caused by a race condition during Xauthority file creation.
https://www.debian.org/security/2020/dsa-4783
https://security.gentoo.org/glsa/202402-02
https://lists.debian.org/debian-lts-announce/2020/11/msg00009.html
https://github.com/sddm/sddm/releases
https://github.com/sddm/sddm/blob/v0.19.0/ChangeLog
https://bugzilla.suse.com/show_bug.cgi?id=CVE-2020-28049
http://lists.opensuse.org/opensuse-security-announce/2020-11/msg00031.html