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.

An issue was discovered in SDDM through 0.17.0. If configured with ReuseSession=true, the password is not checked for users with an already existing session. Any user with access to the system D-Bus can therefore unlock any graphical session. This is related to daemon/Display.cpp and helper/backend/PamBackend.cpp.

Simple Desktop Display Manager (SDDM) before 0.10.0 allows local users to gain root privileges because code running as root performs write operations within a user home directory, and this user may have created links in advance (exploitation requires the user to win a race condition in the ~/.Xauthority chown case, but not other cases).

Simple Desktop Display Manager (SDDM) before 0.10.0 allows local users to log in as user "sddm" without authentication.

daemon/Greeter.cpp in sddm before 0.13.0 does not properly disable the KDE crash handler, which allows local users to gain privileges by crashing a greeter when using certain themes, as demonstrated by the plasma-workspace breeze theme.