CVE 2018-14345
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/
See the 
CVE page on Mitre.org
for more details.
