A vulnerability in sddm was found allowing shell access to the sddm user in case of a crash. Some themes may use KDE components which will automatically load KDE's crash handler. If the greeter were to then somehow crash, that would leave a crash handler allowing other actions, albeit as the locked down SDDM user. Only SDDM users using the breeze theme from plasma-workspace are affected.
Created sddm tracking bugs for this issue:
Affects: fedora-all [bug 1271993]
sddm-0.12.0-5.fc23 has been pushed to the Fedora 23 stable repository. If problems still persist, please make note of it in this bug report.
sddm-0.12.0-5.fc22 has been pushed to the Fedora 22 stable repository. If problems still persist, please make note of it in this bug report.