You can download an updated version of Mull with the security vulnerability fixed, from the DivestOS F-Droid repository: https://divestos.org/fdroid/official/. If you currently have the F-Droid version of Mull installed you will need to uninstall it first.
What would people recommend in the short run as an alternative?
You can get the updated mull from the divestos repo, the issue is fixed there
You can download an updated version of Mull with the security vulnerability fixed, from the DivestOS F-Droid repository: https://divestos.org/fdroid/official/. If you currently have the F-Droid version of Mull installed you will need to uninstall it first.