No Limit on Number of Open Sessions Leads to Memory Exhaustion and Impacts Availability

Summary

No Limit on Number of Open Sessions / Bad Session Close Behaviour in dnf5daemon-server before 5.1.17 allows a malicious user to impact Availability via No Limit on Number of Open Sessions. There is no limit on how many sessions D-Bus clients may create using the `open_session()` D-Bus method. For each session a thread is created in dnf5daemon-server. This spends a couple of hundred megabytes of memory in the process. Further connections will become impossible, likely because no more threads can be spawned by the D-Bus service.