matrix-sdk-base is the base component to build a Matrix client library. Versions 0.14.1 and prior are unable to handle responses that include custom m.room.join_rules values due to a serialization bug. This can be exploited to cause a denial-of-service condition, if a user is invited to a room with non-standard join rules, the crates sync process will stall, preventing further processing for all rooms. This is fixed in version 0.16.0.
The product does not handle or incorrectly handles an exceptional condition.