src/remote/server.cpp in fbserver.exe in Firebird SQL 1.5 before 1.5.6, 2.0 before 2.0.6, 2.1 before 2.1.3, and 2.5 before 2.5 Beta 2 allows remote attackers to cause a denial of service (daemon crash) via a malformed op_connect_request message that triggers an infinite loop or NULL pointer dereference.