charon/sa/tasks/child_create.c in the charon daemon in strongSWAN before 4.3.1 switches the NULL checks for TSi and TSr payloads, which allows remote attackers to cause a denial of service via an IKE_AUTH request without a (1) TSi or (2) TSr traffic selector.
Debian Linux Security Advisory 1899-1 - Several remote vulnerabilities have been discovered in strongswan, an implementation of the IPSEC and IKE protocols.