As noted in bug #1031818, libreswan suffers from a problem with the new ike_pad= feature that was implemented in version 3.6:
During an effort to ignore IKEv2 minor version numbers as required for RFC-5996, complete parse errors of any IKE packets with version 2.1+ were mistakenly accepted for further processing. This causes a crash later on if the IKE packet is mangled (e.g. too short). Openswan turns out not to be vulnerable because it happens to abort on the mismatched IKE length versus packet length before it inspects the rest of the IKE header. And since reading an invalid IKE major aborts further parsing of the IKE header, the length remains at 0, and so it will always mismatch.
As noted, this only affects upstream version 3.6 where the ike_pad= feature was added. Current Fedora and EPEL ship libreswan 3.5.
Code to fix:
Closing as this does not affect anything we ship.
Also note that the upstream also explicitly notes:
While the vulnerable code is present in all libreswan versions, and
some openswan versions, only libreswan version 3.6 exposed this code to
malicious IKE packets due to its new IKE padding feature.