The current version avilable for FreeBSD is vulnerable since 14.08.2017 and has already been patched upstream. The current contains a DoS-Vuln.
Available version: 5.5.3
Patched version: 5.6.0
Fixed a DoS vulnerability in the gmp plugin that was caused by insufficient input validation when verifying RSA signatures, which requires decryption with the operation m^e mod n, where m is the signature, and e and n are the exponent and modulus of the public key. The value m is an integer between 0 and n-1, however, the gmp plugin did not verify this. So if m equals n the calculation results in 0, in which case mpz_export() returns NULL. This result wasn't handled properly causing a null-pointer dereference.
This vulnerability has been registered as CVE-2017-11185 .
Please refer to our blog for details. 
Created attachment 185683 [details]
Testbuilds are fine on 12a, 11a, 10i
I am happy with the patch added.
It can be applied to the ports tree.
Just as a note, the gmp plugin referenced in the code is not enabled on the FreeBSD build of strongSwan.
A commit references this bug:
Date: Wed Aug 23 06:10:20 UTC 2017
New revision: 448590
security/strongswan: update 5.5.3 -> 5.6.0
- the gmp plugin responsible for CVE-2017-11185 is not enabled
in the FreeBSD build
Reported by: firstname.lastname@example.org
Approved by: email@example.com (maintainer)
Update committed, thanks!