The remote Debian host is missing a security-related update.
Google's OSS-Fuzz revealed an exploitable bug in the gmp plugin caused by the patch that fixes CVE-2018-16151 and CVE-2018-16152 (DSA-4305-1). An attacker could trigger it using crafted certificates with RSA keys with very small moduli. Verifying signatures with such keys would cause an integer underflow and subsequent heap buffer overflow resulting in a crash of the daemon. While arbitrary code execution is not completely ruled out because of the heap buffer overflow, due to the form of the data written to the buffer it seems difficult to actually exploit it in such a way.
Upgrade the strongswan packages. For the stable distribution (stretch), this problem has been fixed in version 5.5.1-4+deb9u4.