The remote Debian host is missing a security-related update.
Two vulnerabilities were discovered in strongSwan, an IKE/IPsec suite. - CVE-2018-5388 The stroke plugin did not verify the message length when reading from its control socket. This vulnerability could lead to denial of service. On Debian write access to the socket requires root permission on default configuration. - CVE-2018-10811 A missing variable initialization in IKEv2 key derivation could lead to a denial of service (crash of the charon IKE daemon) if the openssl plugin is used in FIPS mode and the negotiated PRF is HMAC-MD5.
Upgrade the strongswan packages. For the oldstable distribution (jessie), these problems have been fixed in version 5.2.1-6+deb8u6. For the stable distribution (stretch), these problems have been fixed in version 5.5.1-4+deb9u2.