Possibly the IPv6 packet one, CVE-2018-15688? [0] A basic buffer overflow in systemd meant oversized network packets could be used for DoS or "potential code execution".
Good one but not in PID 1, in fact networkd is probably the "most optional" of all systemd components.
CVE-2017-15908
https://nvd.nist.gov/vuln/detail/CVE-2017-15908 says this is a infinite loop DoS in systemd-resolved, so it can't be the (hypothetical) bug described by the GP since it's neither a RCE nor in systemd PID1.