systemd, a system and service manager, (as PID 1) hits an assert and freezes execution when an unprivileged IPC API call is made with spurious data. On version v249 and older the effect is not an assert, but stack overwriting, with the attacker controlled content. From version v250 and newer this is not possible as the safety check causes an assert instead. This IPC call was added in v239, so versions older than that are not affected. Versions 260-rc1, 259.2, 258.5, and 257.11 contain patches. No known workarounds are available.
Per the description of the vulnerability: https://github.com/systemd/systemd/security/advisories/GHSA-gx6q-6f99-m764 NetworkManager is not affected, as it only takes some code from systemd for DHCPv6. This is related to the systemd's D-Bus API, for which NM shares no code at all.