Useful information on things you can do to improve the security of a systemd service.
(Here is a good service implementation: https://github.com/NixOS/nixpkgs/blob/8261f6e94510101738ab45f0b877f2993c7fb069/nixos/modules/services/security/endlessh-go.nix#L98C1-L129)
[root@nixos:~]# systemd-analyze security sshd.service
NAME DESCRIPTION EXPOSURE
✗ RootDirectory=/RootImage= Service runs within the host's root directory 0.1
SupplementaryGroups= Service runs as root, option does not matter
RemoveIPC= Service runs as root, option does not apply
✗ User=/DynamicUser= Service runs as root user 0.4
✗ CapabilityBoundingSet=~CAP_SYS_TIME Service processes may change the system clock 0.2
✗ NoNewPrivileges= Service processes may acquire new privileges 0.2
✓ AmbientCapabilities= Service process does not receive ambient capabilities
✗ PrivateDevices= Service potentially has access to hardware devices 0.2
✗ ProtectClock= Service may write to the hardware clock or system clock 0.2
✗ CapabilityBoundingSet=~CAP_SYS_PACCT Service may use acct() 0.1
✗ CapabilityBoundingSet=~CAP_KILL Service may send UNIX signals to arbitrary processes 0.1
✗ ProtectKernelLogs= Service may read from or write to the kernel log ring buffer 0.2
✗ CapabilityBoundingSet=~CAP_WAKE_ALARM Service may program timers that wake up the system 0.1
✗ CapabilityBoundingSet=~CAP_(DAC_*|FOWNER|IPC_OWNER) Service may override UNIX file/IPC permission checks 0.2
✗ ProtectControlGroups= Service may modify the control group file system 0.2
✗ CapabilityBoundingSet=~CAP_LINUX_IMMUTABLE Service may mark files immutable 0.1
✗ CapabilityBoundingSet=~CAP_IPC_LOCK Service may lock memory into RAM 0.1
✗ ProtectKernelModules= Service may load or read kernel modules 0.2
✗ CapabilityBoundingSet=~CAP_SYS_MODULE Service may load kernel modules 0.2
✗ CapabilityBoundingSet=~CAP_SYS_TTY_CONFIG Service may issue vhangup() 0.1
✗ CapabilityBoundingSet=~CAP_SYS_BOOT Service may issue reboot() 0.1
✗ CapabilityBoundingSet=~CAP_SYS_CHROOT Service may issue chroot() 0.1
✗ PrivateMounts= Service may install system mounts 0.2
✗ SystemCallArchitectures= Service may execute system calls with all ABIs 0.2
✗ CapabilityBoundingSet=~CAP_BLOCK_SUSPEND Service may establish wake locks 0.1
✗ MemoryDenyWriteExecute= Service may create writable executable memory mappings 0.1
✗ RestrictNamespaces=~user Service may create user namespaces 0.3
✗ RestrictNamespaces=~pid Service may create process namespaces 0.1
✗ RestrictNamespaces=~net Service may create network namespaces 0.1
✗ RestrictNamespaces=~uts Service may create hostname namespaces 0.1
✗ RestrictNamespaces=~mnt Service may create file system namespaces 0.1
✗ CapabilityBoundingSet=~CAP_LEASE Service may create file leases 0.1
✗ CapabilityBoundingSet=~CAP_MKNOD Service may create device nodes 0.1
✗ RestrictNamespaces=~cgroup Service may create cgroup namespaces 0.1
✗ RestrictSUIDSGID= Service may create SUID/SGID files 0.2
✗ RestrictNamespaces=~ipc Service may create IPC namespaces 0.1
✗ ProtectHostname= Service may change system host/domainname 0.1
✗ CapabilityBoundingSet=~CAP_(CHOWN|FSETID|SETFCAP) Service may change file ownership/access mode/capabilities unrestricted 0.2
✗ CapabilityBoundingSet=~CAP_SET(UID|GID|PCAP) Service may change UID/GID identities/capabilities 0.3
✗ LockPersonality= Service may change ABI personality 0.1
✗ ProtectKernelTunables= Service may alter kernel tunables 0.2
✗ RestrictAddressFamilies=~AF_PACKET Service may allocate packet sockets 0.2
✗ RestrictAddressFamilies=~AF_NETLINK Service may allocate netlink sockets 0.1
✗ RestrictAddressFamilies=~AF_UNIX Service may allocate local sockets 0.1
✗ RestrictAddressFamilies=~… Service may allocate exotic sockets 0.3
✗ RestrictAddressFamilies=~AF_(INET|INET6) Service may allocate Internet sockets 0.3
✗ CapabilityBoundingSet=~CAP_MAC_* Service may adjust SMACK MAC 0.1
✗ RestrictRealtime= Service may acquire realtime scheduling 0.1
✗ CapabilityBoundingSet=~CAP_SYS_RAWIO Service has raw I/O access 0.2
✗ CapabilityBoundingSet=~CAP_SYS_PTRACE Service has ptrace() debugging abilities 0.3
✗ CapabilityBoundingSet=~CAP_SYS_(NICE|RESOURCE) Service has privileges to change resource use parameters 0.1
[root@intense-eagle:~]# systemd-analyze security sshd.service
NAME DESCRIPTION EXPOSURE
✗ RootDirectory=/RootImage= Service runs within the host's root directory 0.1
SupplementaryGroups= Service runs as root, option does not matter
RemoveIPC= Service runs as root, option does not apply
✗ User=/DynamicUser= Service runs as root user 0.4
✗ CapabilityBoundingSet=~CAP_SYS_TIME Service processes may change the system clock 0.2
✗ NoNewPrivileges= Service processes may acquire new privileges 0.2
✓ AmbientCapabilities= Service process does not receive ambient capabilities
✗ PrivateDevices= Service potentially has access to hardware devices 0.2
✗ ProtectClock= Service may write to the hardware clock or system clock 0.2
✗ CapabilityBoundingSet=~CAP_SYS_PACCT Service may use acct() 0.1
✗ CapabilityBoundingSet=~CAP_KILL Service may send UNIX signals to arbitrary processes 0.1
✗ ProtectKernelLogs= Service may read from or write to the kernel log ring buffer 0.2
✗ CapabilityBoundingSet=~CAP_WAKE_ALARM Service may program timers that wake up the system 0.1
✗ CapabilityBoundingSet=~CAP_(DAC_*|FOWNER|IPC_OWNER) Service may override UNIX file/IPC permission checks 0.2
✗ ProtectControlGroups= Service may modify the control group file system 0.2
✗ CapabilityBoundingSet=~CAP_LINUX_IMMUTABLE Service may mark files immutable 0.1
✗ CapabilityBoundingSet=~CAP_IPC_LOCK Service may lock memory into RAM 0.1
✗ ProtectKernelModules= Service may load or read kernel modules 0.2
✗ CapabilityBoundingSet=~CAP_SYS_MODULE Service may load kernel modules 0.2
✗ CapabilityBoundingSet=~CAP_SYS_TTY_CONFIG Service may issue vhangup() 0.1
✗ CapabilityBoundingSet=~CAP_SYS_BOOT Service may issue reboot() 0.1
✗ CapabilityBoundingSet=~CAP_SYS_CHROOT Service may issue chroot() 0.1
✗ PrivateMounts= Service may install system mounts 0.2
✗ SystemCallArchitectures= Service may execute system calls with all ABIs 0.2
✗ CapabilityBoundingSet=~CAP_BLOCK_SUSPEND Service may establish wake locks 0.1
✗ MemoryDenyWriteExecute= Service may create writable executable memory mappings 0.1
✗ RestrictNamespaces=~user Service may create user namespaces 0.3
✗ RestrictNamespaces=~pid Service may create process namespaces 0.1
✗ RestrictNamespaces=~net Service may create network namespaces 0.1
✗ RestrictNamespaces=~uts Service may create hostname namespaces 0.1
✗ RestrictNamespaces=~mnt Service may create file system namespaces 0.1
✗ CapabilityBoundingSet=~CAP_LEASE Service may create file leases 0.1
✗ CapabilityBoundingSet=~CAP_MKNOD Service may create device nodes 0.1
✗ RestrictNamespaces=~cgroup Service may create cgroup namespaces 0.1
✗ RestrictSUIDSGID= Service may create SUID/SGID files 0.2
✗ RestrictNamespaces=~ipc Service may create IPC namespaces 0.1
✗ ProtectHostname= Service may change system host/domainname 0.1
✗ CapabilityBoundingSet=~CAP_(CHOWN|FSETID|SETFCAP) Service may change file ownership/access mode/capabilities unrestricted 0.2
✗ CapabilityBoundingSet=~CAP_SET(UID|GID|PCAP) Service may change UID/GID identities/capabilities 0.3
✗ LockPersonality= Service may change ABI personality 0.1
✗ ProtectKernelTunables= Service may alter kernel tunables 0.2
✗ RestrictAddressFamilies=~AF_PACKET Service may allocate packet sockets 0.2
✗ RestrictAddressFamilies=~AF_NETLINK Service may allocate netlink sockets 0.1
✗ RestrictAddressFamilies=~AF_UNIX Service may allocate local sockets 0.1
✗ RestrictAddressFamilies=~… Service may allocate exotic sockets 0.3
✗ RestrictAddressFamilies=~AF_(INET|INET6) Service may allocate Internet sockets 0.3
✗ CapabilityBoundingSet=~CAP_MAC_* Service may adjust SMACK MAC 0.1
✗ RestrictRealtime= Service may acquire realtime scheduling 0.1
✗ CapabilityBoundingSet=~CAP_SYS_RAWIO Service has raw I/O access 0.2
✗ CapabilityBoundingSet=~CAP_SYS_PTRACE Service has ptrace() debugging abilities 0.3
✗ CapabilityBoundingSet=~CAP_SYS_(NICE|RESOURCE) Service has privileges to change resource use parameters 0.1
✗ DeviceAllow= Service has no device ACL 0.2
✗ CapabilityBoundingSet=~CAP_NET_ADMIN Service has network configuration privileges 0.2
✗ ProtectSystem= Service has full access to the OS file hierarchy 0.2
✗ ProtectProc= Service has full access to process tree (/proc hidepid=) 0.2
✗ ProcSubset= Service has full access to non-process /proc files (/proc subset=) 0.1
✗ ProtectHome= Service has full access to home directories 0.2
✗ CapabilityBoundingSet=~CAP_NET_(BIND_SERVICE|BROADCAST|RAW) Service has elevated networking privileges 0.1
✗ CapabilityBoundingSet=~CAP_AUDIT_* Service has audit subsystem access 0.1
✗ CapabilityBoundingSet=~CAP_SYS_ADMIN Service has administrator privileges 0.3
✗ PrivateNetwork= Service has access to the host's network 0.5
✗ PrivateUsers= Service has access to other users 0.2
✗ PrivateTmp= Service has access to other software's temporary files 0.2
✗ CapabilityBoundingSet=~CAP_SYSLOG Service has access to kernel logging 0.1
✓ KeyringMode= Service doesn't share key material with other services
✓ Delegate= Service does not maintain its own delegated control group subtree
✗ SystemCallFilter=~@clock Service does not filter system calls 0.2
✗ SystemCallFilter=~@cpu-emulation Service does not filter system calls 0.1
✗ SystemCallFilter=~@debug Service does not filter system calls 0.2
✗ SystemCallFilter=~@module Service does not filter system calls 0.2
✗ SystemCallFilter=~@mount Service does not filter system calls 0.2
✗ SystemCallFilter=~@obsolete Service does not filter system calls 0.1
✗ SystemCallFilter=~@privileged Service does not filter system calls 0.2
✗ SystemCallFilter=~@raw-io Service does not filter system calls 0.2
✗ SystemCallFilter=~@reboot Service does not filter system calls 0.2
✗ SystemCallFilter=~@resources Service does not filter system calls 0.2
✗ SystemCallFilter=~@swap Service does not filter system calls 0.2
✗ IPAddressDeny= Service does not define an IP address allow list 0.2
✓ NotifyAccess= Service child processes cannot alter service state
✗ UMask= Files created by service are world-readable by default 0.1
→ Overall exposure level for sshd.service: 9.6 UNSAFE 😨