Tailscale SSH cannot switch local users with the default systemd capabilities

Environment

• CachyOS (Arch-based), Linux x86_64
• sing-box 1.14.0
• Command-line program running through systemd
• Local user's shell: /bin/fish

Problem

After enabling Tailscale SSH and allowing a normal local user through the SSH policy, connecting from another tailnet device authenticates successfully, but the shell fails to start:

failed to open shell: fork/exec /bin/fish: operation not permitted

The installed service has the same user and capability settings as upstream's release/config/sing-box.service:

User=sing-box
CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_RAW CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH
AmbientCapabilities=CAP_NET_ADMIN CAP_NET_RAW CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH

Verified workaround

Adding this systemd drop-in, which extends the existing capability lists with CAP_SETUID and CAP_SETGID, and restarting the service fixes the problem:

[Service]
CapabilityBoundingSet=CAP_SETUID CAP_SETGID
AmbientCapabilities=CAP_SETUID CAP_SETGID

Read release/config/sing-box.service and compare its CapabilityBoundingSet and AmbientCapabilities with the documented Tailscale SSH failure. Verify that the service can start a shell for a switched local user after the capability configuration is updated, while preserving existing proxy operation.