nebula-mesh: Host advanced overrides allow YAML injection into agent config.yml

internal/configgen/generator.go:86,108,119 interpolates the operator-supplied ListenHost and TunDevice fields raw into a text/template that produces the agent's config.yml. internal/web/advanced.go:20-35 accepts both with only strings.TrimSpace — no character or shape validation.

Exploit

An operator (or attacker with any operator key, given the cross-tenant CRUD advisory) sets adv_tun_device to:

nebula0
lighthouse:
  am_lighthouse: true
  hosts: ["10.0.0.1"]
#

The agent fetches the rendered config on its next signed poll. On config reload, it loads the injected YAML keys: the host self-promotes to lighthouse, attracts mesh traffic, or sets am_relay: true to be selected as a relay. The ListenHost field has the same shape.

Affected

All released versions prior to v0.3.2.

## Threat model - Today: operator can compromise their own host's config (trivially allowed if they own the host, but they can also set lighthouse/relay flags that the operator-create form does NOT expose — privilege uplift within their own tenant). - Combined with the critical /api/v1 authz advisory: any operator key can mutate ANOTHER tenant's host overrides and inject YAML there. - Post-fix of the authz advisory: still relevant — the agent unconditionally trusts whatever config the server hands it, so any future operator-impersonation bug re-amplifies this.

Suggested fix

Two options, either acceptable:

1. Input validation in parseAdvancedFromForm (internal/web/advanced.go): - ListenHost: regex ^[A-Za-z0-9.:\[\]_-]+$ (IPv4/IPv6/hostname) - TunDevice: regex ^[A-Za-z0-9_-]{1,15}$ (Linux IFNAMSIZ caps at 15) Reject invalid input with a form-level error; do not write to the host row.

1. Safer marshalling: switch configgen/generator.go to marshal a typed Go struct via gopkg.in/yaml.v3 (which escapes correctly) instead of text/template string-concat. Larger change, but eliminates this entire injection class.

Option 2 is preferable long-term. Option 1 is the quick fix.

The unsafe_routes advanced field is already netip.Parse{Prefix,Addr}-validated at enroll.go:226-233 — apply the same validation discipline to the other advanced fields.