# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-DEB-SVC-012
title: Only approved services listen on network interfaces
finding:
  title: Services nobody approved accept connections from the network
  description: Every listening socket is an entry point; unknown ones are neither patched nor monitored.
status: active
profiles: [core]
risk:
  severity: medium
assessment: partial
tags: [services, network, attack-surface]

description: |
  Each TCP or UDP socket listening on a non-loopback address must belong to a service the system
  is meant to provide, and listen only on the interfaces where that service is needed.
rationale: |
  The services an attacker can reach from the network define the remote attack surface. Services
  that listen by default (LLMNR in systemd-resolved, development servers, agents) are easy to
  forget and widen it without anyone noticing.
impact: |
  Stopping or rebinding a service makes it unreachable for remote clients; check with its owner
  first.
notes: |
  The check lists the listening sockets outside loopback as `protocol/port process` and always
  passes, so the control ends as `review`: a person must compare the list with the services
  approved for the system role. On a fresh Debian cloud image the list holds `sshd` (tcp/22),
  the DHCP client of `systemd-networkd` (udp/68) and LLMNR in `systemd-resolved` (tcp and
  udp/5355).

audit:
  manual: |
    1. Run `ss -Hltunp` as root.
    2. For each socket not bound to `127.0.0.0/8` or `::1`, check that the process is an approved
       service for this system and that it listens only on the needed addresses.
  checks:
    - id: listeners
      engine: shell
      description: Lists sockets listening outside loopback, for review.
      spec:
        script: |
          command -v ss >/dev/null 2>&1 || { echo "ss not found"; exit 3; }
          ss -Hltunp 2>/dev/null | awk '{
            addr = $5
            if (addr ~ /^(127\.|\[::1\]|\[::ffff:127\.)/) next
            port = addr; sub(/.*:/, "", port)
            proc = $7; sub(/^users:\(\("/, "", proc); sub(/".*/, "", proc)
            print $1 "/" port " " (proc == "" ? "?" : proc)
          }' | sort -u | tr '\n' ' '
          echo

remediation:
  manual: |
    For each socket that is not approved: stop and mask the service (`systemctl mask --now
    <unit>`) or remove its package. For an approved service that listens too widely, bind it to
    the needed address in its configuration, and filter the port with the host firewall.

references:
  - title: ss(8), Debian manual pages
    url: https://manpages.debian.org/trixie/iproute2/ss.8.en.html
    publisher: Debian
    kind: vendor-doc
    accessed: 2026-10-18
  - title: ANSSI-BP-028 v2.0 - Recommandations de configuration d'un système GNU/Linux (R80)
    url: https://messervices.cyber.gouv.fr/documents-guides/fr_np_linux_configuration-v2.0.pdf
    publisher: ANSSI
    kind: guide
    accessed: 2026-10-18
mappings:
  - framework: anssi-linux
    framework_version: "2.0"
    id: R80
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: CM-7(1)
    relation: related
  - framework: cis-debian-12
    framework_version: "2.0.0"
    id: "2.1.4"
    relation: equivalent
  - framework: cis-debian-13
    framework_version: "1.1.0"
    id: "2.1.23"
    relation: equivalent
threats:
  - framework: mitre-attack
    id: T1046
  - framework: mitre-attack
    id: T1210

authors:
  - name: Alice
    github: Sn0wAlice
verified:
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["12"]
    environment: Proxmox VM, Debian 12 genericcloud image, kernel 6.1.0-54-cloud-amd64
    notes: "Check lists non-loopback listeners and ends as review, on fresh and loaded systems, before and after reboot."
  - date: 2026-10-18
    by: Sn0wAlice
    versions: ["13"]
    environment: Proxmox VM, Debian 13 genericcloud image, kernel 6.12.113+deb13-cloud-amd64
    notes: "Check lists non-loopback listeners and ends as review, on fresh and loaded systems, before and after reboot."
introduced_in: 0.1.0
created: 2026-10-18
updated: 2026-10-18