SSH Terrapin Prefix Truncation Weakness

#!/usr/bin/env bash
# terrapin_check.sh
# Purpose: assess likely Terrapin exposure on a Linux SSH server
# Output: VULNERABLE / PATCHED / UNKNOWN
# Exit codes: 0=PATCHED, 1=VULNERABLE, 2=UNKNOWN

set -u

patched() { echo "PATCHED: $1"; exit 0; }
vulnerable() { echo "VULNERABLE: $1"; exit 1; }
unknown() { echo "UNKNOWN: $1"; exit 2; }

have_cmd() { command -v "$1" >/dev/null 2>&1; }

if ! have_cmd sshd; then
  unknown "sshd not found on this host"
fi

# Collect effective algorithms when possible
CIPHERS=""
MACS=""
SSHD_T_OUTPUT="$(sshd -T 2>/dev/null)"
if [ -n "$SSHD_T_OUTPUT" ]; then
  CIPHERS="$(printf '%s\n' "$SSHD_T_OUTPUT" | awk '/^ciphers /{sub(/^ciphers /,""); print; exit}')"
  MACS="$(printf '%s\n' "$SSHD_T_OUTPUT" | awk '/^macs /{sub(/^macs /,""); print; exit}')"
fi

# Helper: see whether config still exposes Terrapin-prone algorithms
config_exposes_vuln="unknown"
if [ -n "$CIPHERS" ] || [ -n "$MACS" ]; then
  has_chacha="no"
  has_cbc="no"
  has_etm="no"

  printf '%s' "$CIPHERS" | grep -q '[email protected]' && has_chacha="yes"
  printf '%s' "$CIPHERS" | grep -Eq '(^|,)[^,]*cbc($|,)' && has_cbc="yes"
  printf '%s' "$MACS" | grep -q -- '[email protected]' && has_etm="yes"

  if [ "$has_chacha" = "yes" ] || { [ "$has_cbc" = "yes" ] && [ "$has_etm" = "yes" ]; }; then
    config_exposes_vuln="yes"
  else
    config_exposes_vuln="no"
  fi
fi

# Parse upstream OpenSSH version if possible
raw_ver="$(sshd -V 2>&1 | head -n1)"
up_major=""
up_minor=""
if printf '%s' "$raw_ver" | grep -q 'OpenSSH_'; then
  ver_part="$(printf '%s' "$raw_ver" | sed -n 's/.*OpenSSH_\([0-9][0-9]*\)\.\([0-9][0-9]*\).*/\1 \2/p' | head -n1)"
  up_major="$(printf '%s' "$ver_part" | awk '{print $1}')"
  up_minor="$(printf '%s' "$ver_part" | awk '{print $2}')"
fi

if [ -n "$up_major" ] && [ -n "$up_minor" ]; then
  if [ "$up_major" -gt 9 ] || { [ "$up_major" -eq 9 ] && [ "$up_minor" -ge 6 ]; }; then
    patched "upstream OpenSSH version is >= 9.6 ($raw_ver)"
  fi
fi

# Distro-specific backport checks
if [ -r /etc/os-release ]; then
  . /etc/os-release
else
  ID=""
  VERSION_ID=""
  VERSION_CODENAME=""
fi

# Debian/Ubuntu package comparisons
if have_cmd dpkg-query && have_cmd dpkg; then
  pkg_ver="$(dpkg-query -W -f='${Version}' openssh-server 2>/dev/null || true)"
  if [ -n "$pkg_ver" ]; then
    if [ "${ID:-}" = "ubuntu" ]; then
      case "${VERSION_CODENAME:-}" in
        focal)
          dpkg --compare-versions "$pkg_ver" ge '1:8.2p1-4ubuntu0.10' && patched "Ubuntu focal openssh-server $pkg_ver includes Terrapin fix"
          ;;
        jammy)
          dpkg --compare-versions "$pkg_ver" ge '1:8.9p1-3ubuntu0.5' && patched "Ubuntu jammy openssh-server $pkg_ver includes Terrapin fix"
          ;;
        lunar)
          dpkg --compare-versions "$pkg_ver" ge '1:9.0p1-1ubuntu8.6' && patched "Ubuntu lunar openssh-server $pkg_ver includes Terrapin fix"
          ;;
        mantic)
          dpkg --compare-versions "$pkg_ver" ge '1:9.3p1-1ubuntu3.1' && patched "Ubuntu mantic openssh-server $pkg_ver includes Terrapin fix"
          ;;
      esac
    fi

    if [ "${ID:-}" = "debian" ]; then
      case "${VERSION_CODENAME:-}" in
        bullseye)
          dpkg --compare-versions "$pkg_ver" ge '1:8.4p1-5+deb11u7' && patched "Debian bullseye openssh-server $pkg_ver includes Terrapin fix"
          ;;
        bookworm)
          dpkg --compare-versions "$pkg_ver" ge '1:9.2p1-2+deb12u9' && patched "Debian bookworm openssh-server $pkg_ver includes Terrapin fix"
          ;;
      esac
    fi
  fi
fi

# RHEL 9.2 EUS example from official advisory
if have_cmd rpm; then
  rpm_ver="$(rpm -q --qf '%{VERSION}-%{RELEASE}' openssh-server 2>/dev/null || true)"
  if [ -n "$rpm_ver" ]; then
    case "${ID:-}" in
      rhel|redhat|rocky|almalinux|centos)
        if [ "${VERSION_ID:-}" = "9.2" ] || printf '%s' "$rpm_ver" | grep -q '\.el9_2'; then
          # String compare is unreliable for RPM EVR; use rpm itself.
          if rpm -q --quiet --whatprovides 'openssh-server >= 8.7p1-30.el9_2.3' 2>/dev/null; then
            patched "RHEL-family 9.2 openssh-server package includes Terrapin fix"
          fi
        fi
        ;;
    esac
  fi
fi

# Config-only mitigation verdicts
if [ "$config_exposes_vuln" = "no" ]; then
  patched "effective sshd config does not expose ChaCha20-Poly1305 or CBC+EtM combinations used by Terrapin"
fi

if [ "$config_exposes_vuln" = "yes" ]; then
  vulnerable "server still offers Terrapin-prone algorithms and no trusted package-level fix was confirmed"
fi

unknown "could not confirm a fixed package level or determine effective algorithm exposure; verify with Nessus 187315 or RUB-NDS Terrapin-Scanner"