TFTP backdoor

# SPDX-FileCopyrightText: 2005 Michel Arboi
# 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

if(description)
{
  script_oid("1.3.6.1.4.1.25623.1.0.18263");
  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:"10.0");
  script_tag(name:"cvss_base_vector", value:"AV:N/AC:L/Au:N/C:C/I:C/A:C");
  script_name("TFTP backdoor");
  script_category(ACT_ATTACK); # nb: Requires info of tftpd_dir_trav.nasl which is in ACT_ATTACK
  script_copyright("Copyright (C) 2005 Michel Arboi");
  script_family("Malware");
  script_dependencies("tftpd_dir_trav.nasl");
  script_require_udp_ports("Services/udp/tftp", 69);
  script_mandatory_keys("tftp/filename_available");

  script_tag(name:"solution", value:"Disinfect your system.");

  script_tag(name:"summary", value:"A TFTP server is running on this port.
  However, while trying to fetch some file, we retrieved an executable file.

  This is probably a backdoor.");

  script_tag(name:"solution_type", value:"Mitigation");
  script_tag(name:"qod_type", value:"remote_vul");

  exit(0);
}

include("port_service_func.inc");

function report_backdoor( port, file, type ) {

  local_var port, file, type, report;

  report = 'A TFTP server is running on this port. However, while trying to fetch '+ file + ', we got a '+ type + ' executable file.\n\nThis is probably a backdoor.';
  security_message( port:port, proto:"udp", data:report );

  if( port == 69 )
    set_kb_item( name:"tftp/backdoor", value:TRUE );

  set_kb_item( name:"tftp/" + port + "/backdoor", value:TRUE );
  exit( 0 );
}

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

nb = 0;

for( i = 0; i < 1000; i++ ) { # <1000 in case somebody gets mad

  fname = get_kb_item( "tftp/" + port + "/filename/" + i );
  if( ! fname ) exit( 0 );

  fcontent = get_kb_item( "tftp/" + port + "/filecontent/" + i );
  if( ! fcontent ) exit( 0 );

  mz = substr( fcontent, 0, 1 );
  ## MS format
  if( mz == 'MZ' || mz == 'ZM' )
    report_backdoor( port:port, file:fname, type:"MS" );

  ## Linux a.out
  # else if( mz == '\x01\x07' ) # object file or impure executable
  #   report_backdoor( port:port, file:fname, type:"a.out OMAGIC" );
  else if( mz == '\x01\x08' ) # pure executable
    report_backdoor( port:port, file:fname, type:"a.out NMAGIC" );
  else if( mz == '\x01\x0B' ) # demand-paged executable
    report_backdoor( port:port, file:fname, type:"a.out ZMAGIC" );
  else if( mz == 'CC' ) # demand-paged executable with the header in the text
    report_backdoor( port:port, file:fname, type:"a.out QMAGIC" );
  # else if( mz == '\x01\x11' ) # core file
  #   report_backdoor( port:port, file:fname, type:"a.out CMAGIC" );
  ## AIX a.out - is this wise?
  else if( mz == '\x01\xDF' )
    report_backdoor( port:port, file:fname, type:"XCOFF32" );
  else if( mz == '\x01\xEF' )
    report_backdoor( port:port, file:fname, type:"XCOFF64" );
  ## ELF
  else if( substr( fcontent, 0, 3 ) == '\x7fELF' )
    report_backdoor( port:port, file:fname, type:"ELF" );
}

exit( 99 );