Network Time Protocol Daemon (ntpd) read_mru_list() Remote DoS

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

include("compat.inc");

if (description)
{
  script_id(95389);
  script_version("1.14");
  script_set_attribute(attribute:"plugin_modification_date", value:"2020/06/12");

  script_cve_id("CVE-2016-7434");
  script_bugtraq_id(94448);
  script_xref(name:"CERT", value:"633847");
  script_xref(name:"EDB-ID", value:"40806");

  script_name(english:"Network Time Protocol Daemon (ntpd) read_mru_list() Remote DoS");
  script_summary(english:"Checks response from a mrulist query.");

  script_set_attribute(attribute:"synopsis", value:
"The remote NTP server is affected by a denial of service
vulnerability.");
  script_set_attribute(attribute:"description", value:
"The remote NTP server is affected by a denial of service vulnerability
due to improper validation of mrulist queries. An unauthenticated,
remote attacker can exploit this, via a specially crafted NTP mrulist
query packet, to terminate the ntpd process.  

Note that the NTP server is reportedly affected by additional
vulnerabilities as well; however, Nessus has not tested for these.");
  script_set_attribute(attribute:"see_also", value:"http://support.ntp.org/bin/view/Main/NtpBug3082");
  script_set_attribute(attribute:"see_also", value:"https://www.nwtime.org/ntp428p9_release/");
  script_set_attribute(attribute:"solution", value:
"Upgrade to NTP version 4.2.8p9 or later.");
  script_set_cvss_base_vector("CVSS2#AV:N/AC:L/Au:N/C:N/I:N/A:P");
  script_set_cvss_temporal_vector("CVSS2#E:POC/RL:OF/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:P/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2016-7434");
  script_set_attribute(attribute:"exploitability_ease", value:"Exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"true");

  script_set_attribute(attribute:"vuln_publication_date", value:"2016/11/21");
  script_set_attribute(attribute:"patch_publication_date", value:"2016/11/21");
  script_set_attribute(attribute:"plugin_publication_date", value:"2016/11/29");

  script_set_attribute(attribute:"plugin_type", value:"remote");
  script_set_attribute(attribute:"cpe", value:"cpe:/a:ntp:ntp");
  script_end_attributes();

  script_category(ACT_ATTACK);
  script_copyright(english:"This script is Copyright (C) 2016-2020 and is owned by Tenable, Inc. or an Affiliate thereof.");
  script_family(english:"Misc.");

  script_dependencies("ntp_open.nasl");
  script_require_keys("NTP/Running");

  exit(0);
}

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

# Make sure NTP server is running
get_kb_item_or_exit('NTP/Running');

# Get ntp server port
port = get_service(svc:"ntp", ipproto:"udp", default:123, exit_on_fail:TRUE);

# Connect to the ntp server
s = open_sock_udp(port);
if (!s) audit(AUDIT_SOCK_FAIL, port);

# CTL_OP_REQ_NONCE
req = 
  '\x1e' +
  '\x0c' +
  '\x00\x01' + 
  '\x00\x00' +
  '\x00\x00' +
  '\x00\x00' +
  '\x00\x00';

send(socket: s, data: req);
res = recv(socket:s, length:1024);

if("nonce" >!< res)
{
  exit(0, 'Failed to get nonce for a mrulist query. Possible reasons : \n' + 
    '- Remote NTP server may have disabled mrulist query.\n' +  
    '- Remote NTP server does not support mrulist query.'); 
}

i = stridx(res, "nonce");
nonce = substr(res, i);
nonce = str_replace(string: nonce, find:'\x0d', replace:'');
nonce = str_replace(string: nonce, find:'\x0a', replace:'');

# CTL_OP_READ_MRU
#
# vulnerable and patched ntpd handle the malformed 'frags'
# parameter differently
data = nonce + ",frags=aaa,limit=1";
req = 
  '\x1e' +
  '\x0a' +
  '\x00\x02' +
  '\x00\x00' +
  '\x00\x00' +
  '\x00\x00' +
  mkword(strlen(data), order: BYTE_ORDER_BIG_ENDIAN);
req += data;

if (strlen(req) % 4)
  req += crap(data:'\x00', length: 4 - strlen(req) % 4);

send(socket: s, data: req);
res = recv(socket:s, length:1024);
close(s);

# Vulnerable ntpd ignores the malformed 'frags'
# parameter, and continues processing the CTL_OP_READ_MRU command.
if(res)
{
  security_report_v4(
    port       : port,
    proto      : "udp",
    severity   : SECURITY_WARNING
  );
}
# Patched ntpd stops processing the CTL_OP_READ_MRU command upon
# encountering a malformed 'frags' parameter.
else
{
  audit(AUDIT_HOST_NOT, 'affected');
}