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Security aspects of time synchronization infrastructure

A  large  number of services on modern corporate network require time to
be  synchronized  within  network  or with absolute time and may fail if
there  are  any  problems  with time synchronization. Below are just few
examples of services and required time precision.

For synchronization within network:

 Expired software licenses/keys: ~weeks
 Directory Synchronization: ~minutes
 Collaboration services: ~minutes
 Scheduling services: ~minutes
 Authentication services (for example Kerberos): ~minutes
 Logging: ~seconds
 Production: ~milliseconds

For synchronization with absolute time:

 Certificates expiration: ~months
 Expired software licenses/keys: ~weeks
 Accounts expiration: ~days
 Allowed logon hours: ~minutes
 Collaboration services: ~minutes
 Scheduling services: ~minutes
 Authentication  for  remote  principials (for example external Kerberos
 trusts): ~minutes
 Authentication with hardware devices (for example eTokens): ~minutes
 Logging: ~seconds
 Production: ~milliseconds

The   aim   of   this   article  is  not  to  show  importance  of  time
synchronization  infrastructure, but to demonstrate important aspects of
time  synchronization infrastructure implementation and to point to some
linked problems from real life.

If  any  of  these  services  are  used  on  corporate  network  and are
considered  as  critical  services,  time synchronization infrastructure
should  also be considered as a critical service. Any inconsistence in a
time synchronization in result of failure, poor administration, sabotage
or  network  attack  may cause depending services to fail resulting in a
lost  of productivity or security and leading to additional expenses. As
a  very  common example, improperly set clocks on user's workstation can
lead  to  inability  to  logon  to his workstation and perform his usual

It's very common that time synchronization infrastructure is missed from
network  design  or  only mechanisms implemented by software vendors are
used  without  reviewing  these  mechanisms from the point of stability,
security and conformance to organization's expectance.

A  main idea behind this article is to show most common problems in time
synchronization  infrastructures  based  on  NTP and SNTP protocols as a
most  widely  implemented in enterprise networks, but problems described
just  slightly  depend  on  network level protocol. If you use different
protocol for time synchronization you still can be interested.

I.      Network synchronization topology

First  question  you  should  ask  yourself  is  how time information is
distributed   within   your   network,   what   is   topology   of  time
synchronization service and reliable this topology is.

RFC 1305 defines Network Time Protocol (NTP) and describes few different
time  synchronization  models. It contains very good review of different
protocols.  NTP  protocol  supports  multiple  masters  model  for  time
synchronization.  This model allows creating partially (of fully) meshed
network  time  topology  with  few,  potentially external, reliable time
sources.  Information  received  from  peers is filtered to provide most
appropriate  result  by  the  means of mathematical statistics. Gathered
information  is  used  to  compute  different  measures  like  skew  and
accuracy and correct clock settings. If any of network nodes has invalid
time  configuration  it  will  not  have  impact to the rest of network,
because  statistical  methods allow filtering out unreliable results. If
there  is  a  host  with  reliable  time  on  the  network (that is host
synchronized  with  some  hardware  source,  like  radio  clocks, cesium
clocks,  GPS  clocks,  etc)  - whole network will be finally, after some
time, synchronized with this host. This distributed mesh topology brings
a kind of inertion to NTP based infrastructure. If you have no access to
whole  network  you cannot make this network to change time immediately.
In this topology public external NTP server can be used as a time source
in relatively safe way.

NTP protocol by itself is not secure - it's UDP based, with fixed source
and   destination   port   and   is   implemented  as  a  single  packet
request-response  service  without any sequence number or another replay
protection.  It  makes  it  easy  to  spoof  NTP packets blindly, but to
succeed  in  this attack you have to spoof replies from the large number
of  internal  hosts.  It's  hard  or  even  impossible without access to
internal  network.  And,  of cause, this topology has no single point of

To implement some kind of security NTP defines message authenticator, to
allow  host to sign its NTP packet. This field is optional and mechanism
of  packet  signing  and key exchange and verification is not defined by
protocol.   It  makes  it  hard  to  implement  this  protection  inside
enterprise  network and almost impossible to implement authenticator for
public time servers.

As  usually,  the  problem  lays  in implementation. To implement NTP in
multiple  master  partially  meshed  topology  it  was designed for, you
should  manually  configure  all  peers  for  all  peers. For additional
security   (if   software   developers   implement  it)  you  must  also
pre-exchange   keys  or  implement  PKI  infrastructure.  After  initial
configuration NTP does not require any maintenance. But, as any manually
configured   service   it   makes   additional   workloads   for  system
administrators  to  read  documentation  and implement initial settings.
Most  administrators  do  not read NTP documentation. That's why usually
only  one  NTP  server  is  configured.  In this single master case time
synchronization  topology  becomes directed tree. In this topology there
is no more mesh stability and failover. If higher level host has invalid
time information, host synchronized with this host may adjust his clocks
to  some  intermediate value. There is still some inertness, but it does
not  protect  your  network  any  more. Tree's root is a single point of
failure and is a good attack target.

To  help  lazy  administrator  to  keep  his network time in most simple
manner  RFC  1769  (Simple  Network Time Protocol, SNTP) was introduced.
SNTP  works  in  single  master  mode  and things are quite simple - you
synchronize  your  clocks  with remote peer, having only network lags to
count  in.  Of cause, directed tree is only available topology with this
protocol.  For  tree topology this protocol is better adopted and is not
less secure. In case root node clocks are changed, whole network will be
synchronized  in  reasonable  amount of time (for NTP it may lead to the
situation different levels of tree have different time). As for networks
communications  security  SNTP  is  absolutely  same - UDP based, single
packet,  both  SRC  and  DST  ports are 123 and status for Authenticator
field is unclear.

II.     Implementations

What  are  requirements  for  secure  NTP  or  SNTP implementation? Main
requirement  is  to  support  Authenticator  field at least for internal
synchronization.  If  Authenticator is not supported only alternative is
to  use  IPSec to sign all synchronization traffic (because this traffic
doesn't  contain  any  sensitive  information to be encrypted AH mode is
preferable).  If  Authenticator  is  supported, either PKI or manual key
configuration  or  both  (and  it's  preferable)  must be supported. PKI
support  is  helpful  to  automate  time  synchronization infrastructure
deployment  in  enterprise network. Manual key configuration is required
to  synchronize  with  public  time  source  (public  time source should
provide public key to prevent spoofing). Of cause, Authenticator must be
implemented  in  a  way  to prevent replay attacks (it's required by RFC

Also,  secure  implementation  should  provide configuration for maximum
correction  time.  RFC  1305  require  these  settings  to be tunable by
network  administrator.  Setting  maximum correction time to lower value
can  cause  problem of client computer to fail time synchronization (for
example  if  host  was  offline  for large period of time). Higher value
opens   possibility   of   attacks   against   services  requiring  time
synchronization  (see  beginning  of  this article), it also may lead to
instability,   because  clocks  skew  and  precision  may  be  invalidly
calculated.  As  a  network  infrastructure  architect  you  must find a
balance. This value for managed network is usually under 15 minutes.

Even  ability to change time for some relatively small value during some
period   can   lead   to   unpleasant   impact   to  stability  of  time
synchronization service, because times values could be malcrafted in way
to  values  for  clocks precision and skew to be incorrectly computed on
synchronized computer leading to increasing skew (drift).

Another  protection is detection of double replies. Double reply usually
means  somebody is trying to spoof NTP/SNTP server reply. It's not clear
how  should  system  react  in  this  case  of double reply (except this
situation  definitely  must  be logged and administrator must be alerted
with  all  possible  ways),  probably  best protection is to ignore both
packets  But  it  makes it possible to prevent time synchronization with
the  flood  of fake NTP/SNTP responses. This kind of attack is extremely
easy but it may require months of time before any negative results.

In  addition, random fraction must be introduced in time synchronization
schedule,  to  make  it  harder  to spoof NTP/SNTP server reply blindly.
Without this fraction remote attacker can blindly spoof NTP server reply
with a single packet.

III.    Example: Windows Active Directory forest network

Windows  2000  Active  directory network is chosen as an example of time
synchronization  infrastructure  for few reasons: it's fully implemented
and  documented  by  vendor, it requires minimum of manual configuration
and it contains both problems and advantages we would like to discuss.

Before    Windows   2000   Microsoft   had   no   time   synchronization
infrastructure.  The  only  available  method  was  manual  (or batched)
synchronization  was  'net time' command. This command required elevated
privileges. It was impossible to synchronize time with user logon script
without giving special permission to set system time to user. Because in
Windows  networks  it's  client  computer  to check allowed logon hours,
giving an ability to change system time to user is not desirable.

In  Windows  2000 Microsoft did a pretty nice job to implement automated
time  synchronization  infrastructure. It's described in details in [3].
This  document  describes  security  impacts  of  time  synchronization.
Microsoft's approach to time synchronization is really serious. SNTP was
chosen  as  a  transport.  Time synchronization supports 3 modes: Domain
mode (Nt5DS), SNTP mode (NTP), and no synchronization mode (NoSync) mode
[6].  In  first  mode  domain  member  computer synchronizes with domain
controller,  domain  controller  synchronizes  with  PDC  emulator,  PDC
emulator  synchronizes  with  domain  controller  in forest root domain,
domain  controllers in forest root domains synchronize with PDC emulator
in  forest  root  domain.  By  default,  in Windows 2000 PDC emulator in
forest root domains is not synchronized and logs an error to application
log). In this mode Authenticator is used and all SNTP packets are signed
with computer keys. For most networks this topology is good enough. It's
fully  automated  and  requires  minimal  administration. It's protected
against  time spoofing attack (not from packet flooding, but this attack
is less significant).

In  last  mode host doesn't synchronize with any external source and use
only  it's own clocks (that is usually it acts as a reliable time source
for network).

The  problem  is  with SNTP mode. In this mode host is synchronized with
external NTP source (RFC 1769 clearly says SNTP shouldn't be used on the
levels  above  subnet  level).  Microsoft  documentation [3], recommends
using  one  the  public  time sources to synchronize clocks of the first
domain   controller  in  first  domain  with  absolute  time.  Microsoft
publishes  a  list  of  publicly  available  time  servers [7]. For SNTP
(external  synchronization)  no  security  was  supported  at  all  - no
key/Authenticator  support,  no  limitation  for  clock  corrections, no
protection  against  double  packets  and scheduled time synchronization
without random fraction. If network is configured in accordance to these
recommendations  it's  possible  to bring whole Windows 2003 forest down
with a single UDP packet.

In  2001  I contacted Microsoft about this problem. Microsoft pointed me
to  [3] (it was hard to find it originally) and recommended to use IPSec
in  accordance  with  [3]  recommendations.  After  my  claim  that it's
impossible  to  use  IPSec  with public time sources, Microsoft accepted
this  issue  as  design  flow  as  [MSRC  1088cb].  To help user to find
documentation [4] and [5] were published. In 2002 Microsoft reported:

-=-= Quote Microsoft Security Response Center [email protected] =-=-

We're happy to report that we've completed all of the (many, many) fixes
and changes as a result of the report you brought us.

At this point, we are in final testing for Windows 2000 SP3, so we can't
get fixes of this magnitude in at this late stage:  we are planning to
include this with Windows 2000 SP4.

-=-= End of Quote =-=-

In 2003 SP4 was released. Documentation about fixes included has nothing
related  to  SNTP.  Also, Windows XP and Windows 2003 were released with
configuration  change  that  time.windows.com  is  not used as a default
external (SNTP) time source.

Initially  I  thought  this  change  was made to allow Microsoft to sign
responses  from  time.windows.com  and to check it's key on Windows host
and  same  change is made to Windows 2000. But testing demonstrated that
time.windows.com  does  NOT  sign SNTP messages. After inability to find
any  information  on  SNTP  fixed  included in SP4 I contacted Microsoft
again  in  2004. After a dialog with MSRC I was able to find some useful
information,  but  it's  probably  incomplete.  At least Microsoft added
undocumented registry parameter for time synchronization service


This  value  allows  to set maximum correction time allowed for external
source  (it's  never  applied  to  internal domain synchronization. This
parameter  doesn't  eliminate  problem  but  it's  possible to use it on
forest's  root  PDC  emulator  to  prevent  most important services from
immediate  attack, but it's still possible to launch attack against time
precision and skew calculation.

Default value is 43200 (12 hours). 12 hours is huge period that allows a
lot of services to be attacked. I recommend value below 60 to be used on
forest's root PDC emulator.

IV.     Design decisions:

For  networks  where  time  synchronization is mission critical consider
using  of  hardware  clocks (for example radio or GPS) for reliable time
sources.  Implement  mesh topology with NTP protocol for high level time
distribution  servers and mission critical hosts (in Windows environment
you  must use third party software and mark these hosts as reliable time
sources  [5], [6] to prevent Windows time service from correcting system
clocks).  Check  if NTP implementation supports Authenticator mechanism.
If  it  does,  exchange  keys  (implement  PKI,  if  required).  If  NTP
implementation  doesn't support Authenticator mechanism, or this support
doesn't  correspond  to  your  needs  implement IPSec in AH mode between
hosts  to  protect  synchronization  traffic. Reliable time sources from
Internet can be used only with average and low security requirements for
mesh  topology with NTP and only with low security requirements for tree
topology with NTP or SNTP.

For  the  rest of network you can use SNTP (for example Windows built-in
time  synchronization)  or  NTP  in  single  master  mode. Protection of
network traffic as described above is also required.

For  NTP/SNTP protocols after initial configuration there is no need for
regular  administration,  but  high level of monitoring is required. Any
synchronization    related    event,   especially   repeated   for   few
synchronization cycles must be carefully checked on everyday basis.

Microsoft created an article Q884776[8] "Configuring the Windows Time
service against a large time offset".

V.      References:

[1]  David  L.  Mills,  Network Time Protocol (Version 3) Specification,
Implementation and Analysis, RFC 1305
[2] D. Mills, Simple Network Time Protocol (SNTP), RFC 1769
[3] The Windows Time Service
[4] The Windows Time Service
[5] Q224799 - Basic Operation of the Windows Time Service
[6] Q223184 - Registry Entries for the W32Time Service
[7] Q262680  -  A List of the Simple Network Time Protocol Time Servers
That Are Available on the Internet
[8] Q884776 Configuring the Windows Time service against a large time offset