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Cisco VPN 3000 Series Concentrators

Cisco Security Notice: Response to BugTraq - Cisco VPN 3000 Gateway MTU Overflow

Document ID: 59691

Revision 1.0

Last Updated 2002 July 15


Cisco Security Procedures


This document is provided to simplify access to Cisco responses to possible product security vulnerability issues posted in public forums for Cisco customers. This does not imply that Cisco perceives each of these issues as an actual product security vulnerability. This notice is provided on an "as is" basis and does not imply any kind of guarantee or warranty. Your use of the information on the page or materials linked from this page are at your own risk. Cisco reserves the right to change or update this page without notice at any time.


Original Report: http://www.securityfocus.com/archive/1/281544 leavingcisco.com. Cisco responded with the following, which is also archived at http://www.securityfocus.com/archive/1/282316 leavingcisco.com.

To: BugTraq 
Subject: Re: Cisco VPN3000 gateway MTU overflow
Date: Jul 15 2002 3:31PM
Author: Pete Davis <psd@cisco.com>

Thanks for the comments on problems you are experiencing. I did not receive
a response to any of the previous messages I sent you, so I have copied the
Bugtraq list, in case you do not receive email sent to your reply to

In general, the best way to report product problems is to open up a case
with the Cisco TAC (Tech Support) and our TAC will work with you to go
through your issues and help find a resolution. If you open a TAC case in
the future and you aren't satisfied, you should feel free to ask for the
case to be escalated. Any problems that are found will have bug IDs assigned
to them so that we can track and fix them for you as soon possible. All open
bugs are visible by all customers in our Bug Tracker tool on cco.cisco.com.

Security related items are handled through our PSIRT group
(psirt cisco com), which helps coordinate getting a fix and response out as
soon as possible including distribution of information to Bugtraq and
various other mailing lists.

Since your issue report appears at first glance to be problem related, I
will do my best to help respond. If you have specific questions or
additional information, please feel free to drop it to me directly and we
will work to help you resolve problems you are experiencing. If you a
specific support case #, please attach this in your email to me as well.
This information will help to make sure that we have all the details on your
specific environment so that if you have found a new problem, we can ensure
it is corrected ASAP.

Our next release will offer some additional nice enhancements regarding
fragmentation, Path MTU Discovery (PMTUD) and Client MTU determination.

1. Ability to choose whether or not large packet fragmentation is performed
before or after encapsulation "pre-fragmentation" as well as whether or not
the Concentrator responds to internal ICMP/PMTUD requests and requests an
internal device to lower its MTU, or otherwise fragments the packet and
clears the DF bit if set. The main reason that this will be a configurable
option is due to the fact that specific service packets of Windows 2000 and
some other OS's do not respond to PMTUD when the request comes from a client
on the same subnet as the server. (Concentrator)
2. Ability to define a maximum Concentrator MTU < default of ~15XX bytes
3. Adjusted setMTU behavior to fix specific NDISWAN/dial-up related issues
that may presently require manual MTU adjustment workaround (Client)

The setMTU application is needed on the client side regardless of anything
on the Concentrator side. In most cases (and we strive for all), running it
manually should be unnecessary. There are some enhancements with our next
release that fix a couple of specific dial-up/NDISWAN issues on the client

If you are interested in specifically trying out our new release and
providing beta feedback as to whether or not your problem is resolved or
would like us to help to resolve your issues, you can feel free to contact
me directly. Since the concentrator does fragment large packets, it would be
interesting to know if you're seeing 1580 byte packets on the wire or the
1580 byte packet fragmented based on the Ethernet MTU of 1518. In order to
investigate and resolve the problems you are reporting, please send me
information concerning your application/environment, MTU on the Client,
Concentrator configuration, device on the public side of the Concentrator
exhibiting problems and a sniffer trace showing 1580 byte packets on the
wire or other problems you are seeing.

I look forward to hearing back from you so that we can help resolve your

Best Regards,
-Pete Davis
Cisco Systems, Inc.
(508) 553-6007


Cisco VPN3000 gateway MTU overflow ==================================

Bug class: Conceptual/bad protocol implementation
Equipments affected: Cisco/VPN 3000 Concentrator with
  software vpn3000-3.5.Rel-k9.bin

  The Cisco VPN3000 gateway lets remote client dictate
  which maximum MTU to use when sending back ESP
  frames, regardless of the transmitting capabilities
  of the physical medium.

* Oversized frames get silently discarded by
  equipements linked to the gateway's public
  interface and retransmissions occur.
* Other disturbances or DoS against neighboring
  equipements may occur, especially as many IP
  stacks on routers and sniffers etc ... are
  poorly implemented.

  We have witnessed this phenomena after establishing
  tunnels with the "VPN dialer" over a modem
  connexion: when the target sends back ethernet
  frames with size close to the max ethernet MTU
  (1500), the gateway encrypts the frames adding
  ESP headers and stupidly tries to send a
  1580-bytes frame back to the client.

-> From the official documentation there is no way
to enforce a maximum MTU on the VPN gateway.
-> Hence: a gateway software patch by Cisco is
necessary: if MTU negociation occurs, the gateway
should set a max-MTU threshold (the physical medium's !).


* client side: For Windows-based OS (likely Unix and
  Linux-based OS too), Cisco released a tool
  called setMTU.exe that can prevent ill MTU
  negociation from happening.

* target side: artificially lowering the max MTU
  on the interfaces.

->  But such a policy is not acceptable:
  The VPN client, as well as remote targets,
  should not have to be aware of
  the gateway's interface configuration !

    The bug does not lie in client software, but
  in the gateway's software.

Master Phi


Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.

Updated: Oct 08, 2004 Document ID: 59691

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