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Cisco IOS vulnerable to DoS via crafted PPTP packet sent to port 1723/tcp

Vulnerability Note VU#656315

Original Release Date: 2001-07-29 | Last Revised: 2004-02-23


Cisco IOS contains a vulnerability that allows an intruder to crash the router.


By sending a specially crafted PPTP packet to port 1723, an intruder can crash a device running a vulnerable version of IOS. Quoting from the Cisco Advisory:

    By sending a crafted PPTP packet to a port 1723, a control PPTP port, it is possible to crash the router. This vulnerability does not require special router configuration. Enabling PPTP is sufficient to expose the vulnerability. The router will crash after it receives a single packet.
For more information, see the Cisco Advisory.


An intruder can cause a vulnerable router to crash.


Upgrade to a later version of IOS as documented in the Cisco Advisory.

Vendor Information


Cisco Systems Inc. Affected

Updated:  July 29, 2001



Vendor Statement


             Cisco Security Advisory: Cisco IOS PPTP vulnerability

Revision 1.0

  For Public Release 2001 July 12 08:00 (UTC -0800)


   Point to Point Tunneling Protocol (PPTP) allows users to tunnel to an
  Internet Protocol (IP) network using a Point to Point Protocol (PPP).
  The protocol is described in RFC2637.

   PPTP implementation using Cisco IOS® software releases contains a
  vulnerability that will crash a router if it receives a malformed or
  crafted PPTP packet. No special conditions or router configuration is

   This vulnerability is present in all Cisco IOS releases that support
  PPTP protocol. PPTP is supported in the following software releases:

     * 12.1 train, releases: T, E, EZ, YA, YD and YC
    * 12.2 train, all releases

   No other Cisco product is vulnerable.

   There is no workaround for this vulnerability.

   This advisory is available at

Affected Products

   This vulnerability is present in all Cisco IOS releases that are
  supporintg PPTP protocol. PPTP is supported in the following releases

     * 11.x train does not supports PPTP
    * 12.1 train, releases: T, E, EZ, YA, YD and YC
    * 12.2 train, all releases

   To determine if a Cisco product is running an affected IOS, log in to
  the device and issue the command show version. Look for the
  "Internetwork Operating System Software" or "IOS (tm)" information,
  which also will have a version number. Other Cisco devices either will
  not have the command "show version" or will give different output.


   By sending a crafted PPTP packet to a port 1723, a control PPTP port,
  it is possible to crash the router. This vulnerability does not
  require special router configuration. Enabling PPTP is sufficient to
  expose the vulnerability. The router will crash after it receives a
  single packet.

   This vulnerability is documented as Cisco Bug ID CSCdt46181


   By repeaditly exploiting this vulnerability it is possible to cause
  permanent Denial of Service (DoS). This denial is not only of the PPTP
  functionality but the whole router will stop functioning.

Software Versions and Fixes

   Each row of the table describes a release train and the platforms or
  products for which it is intended. If a given release train is
  vulnerable, then the earliest possible releases that contain the fix
  and the anticipated date of availability for each are listed in the
  "Rebuild", "Interim", and "Maintenance" columns. A device running a
  release in the given train that is earlier the release in a specific
  column (less than the earliest fixed release) is known to be
  vulnerable. The release should be upgraded at least to the indicated
  release or a later version (greater than the earliest fixed release

   When selecting a release, keep in mind the following definitions:

               Most heavily tested and highly recommended release of any
               label in a given row of the table.

               Constructed from the previous maintenance or major
               release in the same train, it contains the fix for a
               specific defect. Although it receives less testing, it
               contains only the minimal changes necessary to effect the

               Built at regular intervals between maintenance releases
               and receive less testing. Interims should be selected
               only if there is no other suitable release that addresses
               the vulnerability, and interim images should be upgraded
               to the next available maintenance release as soon as
               possible. Interim releases are not available via
               manufacturing, and usually they are not available for
               customer download from CCO without prior arrangement with
               the Cisco TAC.

   In all cases, customers should exercise caution to be certain the
  devices to be upgraded contain sufficient memory and that current
  hardware and software configurations will continue to be supported
  properly by the new release. If the information is not clear, contact
  the Cisco TAC for assistance as shown in the following section.

   More information on IOS release names and abbreviations is available

|        | Description of Image                                            |
| Train  |      or Platform             Availability of Fixed Releases*    |
|      12.1-based Releases           | Rebuild    |Interim** | Maintenance |
| 12.1E  |Core/ISP support: GSR,     |            |          | 12.1(8a)E   |
|        |RSP, c7200                 |            |          |             |
| 12.1EZ |Early Deployment (ED):     |12.1(6)EZ2  |          |             |
|        |special image              |            |          |             |
|        |Early Deployment(ED): VPN, |Not Scheduled                        |
|  12.1T |Distributed Director,      +-------------------------------------+
|        |various platforms          |Upgrade recommended to 12.2(3)       |
|        |                           |Not Scheduled                        |
| 12.1YA |Short-lived early          +-------------------------------------+
|        |deployment release         |Upgrade recommended to 12.2(2)XB     |
| 12.1YC |Short-lived early          |12.1(5)YC1  |          |             |
|        |deployment release         |            |          |             |
| 12.1YD |Short-lived early          |12.1(5)YD2  |          |             |
|        |deployment release         |2001-June-25|          |             |
|       12.2-based Releases          | Rebuild    |Interim** | Maintenance |
|  12.2  |General deployment release |            |12.2(1.1) |12.2(3)      |
|        |for all platforms          |            |          |2001-August  |
|  12.2T |General deployment release |            |          |12.2(4)T     |
|        |for all platforms          |            |          |             |
| 12.2XA |SPLOB                      |            |          |12.2(2)XA    |
| 12.2XD |Short-lived early          |12.2(1)XD1  |          |             |
|        |deployment release         |            |          |             |
| 12.2XE |Short-lived early          |            |          |12.2(1)XE    |
|        |deployment release         |            |          |             |
| 12.2XH |Short-lived early          |            |          |12.2(1)XH    |
|        |deployment release         |            |          |2001-June-25 |
| 12.2XQ |Short-lived early          |            |          |12.2(1)XQ    |
|        |deployment release         |            |          |2001-June-23 |
|                                   Notes                                  |
| * All dates are estimated and subject to change.                         |
|                                                                          |
| ** Interim releases are subjected to less rigorous testing than regular  |
| maintenance releases, and may have serious bugs.                         |

Obtaining Fixed Software

   Cisco is offering free software upgrades to eliminate this
  vulnerability for all affected customers.

   Customers with contracts should obtain upgraded software through their
  regular update channels. For most customers, this means that upgrades
  should be obtained through the Software Center on Cisco's Worldwide
  Web site at

   Customers whose Cisco products are provided or maintained through
  prior or existing agreement with third-party support organizations
  such as Cisco Partners, authorized resellers, or service providers
  should contact that support organization for assistance with the
  upgrade, which should be free of charge.

   Customers who purchase direct from Cisco but who do not hold a Cisco
  service contract and customers who purchase through third party
  vendors but are unsuccessful at obtaining fixed software through their
  point of sale should get their upgrades by contacting the Cisco
  Technical Assistance Center (TAC). TAC contacts are as follows:

     * +1 800 553 2447 (toll-free from within North America)
    * +1 408 526 7209 (toll call from anywhere in the world)
    * e-mail:

   Please have your product serial number available and give the URL
  of this notice as evidence of your entitlement to a free upgrade. Free
  upgrades for non-contract customers must be requested through the TAC.

   Please do not contact either "" or
  "" for software upgrades.


   There is no workaround for this vulnerability.

Exploitation and Public Announcements

   This vulnerability has been reported to us by a customer.

   The Cisco PSIRT received a report about potential exploitation of this
  vulnerability for malicious purpose against one customer.

Status of This Notice: INTERIM

   This is an interim security advisory. Cisco anticipates issuing
  updated versions of this notice at irregular intervals because there
  may be material changes in the facts. The reader is warned that this
  notice may contain inaccurate or incomplete information. Although
  Cisco cannot guarantee the accuracy of all statements in this notice,
  all of the facts have been checked to the best of our ability. Cisco
  anticipates issuing monthly updates of this notice until it reaches
  FINAL status.

   Note: A standalone copy or paraphrase of the text of this security
  advisory that omits the distribution URL in the following section is
  an uncontrolled copy and may lack important information or contain
  factual errors.


   This notice will be posted on Cisco's Worldwide Web site at
  In addition to Worldwide Web posting, a text version of this notice is
  clear-signed with the Cisco PSIRT PGP key and is posted to the
  following e-mail and Usenet news recipients:

    * (includes CERT/CC)
    * Various internal Cisco mailing lists

   Future updates of this notice, if any, will be placed on Cisco's
  Worldwide Web server, but may or may not be actively announced on
  mailing lists or newsgroups. Users concerned about this problem are
  encouraged to check the URL given above for any updates.

Revision History

   Revision 1.0 2001-Jul-12 08:00 UTC -0800 Initial public release

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 Web site at
  This includes instructions for press inquiries regarding Cisco
  security notices.

   All Cisco Security Advisories are available at

   This notice is Copyright 2001 by Cisco Systems, Inc. This notice may
  be redistributed freely after the release date given at the top of the
  text, provided that redistributed copies are complete and unmodified,
  and include all date and version information.

Version: 2.6.2


Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.


The CERT/CC has no additional comments at this time.

If you have feedback, comments, or additional information about this vulnerability, please send us email.

CVSS Metrics

Group Score Vector



Thanks to Cisco for the information contained in their advisory, upon which this document is based.

This document was written by Shawn V. Hernan.

Other Information

CVE IDs: CVE-2001-1183
Severity Metric: 20.79
Date Public: 2001-07-12
Date First Published: 2001-07-29
Date Last Updated: 2004-02-23 22:41 UTC
Document Revision: 5

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