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TUCoPS :: Cisco :: ciacj034.txt

Cisco 7xx TCP Http Vulnerabilities



                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                     Cisco 7xx TCP and HTTP Vulnerabilities

March 12, 1999 19:00 GMT                                          Number J-034
PROBLEM:       Cisco has identified two unrelated security vulnerabilities in
               the software used on the Cisco 7xx series of small-office and
               home-office routers.
                 1) TCP connections to the routers' TELNET ports
                 2) HTTP server
PLATFORM:      7xx series routers, (not the 7xxx series).
DAMAGE:        1) If exploited this vulnerability could cause system reloads
               and therefore result in a denial of service.
               2) Unless the server is explicitly disabled, it can be used to
               make changes to the router configuration, and/or to gain
               information about that configuration.
SOLUTION:      Apply workarounds or install upgrades.
VULNERABILITY  Risk is low on these two vulnerabilities. The TCP bug doesn't
ASSESSMENT:    give attackers access to the router CLI, nor does it give them
               any other way of controlling the router beyond inducing denial
               of service. The HTTP vulnerability can be eliminated by
               disabling the software or upgrade where applicable.

[  Start Cisco Advisory  ]

Cisco 7xx TCP and HTTP Vulnerabilities
Revision 1.1
For release 08:00 AM US/Pacific, Thursday, March 11, 1999

Cisco internal use only until release date

This notice addresses two unrelated security vulnerabilities in the software
used on the Cisco 7xx series of small-office and home-office routers. These
vulnerabilities affect only the 7xx series routers (not the 7xxx series); no
other Cisco product is affected.

The first vulnerability, which has been assigned Cisco bug ID CSCdm03231,
can be used to cause system reloads, and therefore denial of service, using
TCP connections to the routers' TELNET ports.

The second vulnerability has not been assigned a bug ID. 7xx routers running
software versions 3.2(5) through 4.2(3) support a simple HTTP server. This
HTTP server is enabled by default. Unless the server is explicitly disabled,
it can be used to make changes to the router configuration, and/or to gain
information about that configuration. This is intentional behavior, but is
mentioned in this notice because it appears that customers have been caught
unawares by it.

There are configuration workarounds for both of these vulnerabilities.

Who Is Affected
Only networks incorporating 7xx series small-office/home-office routers are
affected by these vulnerabilities. 7xx routers are designed to provide
network connectivity for small remote networks using ISDN BRI lines. If your
network includes 7xx series routers, they are most likely to be found in the
homes of network users, or in remote offices with no more than a few

CSCdm03231 affects all Cisco 7xx routers, running any software version up to
and including release 4.2(3), whose administrators have not taken specific
steps to filter incoming TCP connections. Such filtering is not enabled by
default. Routers running release 4.3(1) or later software are not affected
by CSCdm03231.

The HTTP server is present in all software releases from 3.2(5) through
4.2(3), inclusive. The server is enabled by default in all of these software


Impact of CSCdm03231
- -------------------
CSCdm03231 permits a remote attacker to force a 7xx router to reload,
denying service to the router's home or small office user. It may sometimes
be possible to degrade performance without actually inducing a router
reload. The attack consumes relatively little bandwidth compared to
flooding-based denial of service attacks. In all cases, the router will
recover after the attack stops; the attacker must send traffic continuously
to maintain denial of service. However, if the router has reloaded, the end
user may have to take some installation-dependent action to cause the router
to redial the ISDN connection.

This vulnerability does not give attackers access to the router CLI, nor
does it give them any other way of controlling the router beyond inducing
denial of service.

Impact of the Presence of the HTTP Server
- ----------------------------------------
If the HTTP server is enabled, it can be used to change the router's
configuration, or to retrieve information about that configuration. No
special tools are required.

Software Details
CSCdm03231 affects all software versions earlier than 4.3(1). Customers with
76x or 77x routers should upgrade to release 4.3(1). Because of memory
limitations, release 4.3 is not supported on the 75x routers; customers with
75x routers should use IP filtering as described in the "Workarounds"

The HTTP server is present in all software versions from 3.2(5) through
4.2(3). It is not present in 3.2(4) or earlier releases, nor is it present
in 4.3. Customers with 76x or 77x routers should upgrade to release 4.3(1),
primarily because of the desirability of installing the CSCdm03231 fix. The
HTTP server may be disabled in any software version; disabling the server is
the recommended course of action for customers with 75x routers.

Getting Fixed Software
- ---------------------
Cisco is offering free software upgrades to remedy CSCdm03231 for all
vulnerable customers, regardless of contract status. Customers with service
contracts may upgrade to any software version, although they may only
install the feature sets they've already purchased. Customers without
contracts may upgrade to version 4.3(1).

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

Customers without contracts 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:

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.


Workaround for CSCdm03231
- ------------------------
CSCdm03231 can be made much less useful to attackers by preventing incoming
TCP connections to the router from untrusted hosts. This can be done with
the set ip filter profile command, as in the following example:

     set ip filter tcp source = not trusted-host destination = router block

This example would configure the router to accept incoming TCP connections
only from a single trusted administrative host. More elaborate
configurations, permitting connections of various types from various hosts,
are possible; see the router documentation for more information.

Disabling the HTTP Server
- ------------------------
The HTTP server may be disabled with the system command set clickstart off.

Exploitation and Public Announcements
Cisco knows of no public announcements or discussion of CSCdm03231 before
the date of this notice. Cisco has had no reports of malicious exploitation
of CSCdm03231. CSCdm03231 was found by an outside security company during
laboratory testing.

A simple program is needed to effectively exploit CSCdm03231. Although Cisco
knows of no program available to the public specifically for this purpose,
writing such a program would require very little effort, and only the most
basic of skill. Also, certain publicly-available programs intended for other
purposes could be used or adapted to exploit the vulnerability.

Cisco has had no reports of abuse of the HTTP server on the 7xx series.
However, the potential for abuse is discussed in the product documentation,
and must be considered to be known to potential attackers.

Status of This Notice
This is a final field notice. Although Cisco cannot guarantee the accuracy
of all statements in this notice, all the facts have been checked to the
best of our ability. Cisco does not anticipate issuing updated versions of
this notice unless there is some material change in the facts. Should there
be a significant change in the facts, Cisco may update this notice.

- -----------
This notice will be posted on Cisco's Worldwide Web site at . In addition to
Worldwide Web posting, the initial version of this notice is being sent 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,      Initial release candidate version
 22:00 US/Pacific
 Revision 1.1,      Minor editing of software names
 09:30 US/Pacific

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 Worlwide
Web site at

This includes instructions for press inquiries regarding Cisco security

- -------------------------------------------------------------------------
This notice is copyright 1999 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, including
all date and version information.
- -------------------------------------------------------------------------

[  End Cisco Advisory  ]
CIAC wishes to acknowledge the contributions of Cisco for the
information contained in this bulletin.

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