Advisory ID: cisco-sa-19990311-xxconn
For Public Release 1999 March 11 16:00 UTC (GMT)
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.
This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19990311-xxconn.
This section provides details on affected products.
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 employees.
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.
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 versions.
Routers running release 4.3(1) or later software are not affected by CSCdm03231.
No other Cisco products are currently known to be affected by these vulnerabilities.
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" section.
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.
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
This section describes the impact of these vulnerabilities.
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.
If you are a registered CCO user and you have logged in, you can view the bug details.
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.
When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
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 Technical Assistance Center ("TAC") or your contracted maintenance provider for assistance.
This section describes workarounds for these vulnerabilities
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.
Cisco has made free software available to address this vulnerability for affected customers. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.
Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, customers agree to be bound by the terms of Cisco's software license terms found at http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.
Do not contact either "email@example.com" or "firstname.lastname@example.org" for software upgrades.
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 website at http://www.cisco.com.
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 guidance and assistance with the appropriate course of action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.
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: email@example.com
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.
Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.
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.
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A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.
This advisory will be posted on Cisco's worldwide website at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-19990311-xxconn.
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.
- firstname.lastname@example.org (includes CERT/CC)
- Various internal Cisco mailing lists
Future updates of this advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.
Initial public release.