Cisco Security Advisory: DOCSIS Read-Write Community String Enabled in Non-DOCSIS Platforms Sep 20 2006 05:07PM
Cisco Systems Product Security Incident Response Team (psirt cisco com)
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Cisco Security Advisory:
DOCSIS Read-Write Community String Enabled in Non-DOCSIS Platforms

Document ID: 71255

Advisory ID: cisco-sa-20060920-docsis


Revision 1.0

For Public Release 2006 September 20 1600 UTC (GMT)

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Affected Products
Software Version and Fixes
Obtaining Fixed Software
Exploitation and Public Announcements
Status of this Notice: FINAL
Revision History
Cisco Security Procedures

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A vulnerability exists in certain Cisco IOS software release trains
running on the Cisco IAD2400 series, 1900 Series Mobile Wireless Edge
Routers and Cisco VG224 Analog Phone Gateways. Vulnerable versions may
contain a default hard-coded Simple Network Management Protocol (SNMP)
community string when SNMP is enabled on the device. The default
community string is a result of inadvertently identifying these devices
as supporting Data Over Cable Service Interface Specification (DOCSIS)
compliant interfaces. The consequence of this error is that an
additional read-write community string may be enabled if the device is
configured for SNMP management, allowing a knowledgeable attacker the
potential to gain privileged access to the device.

Cisco is making free software available to address this vulnerability
for affected customers. There are workarounds available to mitigate the
effects of the vulnerability.

This advisory is posted at

Affected Products

The following products are affected if they run a vulnerable release of
Cisco IOS software with the SNMP server enabled. Any version of Cisco
IOS software prior to the versions listed in the Fixed Software section
below may be vulnerable.

To determine if the SNMP server is running on your device and the
default community string is present, issue the "show snmp community"
command while in enable mode at the prompt and look for output similar

Router#show snmp community

Community name: cable-docsis
Community Index: cisco0
Community SecurityName: cable-docsis
storage-type: read-only active

If the SNMP server is disabled on your device, output similar to the
following will be returned:

Router#show snmp community
%SNMP agent not enabled

To determine the software running on a Cisco product, log in to the
device and issue the "show version" command to display the system banner.
Cisco IOS software will identify itself as "Internetwork Operating
System Software" or simply "IOS". On the next line of output, the image
name will be displayed between parentheses, followed by "Version" and
the IOS release name. Other Cisco devices will not have the "show version"
command or will give different output.

The following example identifies a Cisco product running IOS release
12.2(15)MC2 with an installed image name of MWR1900-I-M:

Cisco Internetwork Operating System Software
IOS (tm) 1900 Software (MWR1900-I-M), Version 12.2(15)MC2, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)

Additional information about Cisco IOS release naming can be found at

Vulnerable Products

Cisco devices that may be running with affected Cisco IOS software
releases include:

* Cisco IAD2430 Integrated Access Device
* Cisco IAD2431 Integrated Access Device
* Cisco IAD2432 Integrated Access Device
* Cisco VG224 Analog Phone Gateway
* Cisco MWR 1900 Mobile Wireless Edge Router
* Cisco MWR 1941 Mobile Wireless Edge Router

Products Confirmed Not Vulnerable

* Cisco IAD2420 Integrated Access Device
* Cisco IAD2421 Integrated Access Device
* Cisco IAD2423 Integrated Access Device
* Cisco IAD2424 Integrated Access Device

No other Cisco products are currently known to be affected by this


Implementation of the Data Over Cable Service Interface Specification
(DOCSIS) standards allow for data transmission over physical media used
by cable television providers. Accordingly, RFC 2669
defines the DOCSIS Cable Device MIB, better known as the
DOCS-CABLE-DEVICE-MIB, for which support is required in order to be
considered DOCSIS compliant. That MIB defines the table,
docsDevNmAccessTable, as:

"This table controls access to SNMP objects by network
management stations. If the table is empty, access
to SNMP objects is unrestricted. This table exists only
on SNMPv1 or v2c agents and does not exist on SNMPv3
agents. See the conformance section for details.
Specifically, for v3 agents, the appropriate MIBs and
security models apply in lieu of this table."

In order to comply with the DOCSIS standard and to avoid unrestricted
access to SNMP objects, Cisco devices which support DOCSIS contain a
read-write community string, "cable-docsis".

Inclusion of this SNMP community string is intended only for
DOCSIS-compliant cable-capable devices. A vulnerability exists in the
inadvertent enabling of this community string in Cisco IOS release
trains running on the affected platforms.

Customers running vulnerable versions of Cisco IOS software on those
platforms may be unaware of the additional read-write community string.

This vulnerability is documented in Cisco Bug ID CSCsb04965
on the Cisco IAD2400 series and Cisco VG224 Analog Phone Gateways and
as CSCsb06658 on the 1900 Series Mobile Wireless Edge Routers.


Successful exploitation of the vulnerability may result in full control
of the device.

Software Version and Fixes

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.

Each row of the Cisco IOS software table (below) 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 (the "First Fixed Release") and the anticipated
date of availability for each are listed in the "Rebuild" and
"Maintenance" columns. A device running a release in the given train
that is earlier than the release in a specific column (less than the
First Fixed Release) is known to be vulnerable. The release should be
upgraded at least to the indicated release or a later version (greater
than or equal to the First Fixed Release label).

For more information on the terms "Rebuild" and "Maintenance," consult
the following URL: http://www.cisco.com/warp/public/620/1.html.

| Major Release | Availability of Repaired Releases |
| Affected 12.2-Based Release | Rebuild | Maintenance |
| 12.2MC | 12.2(15)MC2c | |
| 12.2ZJ | Vulnerable; migrate to 12.3(4)T13 or later |
| Affected 12.3-Based Release | Rebuild | Maintenance |
| | 12.3(4)T13 | |
| |---------------------+------------------------|
| | 12.3(7)T11 | |
| |---------------------+------------------------|
| 12.3T | 12.3(8)T10 | |
| |---------------------+------------------------|
| | 12.3(11)T6 | |
| |----------------------------------------------|
| | Vulnerable; for 12.3(14), migrate to |
| | 12.4(1b) or later |
| 12.3XD | Vulnerable; migrate to 12.3(7)T11 or later |
| 12.3XX | Vulnerable; migrate to 12.4(1b) or later |
| 12.3XY | Vulnerable; migrate to 12.4(1b) or later |
| 12.3YA | Vulnerable; contact TAC |
| 12.3YD | Vulnerable; migrate to 12.4(2)T5 or later |
| 12.3YF | Vulnerable; migrate to 12.3(14)YX or later |
| 12.3YG | 12.3(14)YG5 | |
| 12.3YH | Vulnerable; migrate to 12.4(2)T5 or later |
| 12.3YI | Vulnerable; migrate to 12.4(2)T5 or later |
| 12.3YJ | Vulnerable; migrate to 12.3(14)YQ8 or later |
| 12.3YK | Vulnerable; migrate to 12.4(4)T or later |
| 12.3YM | 12.3(14)YM8 | |
| 12.3YQ | 12.3(14)YQ8 | |
| 12.3YS | Vulnerable; migrate to 12.4(4)T or later |
| 12.3YT | Vulnerable; migrate to 12.4(4)T or later |
| 12.3YU | Vulnerable; migrate to 12.4(2)XB or later |
| Affected 12.4-Based Release | Rebuild | Maintenance |
| 12.4 | 12.4(1b) | 12.4(3) |
| 12.4MR | | 12.4(4)MR |
| 12.4T | 12.4(2)T5 | 12.4(4)T |


The effectiveness of any workarounds 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 is the most
appropriate for use in the intended network before it is deployed.

The following workarounds should only be considered as a long term
solution if anti-spoofing methods consistently prevent spoofed source
attacks from entering the network and access-lists provided below are
configured on every potentially affected device.

Disable the SNMP Server

If the SNMP server is not used for any legitimate purposes on the
device, it is a best practice to disable it by issuing the following
commands in configure mode:

no snmp-server

Removing the public community string with the configure command
"no snmp-server community <string> ro" is not sufficient as the SNMP server
will still be running and the device will be vulnerable. The command
"no snmp-server" must be used instead. The SNMP server status may be
verified by using the enable command "show snmp". You should see a
response of "%SNMP agent not enabled".

Note that this workaround may only be viable if SNMP is not used in any
way for managing the device.

Restrict Access via a Community-map

The ability to create a community-map was introduced in Cisco IOS
versions 12.0(23)S, 12.2(25)S and 12.3(2)T. This feature adds the
ability to create a mapping between an SNMP community and an SNMP
context, Engine ID, or security name that is different from the default

When an SNMP community is associated with an SNMP context, whenever a
request is made from this community, it is applied to the context
specified. This feature can also be used to specify the source address
validation for an SNMP community.

Consider the following example:

Router(config)#access-list 65 remark Deny access to community string
Router(config)#access-list 65 deny any
Router(config)#snmp-server community no-access RO 65
Router(config)#snmp mib community-map cable-docsis security-name no-access

The above creates a new community named "no-access". The authorization
to use the community "no-access" is controlled by an access-list, which
in this case denies all hosts from using it. With the addition of the
"snmp mib community-map" command, the current community string of
"cable-docsis" is then under the access restrictions of the new
community string "no-access" preventing any use of the "cable-docsis"
community string.

Control Plane Policing

Cisco IOS release trains 12.0S, 12.2SX, 12.2S, 12.3T, 12.4, and 12.4T
support Control Plane Policing (CoPP) which may be configured to
protect the device from attacks that target the management and control
planes. The following example can be adapted to your network. This
example assumes that all SNMP access is to be restricted to a
management station with the IP address of, and that the
management station need only communicate with router's IP address

access-list 111 deny udp host host eq snmp
access-list 111 permit udp any any eq snmp
access-list 111 deny ip any any
class-map match-all drop-snmp-class
match access-group 111
policy-map drop-snmp-policy
class drop-snmp-class
service-policy input drop-snmp-policy

Please note that in the 12.0S, 12.2S, and 12.2SX Cisco IOS trains the
policy-map syntax is different:

policy-map drop-snmp-policy
class drop-snmp-class
police 32000 1500 1500 conform-action drop exceed-action drop

In the above CoPP examples the ACL entries that match the exploit
packets with the "permit" action result in these packets being
discarded by the policy-map "drop" function, while packets that match
the "deny" action are not affected by the policy-map drop function.

Additional information on the configuration and use of the CoPP feature
can be found at the following URL:


Restrict Access to Trusted Hosts Only

Access Control Lists (ACLs) can be used to deny traffic to the device.
Although Cisco IOS devices have community-string access lists which
check the source address of SNMP requests per community string, they
will not work in this case as the cable-docsis community string is not
able to be modified or deleted via configuration options.

It is possible to permit UDP traffic to the router from trusted IP
addresses with interface ACLs.

Note: Because SNMP is based on UDP, it is possible to spoof the
sender's IP address, which may defeat ACLs that permit communication to
these ports from trusted IP addresses.

The following extended access-list can be adapted to your network. This
example assumes that the router has IP addresses and configured on its interfaces, that all SNMP access is to be
restricted to a management station with the IP address of, and
that the management station need only communicate with IP address

access-list 101 permit udp host host eq snmp
access-list 101 deny udp any host eq snmp
access-list 101 deny udp any host eq snmp
access-list 101 permit ip any any

The access-list must then be applied to all interfaces using the
following configuration commands:

interface FastEthernet 0/0
ip access-group 101 in

Note that UDP traffic to port 161 (SNMP) must be explicitly blocked to
each IP address on the router to prevent the router from accepting and
processing the SNMP packets. Blocking traffic to port 161 from unknown
hosts is considered a best practice. All devices that need to
communicate directly with the router on port 161 will need to be
specifically listed in the above access list.

For devices that have many IP addresses configured, or many hosts that
need to communicate with the router, this may not be a scalable

Infrastructure ACLs (iACL)

Although it is often difficult to block traffic transiting your
network, it is possible to identify traffic which should never be
allowed to target your infrastructure devices and block that traffic at
the border of your network. Infrastructure ACLs are considered a
network security best practice and should be considered as a long-term
addition to good network security as well as a workaround for this
specific vulnerability. The white paper entitled "Protecting Your Core:
Infrastructure Protection Access Control Lists" presents guidelines and
recommended deployment techniques for iACLs:


Obtaining Fixed Software

Cisco will make free software available to address this vulnerability
for affected customers. This advisory will be updated as fixed software
becomes available. 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

Do not contact either "psirt (at) cisco (dot) com [email concealed]" or "security-alert (at) cisco (dot) com [email concealed]"
for software upgrades.

Customers with Service Contracts

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 using Third Party Support Organizations

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 without Service Contracts

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: tac (at) cisco (dot) com [email concealed]

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.

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

This vulnerability was reported to Cisco by a customer.

Status of this Notice: FINAL


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 is posted on Cisco's worldwide website 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.

* cust-security-announce (at) cisco (dot) com [email concealed]
* first-teams (at) first (dot) org [email concealed]
* bugtraq (at) securityfocus (dot) com [email concealed]
* vulnwatch (at) vulnwatch (dot) org [email concealed]
* cisco (at) spot.colorado (dot) edu [email concealed]
* cisco-nsp (at) puck.nether (dot) net [email concealed]
* full-disclosure (at) lists.grok.org (dot) uk [email concealed]
* comp.dcom.sys.cisco (at) newsgate.cisco (dot) com [email concealed]

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.

Revision History

| Revision | | Initial |
| 1.0 | 2006-September-20 | 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 website at
This includes instructions for press inquiries regarding Cisco security
notices. All Cisco security advisories are available at

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All contents are Copyright 1992-2006 Cisco Systems, Inc. All rights
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Updated: Sep 20, 2006 Document ID: 71255

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