[VulnWatch] Cisco Security Advisory: Multiple IOS IPS Vulnerabilities



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Cisco Security Advisory: Multiple IOS IPS Vulnerabilities

Advisory ID: cisco-sa-20070213-iosips

http://www.cisco.com/warp/public/707/cisco-sa-20070213-iosips.shtml

Revision 1.0

For Public Release 2007 February 13 1600 UTC (GMT)

- -----------------------------------------------------------------------

Summary
=======

The Intrusion Prevention System (IPS) feature set of Cisco IOS
contains several vulnerabilities. These include:

* Fragmented IP packets may be used to evade signature inspection.
* IPS signatures utilizing the regular expression feature of the
ATOMIC.TCP signature engine may cause a router to crash resulting
in a denial of service.

There are mitigations and workarounds for these vulnerabilities. Cisco
has made free software available to address these vulnerabilities for
affected customers.

This advisory is posted at
http://www.cisco.com/warp/public/707/cisco-sa-20070213-iosips.shtml.

Affected Products
=================

Vulnerable Products
+------------------

The following Cisco IOS release trains with the IPS feature set enabled
are vulnerable to the fragmented packet evasion vulnerability:

* 12.3T, except versions 12.3(2)T, 12.3(4)T, and 12.3(7)T
* 12.4
* 12.4T
* 12.4XE

The following Cisco IOS release trains with the IPS feature set enabled
are vulnerable to the ATOMIC.TCP regular expression denial of service
vulnerability:

* 12.3T, except versions 12.3(2)T, 12.3(4)T, and 12.3(7)T
* 12.3XQ, 12.3XR, 12.3XS, 12.3XW, 12.3XX, 12.3XY
* 12.3YA, 12.3YD, 12.3YG, 12.3YH, 12.3YI, 12.3YJ, 12.3YK, 12.3YM,
12.3YQ, 12.3YS, 12.3YT, 12.3YX, 12.3YZ
* 12.4
* 12.4MR
* 12.4T
* 12.4XA, 12.4XB

To determine if the IPS feature set is active on an IOS device, use the
"show ip ips configuration" command. This command will list the
interfaces configured to use IPS inspection. You will then need to
further check the status of each interface to confirm if they are
enabled or not.

router#show ip ips configuration
Configured Config Locations: -none-
Last signature default load time: 18:46:50 UTC Jan 5 2007
Last signature delta load time: -none-
Last event action (SEAP) load time: -none-
IPS Auto Update is not currently configured
IPS fail closed is disabled
Fastpath ips is enabled
Quick run mode is enabled
Event notification through syslog is enabled
Event notification through SDEE is enabled
Total Active Signatures: 85
Total Inactive Signatures: 61
IPS Rule Configuration
IPS name test
IPS Category CLI is not configured
Interface Configuration
Interface FastEthernet0/0
Inbound IPS rule is test
Outgoing IPS rule is not set

router#show ip interface FastEthernet0/0
FastEthernet0/0 is up, line protocol is up


In the above example, interface FastEthernet0/0 is configured to use
IPS and is shown to be enabled.

Products Confirmed Not Vulnerable
+--------------------------------

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

Details
=======

Cisco IOS Intrusion Prevention System (IPS) is an inline, deep-packet
inspection-based feature that enables Cisco IOS software to mitigate
network attacks. Cisco IOS IPS enables the network to defend itself
with the intelligence to identify, classify, and stop or block certain
malicious or damaging traffic in real time. The IOS IPS feature set
contains multiple vulnerabilities. Only IOS images containing the IPS
feature set are affected by these vulnerabilities.

Fragmented Packet Evasion Vulnerability
+--------------------------------------

Some of the IPS signatures utilize regular expressions. Due to a
vulnerability, an attacker can evade those IPS signatures by sending
malicious network traffic as IP fragments. This may result in potential
malicious traffic bypassing signature inspection and possibly allow the
exploitation of protected systems. IPS signatures which do not utilize
regular expressions are not affected by this vulnerability. All IP
protocols (e.g. TCP, UDP, ICMP) are affected by this vulnerability.
There is a mitigation for this vulnerability. This vulnerability is
documented in Cisco Bug ID CSCsg15598.

ATOMIC.TCP Regular Expression Denial of Service Vulnerability
+------------------------------------------------------------

Certain network traffic can trigger IPS signatures which use the
regular expression feature of the ATOMIC.TCP signature engine which may
cause the IOS IPS device to crash. This may cause a denial of service
resulting in disruption network traffic. Signature 3123.0 (Netbus Pro
Traffic) has been demonstrated to trigger this vulnerability. There is
a workaround for this vulnerability. This vulnerability is documented
in Cisco Bug ID CSCsa53334.

Vulnerability Scoring Details
+----------------------------

Cisco is providing scores for the vulnerabilities in this advisory
based on the Common Vulnerability Scoring System (CVSS).

Cisco will provide a base and temporal score. Customers can then
compute environmental scores to assist in determining the impact of the
vulnerability in individual networks.

Cisco PSIRT will set the bias in all cases to normal. Customers are
encouraged to apply the bias parameter when determining the
environmental impact of a particular vulnerability.

CVSS is a standards based scoring method that conveys vulnerability
severity and helps determine urgency and priority of response.

Cisco has provided an FAQ to answer additional questions regarding CVSS
at http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html.

Cisco has also provided a CVSS calculator to help compute the
environmental impact for individual networks at
http://intellishield.cisco.com/security/alertmanager/cvss.

CSCsg15598 - DYIDS: Fragmentation prevents signature recognition

CVSS Base Score: 4.7
Access Vector: Remote
Access Complexity: Low
Authentication: Not Required
Confidentiality Impact: Partial
Integrity Impact: Partial
Availability Impact: None
Impact Bias: Normal

CVSS Temporal Score: 3.9
Exploitability: Functional
Remediation Level: Official Fix
Report Confidence: Confirmed


CSCsa53334 - bus error in single_pkt_regex

CVSS Base Score: 3.3
Access Vector: Remote
Access Complexity: Low
Authentication: Not Required
Confidentiality Impact: None
Integrity Impact: None
Availability Impact: Complete
Impact Bias: Normal

CVSS Temporal Score: 2
Exploitability: Functional
Remediation Level: Official Fix
Report Confidence: Confirmed



Impact
======

Successful exploitation of the fragmented packet evasion vulnerability
may result in an attacker being able to evade detection by an IOS IPS
device. This could allow protected systems to be covertly attacked.

Successful exploitation of the ATOMIC.TCP regular expression denial of
service vulnerability may cause an IOS IPS device to crash.

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 | | |
| 12.3-Based | Rebuild | Maintenance |
| Release | | |
|------------------+------------------------------------------|
| | All 12.3(2)T, 12.3(4)T, and 12.3(7)T |
| | releases are not vulnerable |
| |------------------------------------------|
| | All 12.3(8)T releases are vulnerable |
| |------------------------------------------|
| | 12.3(11)T10 | |
| 12.3T |----------------------------+-------------|
| | 12.3(14)T7 | |
| |------------------------------------------|
| | Limited platform support is available: |
| | contact TAC |
| |------------------------------------------|
| | Please migrate to 12.4(12) or later |
|------------------+------------------------------------------|
| 12.3XQ | Vulnerable; migrate to 12.4(12) or later |
|------------------+------------------------------------------|
| 12.3XR | Vulnerable; contact TAC |
|------------------+------------------------------------------|
| 12.3XS | Vulnerable; migrate to 12.4(12) or later |
|------------------+------------------------------------------|
| 12.3XW | Vulnerable; migrate to 12.3(11)YF or |
| | later |
|------------------+------------------------------------------|
| 12.3XX | Vulnerable; migrate to 12.4(12) or later |
|------------------+------------------------------------------|
| 12.3XY | Vulnerable; migrate to 12.4(12) or later |
|------------------+------------------------------------------|
| 12.3YA | Vulnerable; contact TAC |
|------------------+------------------------------------------|
| 12.3YD | Vulnerable; migrate to 12.4(2)T3 or |
| | later |
|------------------+------------------------------------------|
| 12.3YG | Vulnerable; migrate to 12.4(2)T3 or |
| | later |
|------------------+------------------------------------------|
| 12.3YH | Vulnerable; migrate to 12.4(2)T3 or |
| | later |
|------------------+------------------------------------------|
| 12.3YI | Vulnerable; migrate to 12.4(2)T3 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)YM5 | |
|------------------+----------------------------+-------------|
| 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 |
|------------------+------------------------------------------|
| 12.3YX | 12.3(14)YX3 | |
|------------------+----------------------------+-------------|
| 12.3YZ | 12.3(11)YZ | |
|------------------+----------------------------+-------------|
| Affected | | |
| 12.4-Based | Rebuild | Maintenance |
| Release | | |
|------------------+----------------------------+-------------|
| | 12.4(1c) | |
| |----------------------------+-------------|
| | 12.4(3b) | 12.4(5) |
| |----------------------------+-------------|
| 12.4 | 12.4(7e); available | |
| | 26-Mar-07 | |
| |------------------------------------------|
| | All 12.4(8) releases are vulnerable |
| |------------------------------------------|
| | 12.4(10b) | 12.4(12) |
|------------------+----------------------------+-------------|
| 12.4MR | 12.4(6)MR1 | |
|------------------+----------------------------+-------------|
| | 12.4(2)T3 | 12.4(4)T |
| |----------------------------+-------------|
| | | 12.4(6)T |
| 12.4T |------------------------------------------|
| | 12.4(9)T3; available 9-Apr-07 |
| |------------------------------------------|
| | 12.4(11)T1 |
|------------------+------------------------------------------|
| 12.4XA | 12.4(2)XA2 | |
|------------------+----------------------------+-------------|
| 12.4XB | 12.4(2)XB3 | |
|------------------+------------------------------------------|
| 12.4XE | Vulnerable; contact TAC |
+-------------------------------------------------------------+

Workarounds
===========

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

Additional mitigations that can be deployed on Cisco devices within the
network are available in the Cisco Applied Intelligence companion
document for this advisory:
http://www.cisco.com/warp/public/707/cisco-air-20070213-iosips.shtml

Fragmented Packet Evasion Vulnerability
+--------------------------------------

There is a mitigation for the fragmented packet evasion vulnerability.
The "fragments" keyword of IOS transit Access Control Lists (ACL) can be
used to prohibit fragmented IP packets from transiting an IOS device.
More information about filtering IP fragments can be found here:

* http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801afc76.shtml#frag

Blocking IP fragments may have adverse affects on some protocols (like
HTTP, FTP and Kerberos/Active Directory), so this workaround should be
used with caution.

access-list 100 deny tcp any 10.1.1.0 0.0.0.255 fragments
access-list 100 deny udp any 10.1.1.0 0.0.0.255 fragments


ATOMIC.TCP Regular Expression Denial of Service Vulnerability
+------------------------------------------------------------

There is a workaround for the ATOMIC.TCP regular expression denial of
service vulnerability by deleting IPS signature 3123.0 from the IOS IPS
Signature Definition File (SDF). Disabling signature 3123.0 is alone
not sufficient for the workaround to be effective. The following
commands will delete signature 3123.0 from an IOS IPS device.

router#configure terminal
router(config)#ip ips signature 3123 delete
%IPS Signature 3123:0 is marked for deletion
%IPS The signature will be deleted when signatures are reloaded or saved
router(config)#interface FastEthernet0/0
router(config)#no ip ips test in
router(config)#ip ips test in
router(config)#exit


In the above example, signature 3123.0 is first deleted from the
Signature Definition File, then the IPS instance running on interface
FastEthernet0/0 is stopped and started to reinitialize the IPS state to
reflect the signature deletion. If the IPS feature set is configured on
multiple interfaces, then these steps must be completed for each
affected interface.

To determine if signature 3123 is active, use the "show ip ips signature"
command.

router#show ip ips signatures | include 3123
3123:0 N A MED 0 0 0 100 30 FA N S46


In the command output above, the N after 3123.0 indicates that the
signature is present in the configuration but not enabled. Once the
signature has been deleted, rerunning the "show ip ips signature" command
shows:

router#show ip ips signatures | include 3123
3123:0 N* A MED 0 0 0 100 30 FA N S46


In the command output above, the N* after 3123.0 indicates that the
signature has been deleted from the configuration. The IPS feature set
must be restarted on each interface configured for IPS to complete the
workaround. Once completed, the output of the "show ip ips signature"
command will show:

router#show ip ips signatures | include 3123

router#


This vulnerability may affect any IPS signature using the regular
expression functionality of the ATOMIC.TCP engine. Currently, Cisco
only ships one signature configured this way (3123.0). If custom
signatures have been added to an IOS IPS device configured use the
ATOMIC.TCP engine with a regular expression, these signatures must also
be deleted from the IPS configuration to ensure the effectiveness of
the workaround.

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
http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Do not contact either "psirt@xxxxxxxxx" or "security-alert@xxxxxxxxx"
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@xxxxxxxxx

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 vulnerabilities described in this advisory.

The fragmented packet evasion vulnerability was discovered internally
by Cisco.

The ATOMIC.TCP regular expression denial of service vulnerability was
reported to Cisco by a customer.

Status of This Notice: FINAL
============================

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY
KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE
INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT
YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS
DOCUMENT AT ANY TIME.

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.

Distribution
============

This advisory is posted on Cisco's worldwide website at:
http://www.cisco.com/warp/public/707/cisco-sa-20070213-iosips.shtml

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@xxxxxxxxx
* first-teams@xxxxxxxxx
* bugtraq@xxxxxxxxxxxxxxxxx
* vulnwatch@xxxxxxxxxxxxx
* cisco@xxxxxxxxxxxxxxxxx
* cisco-nsp@xxxxxxxxxxxxxxx
* full-disclosure@xxxxxxxxxxxxxxxxx
* comp.dcom.sys.cisco@xxxxxxxxxxxxxxxxxx

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 1.0 | 2007-February-13 | 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 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.

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All contents are Copyright 1992-2007 Cisco Systems, Inc. All rights
reserved.
- -----------------------------------------------------------------------

Updated: Feb 13, 2007 Document ID: 81545

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