[NEWS] Scanning for Cisco Provided SSH Can Cause a Crash

From: support@securiteam.com
Date: 06/28/02


From: support@securiteam.com
To: list@securiteam.com
Date: Fri, 28 Jun 2002 14:05:21 +0200 (CEST)

The following security advisory is sent to the securiteam mailing list, and can be found at the SecuriTeam web site: http://www.securiteam.com
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  Scanning for Cisco Provided SSH Can Cause a Crash
------------------------------------------------------------------------

SUMMARY

While fixing vulnerabilities mentioned in the Cisco Security Advisory:
Multiple SSH Vulnerabilities (
<http://www.cisco.com/warp/public/707/SSH-multiple-pub.html>
http://www.cisco.com/warp/public/707/SSH-multiple-pub.html) we
inadvertently introduced an instability in some products. When an attacker
tries to exploit the vulnerability the SSH module will consume too much of
the processor's time, effectively causing a DoS. In some cases, the device
will reboot. In order to be exposed SSH must be enabled on the device.

DETAILS

Affected product lines are:
 * All devices running Cisco IOSŪ Software supporting SSH. This includes
routers and switches running Cisco IOS Software.
 * Catalyst 6000 switches running CatOS.
 * Cisco PIX Firewall.
 * Cisco 11000 Content Service Switch family.

No other Cisco product is vulnerable. It is possible to mitigate this
vulnerability by preventing, or having control over, the SSH traffic.

Details:
While fixing the vulnerabilities listed in
<http://www.cisco.com/warp/public/707/SSH-multiple-pub.html>
http://www.cisco.com/warp/public/707/SSH-multiple-pub.html (Cisco Security
Advisory: Multiple SSH Vulnerabilities) an instability is introduced in
some products. When exposed to an overly large packet, the SSH process
will consume a large portion of the processor's instruction cycles,
effectively causing a DoS. The capability to create such a packet is
available in publicly available exploit code. In some cases, this
availability attack may result in a reboot of the device. In order to be
exposed SSH must be enabled on the device.

Impact:
By repeatedly exploiting this vulnerability an attacker can cause a denial
of service, though Cisco products remain unaffected to the exploits that
are trying to exploit vulnerabilities listed in
<http://www.cisco.com/warp/public/707/SSH-multiple-pub.html>
http://www.cisco.com/warp/public/707/SSH-multiple-pub.html.

Software Versions and Fixes:
A table containing affected version and their appropriate fixes can be
found at:
 <http://www.cisco.com/warp/public/707/SSH-scanning.shtml#Software>
http://www.cisco.com/warp/public/707/SSH-scanning.shtml#Software

Obtaining Fixed Software:
Cisco is offering free software upgrades to remedy this vulnerability for
all affected customers. Customers may only install and expect support for
the feature sets they have purchased.

Customers with service contracts should obtain upgraded software through
their regular update channels to any software release containing the
feature sets they have purchased. For most customers, this means that
upgrades should be obtained through the Software Center on Cisco's
worldwide website at <http://www.cisco.com> 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 assistance with the upgrade, which should be free
of charge.

Customers who purchased directly 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 obtain fixed software by contacting the Cisco TAC. In those
cases, customers may only upgrade to a later version of the same release
as indicated by the applicable row in the Software Versions and Fixes
table.

Cisco 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@cisco.com

See <http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml>
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.

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 "psirt@cisco.com" or
"security-alert@cisco.com" for software upgrades.

Workarounds:
It is possible to mitigate this vulnerability in two ways:
 * Block all SSH connections on the border on your network, or
 * On each individual device allow SSH connections only from the required
IP addresses and block all others.

Blocking all SSH connections and all other protocols that are not supposed
to come from the outside, on the network edge should be an integral part
of the network security best practice.

ADDITIONAL INFORMATION

The information has been provided by .

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