Vulnerability Note VU#411332

Cisco IOS Interface Blocked by IPv4 Packet

Overview

A denial-of-service vulnerability exists in Cisco's Internetwork Operating System (IOS). This vulnerability may allow remote attackers to conduct denial-of-service attacks on an affected device.

I. Description

Cisco IOS is a very widely deployed network operating system. A vulnerability in IOS could allow an intruder to execute a denial-of-service attack against an affected device. Cisco has published an advisory on this topic, available at http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml. We strongly encourage sites using IOS to read this document and take appropriate action.

II. Impact

By sending specially crafted IPv4 packets to an interface on a vulnerable device, an intruder can cause the device to stop processing packets destined to that interface. Quoting from Cisco's advisory:

"A device receiving these specifically crafted IPv4 packets will force the inbound interface to stop processing traffic. The device may stop processing packets destined to the router, including routing protocol packets and ARP packets. No alarms will be triggered, nor will the router reload to correct itself. This issue can affect all Cisco devices running Cisco IOS software. This vulnerability may be exercised repeatedly resulting in loss of availability until a workaround has been applied or the device has been upgraded to a fixed version of code."

III. Solution

Apply a patch as described in the Cisco Advisory.

Until a patch can be applied, you can mitigate the risks presented by this vulnerability by judicious use of access control lists (ACLs). The correct use of ACLs depends of your network topology. Additionally, ACLs may degrade performace on some systems. We recommend reviewing the following before applying ACLs

http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#workarounds
http://www.cisco.com/warp/public/707/racl.html
http://www.cisco.com/warp/public/707/iacl.html

It is important to understand your network requirements before making changes.

Systems Affected

Vendor	Status	Date Updated
Cisco Systems Inc.	Vulnerable	17-Jul-2003

References

http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml#workarounds
http://www.cisco.com/warp/public/707/racl.html
http://www.cisco.com/warp/public/707/iacl.html
http://www.cisco.com/en/US/products/sw/iosswrel/products_ios_cisco_ios_software_category_home.html
http://www.cisco.com/en/US/products/sw/iosswrel/products_ios_cisco_ios_software_releases.html
http://www.cisco.com/en/US/products/sw/iosswrel/ios_release_information_listing.html
http://www.cisco.com/univercd/cc/td/doc/product/software/
http://www.cisco.com/public/sw-center/sw-ios.shtml

Credit

Thanks to Cisco Systems for reporting this vulnerability.

This document was written by Shawn V Hernan.

Other Information

Date Public	07/16/2003
Date First Published	07/17/2003 09:21:16 AM
Date Last Updated	07/17/2003
CERT Advisory	CA-2003-15
CVE Name	CAN-2003-0567
Metric	73.50
Document Revision	24

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