Cisco Cisco IOS Software Release 12.2

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A document that describes how the Internet Control Message Protocol (ICMP) could be used to 
perform a number of Denial of Service (DoS) attacks against the Transmission Control Protocol 
(TCP) has been made publicly available. This document has been published through the Internet 
Engineering Task Force (IETF) Internet Draft process, and is entitled “ICMP Attacks Against TCP” 
(draft-gont-tcpm-icmp-attacks-03.txt).

These attacks, which only affect sessions terminating or originating on a device itself, can be of 
three types:

1. Attacks that use ICMP “hard” error messages 
2. Attacks that use ICMP “fragmentation needed and Don’t Fragment (DF) bit set” messages, also 
known as Path Maximum Transmission Unit Discovery (PMTUD) attacks 
3. Attacks that use ICMP “source quench” messages

Successful attacks may cause connection resets or reduction of throughput in existing connections, 
depending on the attack type.

Multiple Cisco products are affected by the attacks described in this Internet draft.

Cisco has made free software available to address these vulnerabilities. In some cases there are 
workarounds available to mitigate the effects of the vulnerability.

This advisory is posted at 

The disclosure of these vulnerabilities is being coordinated by the National Infrastructure Security 
Coordination Centre (NISCC), based in the United Kingdom. NISCC is working with multiple 
vendors whose products are potentially affected. Its posting can be found at: 

. 

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Symptoms: A Cisco router may forward packets that come in on a subinterface that is in an 
administratively shut down state.

Conditions: This symptom is observed on a Cisco router that is configured with Frame Relay 
encapsulation.

Workaround: There is no workaround. 

Cisco IOS Release 12.2(26c) is a rebuild release for Cisco IOS Release 12.2(26). The caveats in this 
section are resolved in Cisco IOS Release 12.2(26c) but may be open in previous Cisco IOS releases.

The following information is provided for each caveat:

Symptoms: A description of what is observed when the caveat occurs. 

Conditions: The conditions under which the caveat has been known to occur. 

Workaround: Solutions, if available, to counteract the caveat.