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Cisco IOS fails to properly process TCP packets

Vulnerability Note VU#217912

Original Release Date: 2007-01-24 | Last Revised: 2007-02-02


The Cisco IOS Transmission Control Protocol listener contains a memory leak.


Cisco IOS is an operating system that is used on Cisco network devices. The Cisco IOS software can run Transmission Control Protocol (TCP) servers that allow administrators to connect to the devices for management or monitoring purposes.

A vulnerability exists in the IOS TCP listener. An attacker may be able to craft an IPv4 TCP packet that causes IOS to leak memory when processed. An attacker may be able to exploit this vulnerability to create a denial-of-service condition. The specially crafted TCP can be delivered outside the context of a fully established TCP 3-way handshake.


A remote, unauthenticated attacker with the ability to supply specially crafted TCP packets could cause memory to leak in the affected service. Sustained exploitation could eventually cause a denial of service due to shortage of memory in the affected device and would require a system reset to recover. An attacker could send spoofed TCP packets to trigger this vulnerability making an attack harder to detect or stop.

Note that Cisco states that this vulnerability only applies to traffic destined to the affected device. Traffic transiting the device will not trigger this vulnerability.


See the Software Version and Fixes section of Cisco Security Advisory 200701 for information on available upgrades.

Restrict Access

Restricting public access to TCP servers running on vulnerable systems may mitigate this vulnerability. Access control lists, management VLANs, or alternate connection methods such as modem or console ports can be used to allow restricted access to the device.

Disable Services

Disabling all services that accept incoming TCP connections on a vulnerable device may prevent this vulnerability from being exploited.

For more information about these and other workarounds, see the Workarounds section of Cisco Security Advisory 20070124.

Vendor Information


Cisco Systems, Inc. Affected

Updated:  January 24, 2007



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.


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CVSS Metrics

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Thanks to Cisco for information that was used in this report.

This document was written by Ryan Giobbi.

Other Information

CVE IDs: None
Severity Metric: 7.53
Date Public: 2007-01-24
Date First Published: 2007-01-24
Date Last Updated: 2007-02-02 19:51 UTC
Document Revision: 40

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