Cisco Security Advisory: Cisco IOS Software TCP Denial of Service Vulnerability

Cisco Systems Product Security Incident Response Team psirt at
Thu Aug 12 22:26:32 UTC 2010

Hash: SHA1

Cisco Security Advisory: Cisco IOS Software TCP Denial of Service

Advisory ID: cisco-sa-20100812-tcp

Revision 1.0

For Public Release 2010 August 12 2130 UTC (GMT)



Cisco IOS Software Release, 15.1(2)T is affected by a denial of
service (DoS) vulnerability during the TCP establishment phase. The
vulnerability could cause embryonic TCP connections to remain in a
SYNRCVD or SYNSENT state. Enough embryonic TCP connections in these
states could consume system resources and prevent an affected device
from accepting or initiating new TCP connections, including any
TCP-based remote management access to the device.

No authentication is required to exploit this vulnerability. An attacker
does not need to complete a three-way handshake to trigger this
vulnerability; therefore, this this vunerability can be exploited using
spoofed packets. This vulnerability may be triggered by normal network

Cisco has released Cisco IOS Software Release 15.1(2)T0a to address this

This advisory is posted at

Affected Products

This vulnerability affects only Cisco IOS Software Release 15.1(2)T. No
other Cisco IOS Software Releases are affected. Cisco IOS XE Software,
Cisco IOS XR Software, and Cisco NX-OS Software are not affected by this

Vulnerable Products

A Cisco device is vulnerable when it is running Cisco IOS Software
Release 15.1(2)T. To determine the Cisco IOS Software Release that is
running on a Cisco product, administrators can log in to the device
and issue the "show version" command to display the system banner.
The system banner confirms that the device is running Cisco IOS
Software by displaying text similar to "Cisco Internetwork Operating
System Software" or "Cisco IOS Software." The image name displays in
parentheses, followed by "Version" and the Cisco IOS Software Release
name. Other Cisco devices do not have the "show version" command or may
provide different output.

The following example identifies a Cisco product that is running
Cisco IOS Software Release 15.1(2)T with an installed image name of

    Router#show version
    Cisco IOS Software, 2800 Software (C2800NM-ENTSERVICES-M), Version 15.1(2)T,
        RELEASE SOFTWARE (fc1)
    Technical Support:
    Copyright (c) 1986-2010 by Cisco Systems, Inc.
    Compiled Mon 19-Jul-10 16:38 by prod_rel_team

    <output truncated>

Additional information about Cisco IOS Software Release naming
conventions is available in the White Paper: Cisco IOS Reference Guide.

Products Confirmed Not Vulnerable

No other Cisco IOS Software versions are affected by this vulnerability.

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


TCP provides reliable data transmission services in packet-switched
network environments. TCP corresponds to the transport layer (Layer
4) of the OSI reference model. Among the services TCP provides are
stream data transfer, reliability, efficient flow control, full-duplex
operation, and multiplexing.

When TCP connections are terminated in Cisco IOS Software, they are
allocated a transmission control block (TCB). All allocated TCBs,
associated TCP port numbers, and the TCP state are displayed in the
output of the "show tcp brief all" command-line interface (CLI) command.

Cisco IOS Software version 15.1(2)T contains a vulnerability that could
cause an embryonic TCP connection to remain in SYNRCVD or SYNSENT
state without a further TCP state transition. Examining the output of
the "show tcp brief all" command multiple times will indicate if TCP
sessions remain in one of these states.

This vulnerability is triggered only by TCP traffic that is terminated
by or originated from the device. Transit traffic will not trigger this

Both connections to and from the router could trigger this
vulnerability. An example of a connection to the router is that you may
still be able to ping the device, but fail to establish a TELNET or SSH
connection to the device. For example, an administrator may still be
able to ping the device but fail to establish a Telnet or SSH connection
to the device. Administrators who attempt a Telnet or a SSH connection
to a remote device from the CLI prompt will encounter a hung session
and the "Trying <ip address|hostname> ..." prompt. The connection
that is initiated or terminated by the router can be removed from the
socket table by clearing the associated TCB with the "clear tcp tcb
0x<address>" command.

Devices could be vulnerable if examining the output of the CLI command
"debug ip tcp transactions", displays the error messages "connection
queue limit reached: port <port number>" or "No wild listener: port
<port number>".

Devices could also be vulnerable if output from repetitive show tcp
brief all CLI commands indicates many TCBs in the state SYNRCVD or

The following example shows a device that has several HTTP, SSH, and
Telnet sessions in the TCP SYNRCVD state:

    Example#show tcp brief all
    TCB       Local Address               Foreign Address             (state)
    07C2D6C8              SYNRCVD
    07C38128               SYNRCVD
    07C2DD60              SYNRCVD
    07C2A8A0               SYNRCVD

    <output truncated>

Any TCP sessions can be cleared by clearing the associated TCB with
"clear tcp tcb 0x<address>". Alternatively Administrators can clear all
TCBs at once by issuing "clear tcp tcb *".

Note: This will clear all active and hung TCP connections.

This vulnerability is documented in the Cisco bug ID CSCti18193. This
vulnerability has been assigned Common Vulnerabilities and Exposures
(CVE) ID CVE-2010-2827.

Some TCP application specific information is provided in the following

Telnet and SSH

Telnet can not be explicitly disabled on a Cisco IOS device. Configuring
"transport input none" on the vty lines of a vulnerable device will
prevent it from being exploited on TCP port 23. However, if the Cisco
IOS SSH server feature is configured on the device, "transport input
none" will not prevent the device from being exploited on TCP port 22.

Configuration of vty access control lists can partially mitigate this
vulnerability because the vulnerability can be exploited using spoofed
IP source addresses.

Border Gateway Protocol

Routers that are configured with Border Gateway Protocol (BGP) can be
protected further by using the Generalized Time to Live (TTL) Security
Mechanism (GTSM) feature. GTSM allows users to configure the expected
TTL of a packet between a source and destination address. Packets that
fail the GTSM check will be dropped before TCP processing occurs, which
prevents an attacker from exploiting this vulnerability through BGP.
GTSM is implemented with the command "ttl-security hops".

Further information on protecting BGP can be found in
"Protecting Border Gateway Protocol for the Enterprise"

TCP MD5 Authentication for BGP does not prevent this vulnerability from
being exploited.

Vulnerability Scoring Details

Cisco has provided a score for the vulnerability in this advisory based
on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in
this Security Advisory is done in accordance with CVSS version 2.0.

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

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

Cisco has provided an FAQ to answer additional questions regarding CVSS

Cisco has also provided a CVSS calculator to help compute the
environmental impact for individual networks at:

* CSCti18193 ("TCP connections never timeout in IOS 15.1(2)T")

CVSS Base Score - 7.8
    Access Vector -            Network
    Access Complexity -        Low
    Authentication -           None
    Confidentiality Impact -   None
    Integrity Impact -         None
    Availability Impact -      Complete

CVSS Temporal Score - 6.4
    Exploitability -           Functional
    Remediation Level -        Official-Fix
    Report Confidence -        Confirmed


Successful exploitation of this vulnerability may prevent some TCP
applications on Cisco IOS Software from accepting any new connections.
Exploitation could also prevent remote access to the affected system
via the vtys. Remote access to the affected device via out-of-band
connectivity to the console port should still be available.

Software Versions and Fixes

When considering software upgrades, also consult 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) names a Cisco IOS
release train. If a release train is vulnerable, then the earliest
possible releases that contain the fix (along with the anticipated date
of availability for each, if applicable) are listed in the "First Fixed
Release" column of the table. The "Recommended Release" column indicates
the releases which have fixes for all the published vulnerabilities
at the time of this Advisory. 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. Cisco recommends
upgrading to a release equal to or later than the release in the
"Recommended Releases" column of the table.

|   Major    | Availability of Repaired |
|  Release   |         Releases         |
|  Affected  |                          |
| 12.x-Based |   First Fixed Release    |
|  Releases  |                          |
| 12.0 -     | 12.0 through 12.4 based  |
| 12.4       | releases are not         |
|            | affected                 |
|  Affected  |                          |
| 15.0-Based |   First Fixed Release    |
|  Releases  |                          |
| 15.0       | There are no affected    |
|            | 15.0 based releases      |
|  Affected  |                          |
| 15.1-Based |   First Fixed Release    |
|  Releases  |                          |
|            | 15.1(2)T0a               |
|            |                          |
|            | 15.1(2)T1; available on  |
|            | 20-AUG-2010              |
| 15.1T      |                          |
|            | Releases prior to 15.1   |
|            | (2)T are not vulnerable. |
|            | The vulnerability is     |
|            | first fixed in release   |
|            | 15.1(2)T0a.              |


The only complete workaround to mitigate this vulnerability is to
disable the specific features that make a device vulnerable, if this
action is feasible.

Allowing only legitimate devices to connect to affected devices will
help limit exposure to this vulnerability. Refer to the following
Control Plane Policing and Configuring Infrastructure Access Lists
subsections for further details. Because a TCP three-way handshake
is not required, the mitigation must be combined with anti-spoofing
measures on the network edge to increase effectiveness.

Additional mitigations that can be deployed on Cisco devices within the
network are available in the Cisco Applied Mitigation Bulletin companion
document for this advisory, which is available at the following link:

Cisco Guide to Harden Cisco IOS Devices

The Cisco Guide to Harden Cisco IOS Devices provides examples of many
useful techniques to mitigate TCP state manipulation vulnerabilities.
These include:

  * Infrastructure Access Control Lists (iACL)
  * Receive Access Control Lists (rACL)
  * Transit Access Control Lists (tACL)
  * vty Access Control Lists
  * Control Plane Policing (CoPP)
  * Control Plane Protection (CPPr)

For more information on these topics, consult
"Cisco Guide to Harden Cisco IOS Devices"


For devices that need to offer TCP services, administrators can use
CoPP to block TCP traffic from untrusted sources that is destined
to the affected device. Cisco IOS Software Releases 12.0S, 12.2SX,
12.2S, 12.3T, 12.4, and 12.4T support the CoPP feature. CoPP may be
configured on a device to protect the management and control planes
and minimize the risk and effectiveness of direct infrastructure
attacks by explicitly permitting only authorized traffic sent to
infrastructure devices in accordance with existing security policies and
configurations. The following example can be adapted to specific network

    !-- The network and the host are trusted.
    !-- Everything else is not trusted. The following access list is used
    !-- to determine what traffic needs to be dropped by a control plane
    !-- policy (the CoPP feature.) If the access list matches (permit),
    !-- then traffic will be dropped. If the access list does not
    !-- match (deny), then traffic will be processed by the router.
    !-- Note that TCP ports 22 and 23 are examples; this 
    !-- configuration needs to be expanded to include all used
    !-- TCP ports.
    access-list 100 deny tcp any eq 22
    access-list 100 deny tcp any eq 23
    access-list 100 deny tcp host any eq 22
    access-list 100 deny tcp host any eq 23
    access-list 100 permit tcp any any

    !-- Permit (Police or Drop)/Deny (Allow) all other Layer3 and Layer4
    !-- traffic in accordance with existing security policies and
    !-- configurations for traffic that is authorized to be sent
    !-- to infrastructure devices.
    !-- Create a class map for traffic that will be policed by
    !-- the CoPP feature.
    class-map match-all drop-tcp-class
      match access-group 100

    !-- Create a policy map that will be applied to the
    !-- Control Plane of the device, and add the "drop-tcp-traffic"
    !-- class map.
    policy-map control-plane-policy
     class drop-tcp-class

    !-- Apply the policy map to the control plane of the
    !-- device.
     service-policy input control-plane-policy

Warning: Because a TCP three-way handshake is not required to exploit
this vulnerability, it is possible to spoof the IP address of the
sender, which could defeat access control lists (ACLs) that permit
communication to these ports from trusted IP addresses.

In the preceding CoPP example, the access control entries (ACEs)
that match the potential exploit packets with the "permit" action
result in these packets being discarded by the policy-map "drop"
function, while packets that match the "deny" action (not shown)
are not affected by the policy-map drop function. Additional
information on the configuration and use of the CoPP feature can
be found at "Control Plane Policing Implementation Best Practices"
and "Control Plane Policing"

Configuring iACLs

Although it is often difficult to block traffic that transits a
network, it is possible to identify traffic that should never be
allowed to target infrastructure devices and block that traffic
at the border of your network. Infrastructure ACLs are considered
a network security best practice and should be considered as a
long-term addition to good network security as well as a workaround
for this specific vulnerability. The white paper "Protecting
Your Core: Infrastructure Protection Access Control Lists"
presents guidelines and recommended deployment
techniques for infrastructure protection ACLs.

BGP Considerations

GTSM can help prevent exploitation of this vulnerability by
means of the BGP port because packets that originate from
devices that do not pass the TTL check configured by GTSM are
dropped before any TCP processing occurs. For information
on GTSM refer to "BGP Support for TTL Security Check"
and "BGP Time To Live Security Check"

Embedded Event Manager (EEM)

A Cisco IOS Embedded Event Manager (EEM) policy that is based on Tool
Command Language (Tcl) can be used on vulnerable Cisco IOS devices to
identify and detect a hung, extended, or indefinite TCP connection
that is caused by this vulnerability. The policy allows administrators
to monitor TCP connections on a Cisco IOS device. When Cisco IOS EEM
detects potential exploitation of this vulnerability, the policy can
trigger a response by sending a syslog message or a Simple Network
Management Protocol (SNMP) trap to clear the TCP connection. The example
policy provided in this document is based on a Tcl script that monitors
and parses the output from two commands at defined intervals, produces a
syslog message when the monitor threshold reaches its configured value,
and can reset the TCP connection.

The Tcl script is available for download at the "Cisco
Beyond: Embedded Event Manager (EEM) Scripting Community"
( at the following link,
and the device sample configuration is provided below.

    !-- Location where the Tcl script will be stored
    event manager directory user policy disk0:/eem

    !-- Define variable and set the monitoring interval
    !-- as an integer (expressed in seconds)
    event manager environment EEM_MONITOR_INTERVAL 60

    !-- Define variable and set the threshold value as
    !-- an integer for the number of retransmissions
    !-- that determine if the TCP connection is hung
    !-- (a recommended value to use is 15)
    event manager environment EEM_MONITOR_THRESHOLD 15

    !-- Define variable and set the value to "yes" to
    !-- enable the clearing of hung TCP connections
    event manager environment EEM_MONITOR_CLEAR yes

    !-- Define variable and set to the TCP connection
    !-- state or states that script will monitor, which
    !-- can be a single state or a space-separated list
    !-- of states
    event manager environment EEM_MONITOR_STATES SYNRCVD SYNSENT

    !-- Register the script as a Cisco EEM policy
    event manager policy monitor-sockets.tcl

Obtaining Fixed Software

Cisco has released free software updates that address this
vulnerability. 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,
or as otherwise set forth at Downloads at

Do not contact psirt at or security-alert at for software

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

Customers without Service Contracts

Customers who purchase direct from Cisco but do not hold a Cisco service
contract, and customers who purchase through third-party vendors but are
unsuccessful in obtaining fixed software through their point of sale
should acquire 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 at

Customers should have their product serial number available and be
prepared to give the URL of this notice as evidence of entitlement to a
free upgrade. Free upgrades for non-contract customers must be requested
through the TAC.

Refer to
for additional TAC contact information, including localized telephone
numbers, and instructions and e-mail addresses for use in various

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious
use of the vulnerability described in this advisory.

This vulnerability was reported to Cisco by a customer.

Status of this Notice: FINAL


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.


This advisory is posted on Cisco's worldwide website at:

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 at
  * first-bulletins at
  * bugtraq at
  * vulnwatch at
  * cisco at
  * cisco-nsp at
  * full-disclosure at
  * at

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  | 2010-August-12  | 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
This includes instructions for press inquiries regarding
Cisco security notices. All Cisco security advisories are available at

Copyright 2008-2010 Cisco Systems, Inc. All rights reserved.

Updated: Aug 12, 2010                             Document ID: 112099
Version: GnuPG v1.4.10 (GNU/Linux)


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