Cisco Security Advisory: Crafted TCP Packet Can Cause Denial of Service

Document ID: 72318
Advisory ID: cisco-sa-20070124-crafted-tcp
http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml
1.0
For Public Release 2007 January 24 1600 UTC (GMT)

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Contents
Summary
Affected Products
Details
Vulnerability Scoring Details
Impact
Software Version and Fixes
Workarounds
Obtaining Fixed Software
Exploitation and Public Announcements
Status of this Notice:FINAL
Distribution
Revision History
Cisco Security Procedures


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Summary
The Cisco IOS Transmission Control Protocol (TCP) listener in certain versions of Cisco IOS software is vulnerable to a remotely-exploitable memory leak that may lead to a denial of service condition.

This vulnerability only applies to traffic destined to the Cisco IOS device. Traffic transiting the Cisco IOS device will not trigger this vulnerability.

Cisco has made free software available to address this vulnerability for affected customers.

This issue is documented as Cisco bug ID CSCek37177 ( registered customers only) .

There are workarounds available to mitigate the effects of the vulnerability.

This advisory is posted at http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml.

Affected Products
Vulnerable Products
This issue affects all Cisco devices running Cisco IOS software. To be affected, devices must be configured to process Internet Protocol version 4 (IPv4) packets and receive TCP packets. Devices which run only Internet Protocol version 6 (IPv6) are not affected.

This vulnerability is present in all unfixed versions of Cisco IOS software, including versions 9.x, 10.x, 11.x and 12.x.

To determine the software running on a Cisco product, log in to the device and issue the "show version" command to display the system banner. Cisco IOS software will identify itself as "Internetwork Operating System Software" or simply "IOS". On the next line of output, the image name will be displayed between parentheses, followed by "Version" and the IOS release name. Other Cisco devices will not have the "show version" command or will give different output.

The following example identifies a Cisco product running Cisco IOS release 12.2(14)S16 with an installed image name of C7200-IS-M:

Cisco Internetwork Operating System Software
IOS (tm) 7200 Software (C7200-IS-M), Version 12.2(14)S16, RELEASE SOFTWARE (fc1)The release train label is "12.2".

The next example shows a product running IOS release 12.3(7)T12 with an image name of C7200-IK9S-M:

Cisco IOS Software, 7200 Software (C7200-IK9S-M), Version 12.3(7)T12, RELEASE SOFTWARE (fc1)
Additional information about Cisco IOS Banners is available at:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_white_paper09186a008018305e.shtml#3

Products Confirmed Not Vulnerable
Cisco products that do not run IOS are unaffected by this vulnerability.

Cisco IOS-XR is not affected.

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

Details
TCP is the transport layer protocol designed to provide connection-oriented, reliable delivery of a data stream. To accomplish this, TCP uses a mixture of flags to indicate state and sequence numbers to identify the order in which the packets are to be reassembled. TCP also provides a number, called an acknowledgement number, that is used to indicate the sequence number of the next packet expected. The full specification of the TCP protocol can be found at http://www.ietf.org/rfc/rfc0793.txt .

Cisco IOS devices that are configured to receive TCP packets are exposed to this issue. This Advisory does not apply to traffic that is transiting the device.

Certain crafted packets destined to an IPv4 address assigned to a physical or virtual interface on a Cisco IOS device may cause the device to leak a small amount of memory. Over time, such a memory leak may lead to memory exhaustion and potentially degraded service.

Although this is an issue with TCP, it is not required to complete the TCP 3-way handshake in order for the memory leak to be triggered. Therefore, TCP packets with a spoofed source address may trigger the leak.

The following document contains additional information on how to identify if your router is suffering from a memory leak in Processor memory:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_tech_note09186a00800a6f3a.shtml#tshoot2

Vulnerability Scoring Details
Cisco is providing scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). Cisco will provide a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

Cisco PSIRT will set the bias in all cases to normal. Customers are encouraged to apply the bias parameter when determining the environmental impact of a particular vulnerability.

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

Cisco has provided an FAQ to answer additional questions regarding CVSS at:

http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks: http://intellishield.cisco.com/security/alertmanager/cvss

CSCek37177 - malformed tcp packets deplete processor memory ( registered customers only)

Calculate the environmental score of CSCek37177

CVSS Base Score - 3.3

Access Vector
Access Complexity
Authentication
Confidentiality Impact
Integrity Impact
Availability Impact
Impact Bias

Remote
Low
Not Required
None
None
Complete
Normal

CVSS Temporal Score - 2.7

Exploitability
Remediation Level
Report Confidence

Functional
Official Fix
Confirmed



Impact
Successful exploitation of the vulnerability may result in a small amount of processor memory to leak, which may lead to degraded service. This issue will not resolve over time, and will require a device reset to recover the leaked memory.

This vulnerability only applies to traffic destined to the Cisco IOS device. Traffic transiting the device will not trigger this issue.

Software Version and Fixes
When considering software upgrades, also consult http://www.cisco.com/go/psirt 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) describes a release train and the platforms or products for which it is intended. If a given release train is vulnerable, then the earliest possible releases that contain the fix (the "First Fixed Release") and the anticipated date of availability for each are listed in the "Rebuild" and "Maintenance" columns. 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. The release should be upgraded at least to the indicated release or a later version (greater than or equal to the First Fixed Release label).

For more information on the terms "Rebuild" and "Maintenance," consult the following URL:

http://www.cisco.com/warp/public/620/1.html.

Note: There are three IOS security advisories and one field notice being published on January 24, 2007. Each advisory lists only the releases which fix the issue described in the advisory. A combined software table is available at http://www.cisco.com/warp/public/707/cisco-sa-20070124-bundle.shtml and can be used to choose a software release which fixes all security vulnerabilities published as of January 24, 2007. Links for the advisories and field notice are listed here.

http://www.cisco.com/warp/public/707/cisco-sa-20070124-IOS-IPv6.shtml

http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml

http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-ip-option.shtml

http://www.cisco.com/warp/public/770/fn62613.shtml

Requests for software rebuilds to include the change for Daylight Savings Time (DST) that will be implemented in March 2007 should be directed through the Technical Assistance Center (TAC), and this advisory should be used as reference.

Major Release
Availability of Repaired Releases

Affected 12.0-Based Release
Rebuild
Maintenance

12.0
Vulnerable; migrate to 12.2(37) or later

12.0DA
Vulnerable; migrate to 12.2(10)DA5 or later

12.0DB
Vulnerable; migrate to 12.3(4)T13 or later

12.0DC
Vulnerable; migrate to 12.3(4)T13 or later

12.0S
12.0(31)S6

12.0(32)S4

12.0SC
Vulnerable; migrate to 12.3(13a)BC6 or later

12.0SL
Vulnerable; migrate to 12.0(31)S6 or later

12.0SP
Vulnerable; migrate to 12.0(31)S6 or later

12.0ST
Vulnerable; migrate to 12.0(31)S6 or later

12.0SX
12.0(25)SX11

12.0SY
12.0(32)SY

12.0SZ
Vulnerable; migrate to 12.0(31)S6 or later

12.0T
Vulnerable; migrate to 12.2(37) or later

12.0W
Not vulnerable

12.0WC
12.0(5)WC15

12.0WT
Not vulnerable

12.0XA
Vulnerable; migrate to 12.2(37) or later

12.0XB
Vulnerable; migrate to 12.2(37) or later

12.0XC
Vulnerable; migrate to 12.2(37) or later

12.0XD
Vulnerable; migrate to 12.2(37) or later

12.0XE
Vulnerable; migrate to 12.1(26)E7 or later

12.0XF
Not vulnerable

12.0XG
Vulnerable; migrate to 12.2(37) or later

12.0XH
Vulnerable; migrate to 12.2(37) or later

12.0XI
Vulnerable; migrate to 12.2(37) or later

12.0XJ
Vulnerable; migrate to 12.2(37) or later

12.0XK
Vulnerable; migrate to 12.2(37) or later

12.0XL
Vulnerable; migrate to 12.2(37) or later

12.0XM
Vulnerable; migrate to 12.2(37) or later

12.0XN
Vulnerable; migrate to 12.2(37) or later

12.0XQ
Vulnerable; migrate to 12.2(37) or later

12.0XR
Vulnerable; migrate to 12.2(37) or later

12.0XS
Vulnerable; migrate to 12.1(26)E7 or later

12.0XV
Vulnerable; migrate to 12.2(37) or later

12.0XW
Not vulnerable

Affected 12.1-Based Release
Rebuild
Maintenance

12.1
Vulnerable; migrate to 12.2(37) or later

12.1 AA
Vulnerable; migrate to 12.2(37) or later

12.1 AX
Vulnerable; migrate to 12.2(25)EY4 or later

12.1 AY
Vulnerable; migrate to 12.1(22)EA8 or later

12.1 AZ
Vulnerable; migrate to 12.1(22)EA8 or later

12.1 CX
Vulnerable; migrate to 12.2(37) or later

12.1 DA
Vulnerable; migrate to 12.2(10)DA5 or later

12.1 DB
Vulnerable; migrate to 12.3(4)T13 or later

12.1 DC
Vulnerable; migrate to 12.3(4)T13 or later

12.1E
12.1(26)E7

12.1(27b)E1

12.1EA
12.1(22)EA8

12.1EB
Vulnerable; contact TAC

12.1EC
Vulnerable; migrate to 12.3(13a)BC6 or later

12.1EO
12.1(19)EO6; available on 31-Jan-07

12.1(20)EO3

12.1EU
Vulnerable; migrate to 12.2(25)EWA6 or later

12.1EV
Vulnerable; migrate to 12.2(27)SV4 or later

12.1EW
Vulnerable; migrate to 12.2(25)EWA6 or later

12.1EX
Vulnerable; migrate to 12.1(26)E7 or later

12.1EY
Vulnerable; migrate to 12.1(26)E7 or later

12.1EZ
Vulnerable; migrate to 12.1(26)E7 or later

12.1T
Vulnerable; migrate to 12.2(37) or later

12.1XA
Vulnerable; migrate to 12.2(37) or later

12.1XB
Vulnerable; migrate to 12.2(37) or later

12.1XC
Vulnerable; migrate to 12.2(37) or later

12.1XD
Vulnerable; migrate to 12.2(37) or later

12.1XE
Vulnerable; migrate to 12.1(26)E7 or later

12.1XF
Vulnerable; migrate to 12.3(19) or later

12.1XG
Vulnerable; migrate to 12.3(19) or later

12.1XH
Vulnerable; migrate to 12.2(37) or later

12.1XI
Vulnerable; migrate to 12.2(37) or later

12.1XJ
Vulnerable; migrate to 12.3(19) or later

12.1XL
Vulnerable; migrate to 12.3(19) or later

12.1XM
Vulnerable; migrate to 12.3(19) or later

12.1XP
Vulnerable; migrate to 12.3(19) or later

12.1XQ
Vulnerable; migrate to 12.3(19) or later

12.1XR
Vulnerable; migrate to 12.3(19) or later

12.1XS
Vulnerable; migrate to 12.2(37) or later

12.1XT
Vulnerable; migrate to 12.3(19) or later

12.1XU
Vulnerable; migrate to 12.3(19) or later

12.1XV
Vulnerable; migrate to 12.3(19) or later

12.1XW
Vulnerable; migrate to 12.2(37) or later

12.1XX
Vulnerable; migrate to 12.2(37) or later

12.1XY
Vulnerable; migrate to 12.2(37) or later

12.1XZ
Vulnerable; migrate to 12.2(37) or later

12.1YA
Vulnerable; migrate to 12.3(19) or later

12.1YB
Vulnerable; migrate to 12.3(19) or later

12.1YC
Vulnerable; migrate to 12.3(19) or later

12.1YD
Vulnerable; migrate to 12.3(19) or later

12.1YE
Vulnerable; migrate to 12.3(19) or later

12.1YF
Vulnerable; migrate to 12.3(19) or later

12.1YH
Vulnerable; migrate to 12.3(19) or later

12.1YI
Vulnerable; migrate to 12.3(19) or later

12.1YJ
Vulnerable; migrate to 12.1(22)EA8 or later

Affected 12.2-Based Release
Rebuild
Maintenance

12.2
12.2(37)

12.2B
Vulnerable; migrate to 12.3(4)T13 or later

12.2BC
Vulnerable; migrate to 12.3(13a)BC6 or later

12.2BW
Vulnerable; migrate to 12.3(19) or later

12.2BY
Vulnerable; migrate to 12.3(4)T13 or later

12.2BZ
Vulnerable; migrate to 12.3(7)XI8 or later

12.2CX
Vulnerable; migrate to 12.3(13a)BC6 or later

12.2CY
Vulnerable; migrate to 12.3(13a)BC6 or later

12.2CZ
Vulnerable; contact TAC

12.2DA
12.2(10)DA5

12.2(12)DA10

12.2DD
Vulnerable; migrate to 12.3(4)T13 or later

12.2DX
Vulnerable; migrate to 12.3(4)T13 or later

12.2EU
Vulnerable; migrate to 12.2(25)EWA6 or later

12.2EW
Vulnerable; migrate to 12.2(25)EWA6 or later

12.2EWA
12.2(25)EWA6

12.2EX
12.2(25)EX1

12.2EY
12.2(25)EY4

12.2EZ
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2FX
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2FY
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2FZ
All 12.2FZ releases are fixed

12.2IXA
Vulnerable; contact TAC

12.2IXB
Vulnerable; contact TAC

12.2IXC
Vulnerable; contact TAC

12.2JA
Vulnerable; migrate to 12.3(8)JA2 or later

12.2JK
Vulnerable; migrate to 12.4(4)T4 or later

12.2MB
Vulnerable; migrate to 12.2(25)SW8 or later

12.2MC
Vulnerable; migrate to 12.3(11)T11 or later

12.2S
12.2(25)S12; Available 12-Feb-07

12.2SB
12.2(28)SB2
12.2(31)SB

12.2SBC
12.2(27)SBC5

12.2SE
12.2(35)SE

12.2SEA
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2SEB
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2SEC
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2SED
Vulnerable; migrate to 12.2(25)SEE1 or later

12.2SEE
12.2(25)SEE1

12.2SEF
12.2(25)SEF1

12.2SEG
All 12.2SEG releases are fixed

12.2SG
12.2(37)SG; Available 25-Apr-07

12.2SGA
All 12.2SGA releases are fixed

12.2SO
12.2(18)SO7

12.2SRA
All 12.2SRA releases are fixed

12.2SRB
All 12.2SRB releases are fixed

12.2SU
Vulnerable; migrate to 12.4(8) or later

12.2SV
12.2(27)SV4

12.2(28)SV1

12.2(29)SV1

12.2SW
12.2(25)SW8

12.2SX
Vulnerable; migrate to 12.2(18)SXD7a or later

12.2SXA
Vulnerable; migrate to 12.2(18)SXD7a or later

12.2SXB
Vulnerable; migrate to 12.2(18)SXD7a or later

12.2SXD
12.2(18)SXD7a

12.2SXE
12.2(18)SXE6

12.2SXF
12.2(18)SXF5

12.2SY
Vulnerable; migrate to 12.2(18)SXD7a or later

12.2SZ
Vulnerable; migrate to 12.2(25)S12 or later; Available 12-Feb-07

12.2T
Vulnerable; migrate to 12.3(19) or later

12.2TPC
Vulnerable; contact TAC

12.2XA
Vulnerable; migrate to 12.3(19) or later

12.2XB
Vulnerable; migrate to 12.3(19) or later

12.2XC
Vulnerable; migrate to 12.3(4)T13 or later

12.2XD
Vulnerable; migrate to 12.3(19) or later

12.2XE
Vulnerable; migrate to 12.3(19) or later

12.2XF
Vulnerable; migrate to 12.3(13a)BC6 or later

12.2XG
Vulnerable; migrate to 12.3(19) or later

12.2XH
Vulnerable; migrate to 12.3(19) or later

12.2XI
Vulnerable; migrate to 12.3(19) or later

12.2XJ
Vulnerable; migrate to 12.3(19) or later

12.2XK
Vulnerable; migrate to 12.3(19) or later

12.2XL
Vulnerable; migrate to 12.3(19) or later

12.2XM
Vulnerable; migrate to 12.3(19) or later

12.2XN
Vulnerable; migrate to 12.3(19) or later

12.2XQ
Vulnerable; migrate to 12.3(19) or later

12.2XR
Vulnerable; migrate to 12.3(19) or later

12.2XS
Vulnerable; migrate to 12.3(19) or later

12.2XT
Vulnerable; migrate to 12.3(19) or later

12.2XU
Vulnerable; migrate to 12.3(19) or later

12.2XV
Vulnerable; migrate to 12.3(19) or later

12.2XW
Vulnerable; migrate to 12.3(19) or later

12.2YA
Vulnerable; migrate to 12.3(19) or later

12.2YB
Vulnerable; migrate to 12.3(19) or later

12.2YC
Vulnerable; migrate to 12.3(19) or later

12.2YD
Vulnerable; migrate to 12.3(11)T11 or later

12.2YE
Vulnerable; migrate to 12.2(25)S12 or later; Available 12-Feb-07

12.2YF
Vulnerable; migrate to 12.3(19) or later

12.2YG
Vulnerable; migrate to 12.3(19) or later

12.2YH
Vulnerable; migrate to 12.3(19) or later

12.2YJ
Vulnerable; migrate to 12.3(19) or later

12.2YK
Vulnerable; migrate to 12.3(4)T13 or later

12.2YL
Vulnerable; migrate to 12.3(4)T13 or later

12.2YM
Vulnerable; migrate to 12.3(4)T13 or later

12.2YN
Vulnerable; migrate to 12.3(4)T13 or later

12.2YO
Not vulnerable

12.2YP
Vulnerable; migrate to 12.3(19) or later

12.2YQ
Vulnerable; migrate to 12.3(4)T13 or later

12.2YR
Vulnerable; migrate to 12.3(4)T13 or later

12.2YS
Not vulnerable

12.2YT
Vulnerable; migrate to 12.3(19) or later

12.2YU
Vulnerable; migrate to 12.3(4)T13 or later

12.2YV
Vulnerable; migrate to 12.3(4)T13 or later

12.2YW
Vulnerable; migrate to 12.3(4)T13 or later

12.2YX
Vulnerable; migrate to 12.4(8) or later

12.2YY
Vulnerable; migrate to 12.3(4)T13 or later

12.2YZ
Vulnerable; migrate to 12.2(25)S12 or later; Available 12-Feb-07

12.2ZA
Vulnerable; migrate to 12.2(18)SXD7a or later

12.2ZB
Vulnerable; migrate to 12.3(4)T13 or later

12.2ZC
Vulnerable; migrate to 12.3(4)T13 or later

12.2ZD
Vulnerable; contact TAC

12.2ZE
Vulnerable; migrate to 12.3(19) or later

12.2ZF
Vulnerable; migrate to 12.3(4)T13 or later

12.2ZG
Vulnerable; contact TAC

12.2ZH
Vulnerable; contact TAC

12.2ZJ
Vulnerable; migrate to 12.3(4)T13 or later

12.2ZL
Vulnerable; contact TAC

12.2ZN
Vulnerable; migrate to 12.3(4)T13 or later

12.2ZP
Vulnerable; migrate to 12.4(8) or later

Affected 12.3-Based Release
Rebuild
Maintenance

12.3
12.3(10f)
12.3(19)

12.3B
Vulnerable; migrate to 12.3(11)T11 or later

12.3BC
12.3(13a)BC6

12.3(17a)BC2

12.3BW
Vulnerable; migrate to 13.3(11)T11 or later

12.3JA
12.3(8)JA2

12.3JEA
All 12.3JEA releases are fixed

12.3JEB
All 12.3JEB releases are fixed

12.3JK
12.3(2)JK2

12.3JX
12.3(7)JX4
12.3(11)JX

12.3T
12.3(4)T13

12.3(11)T11

12.3TPC
Vulnerable; contact TAC

12.3XA
Vulnerable; contact TAC

12.3XB
Vulnerable; migrate to 12.3(11)T11 or later

12.3XC
Vulnerable; contact TAC

12.3XD
Vulnerable; migrate to 12.3(11)T11 or later

12.3XE
Vulnerable; contact TAC

12.3XF
Vulnerable; migrate to 12.3(11)T11 or later

12.3XG
Vulnerable; contact TAC

12.3XH
Vulnerable; migrate to 12.3(11)T11 or later

12.3XI
12.3(7)XI8

12.3XJ
Vulnerable; migrate to 12.3(14)YX2 or later

12.3XK
Vulnerable; migrate to 12.4(8) or later

12.3XQ
Vulnerable; migrate to 12.4(8) or later

12.3XR
Vulnerable; contact TAC

12.3XS
Vulnerable; migrate to 12.4(8) or later

12.3XU
Vulnerable; migrate to 12.4(2)T5 or later

12.3XW
Vulnerable; migrate to 12.3(14)YX2 or later

12.3XX
Vulnerable; migrate to 12.4(8) or later

12.3XY
Vulnerable; migrate to 12.4(8) or later

12.3YA
Vulnerable; contact TAC

12.3YD
Vulnerable; migrate to 12.4(2)T5 or later

12.3YF
Vulnerable; migrate to 12.3(14)YX2 or later

12.3YG
Vulnerable; migrate to 12.4(2)T5 or later

12.3YH
Vulnerable; migrate to 12.4(2)T5 or later

12.3YI
Vulnerable; migrate to 12.4(2)T5 or later

12.3YJ
Vulnerable; migrate to 12.3(14)YQ8 or later

12.3YK
Vulnerable; migrate to 12.4(4)T4 or later

12.3YM
12.3(14)YM8

12.3YQ
12.3(14)YQ8

12.3YS
Vulnerable; migrate to 12.4(4)T4 or later

12.3YT
Vulnerable; migrate to 12.4(4)T4 or later

12.3YU
Vulnerable; contact TAC

12.3YX
12.3(14)YX2

12.3YZ
12.3(11)YZ1

Affected 12.4-Based Release
Rebuild
Maintenance

12.4
12.4(3e)

12.4(7b)
12.4(8)

12.4MR
12.4(6)MR1

12.4SW
All 12.4SW releases are fixed

12.4T
12.4(2)T5

12.4(4)T4

12.4(6)T3
12.4(9)T

12.4XA
Vulnerable; migrate to 12.4(6)T3

12.4XB
Vulnerable; contact TAC

12.4XC
12.4(4)XC3

12.4XD
12.4(4)XD4

12.4XE
All 12.4XE releases are fixed

12.4XG
All 12.4XG releases are fixed

12.4XJ
All 12.4XJ releases are fixed

12.4XP
All 12.4XP releases are fixed

12.4XT
All 12.4XT releases are fixed



Workarounds
Additional mitigations that can be deployed on Cisco devices within the network are available in the Cisco Applied Intelligence companion document for this advisory:

http://www.cisco.com/warp/public/707/cisco-air-20070124-crafted-tcp.shtml

Note: Configuring VTY access-class filters is not an effective mitigation strategy for this vulnerability.

Infrastructure ACLs (iACL)
Although it is often difficult to block traffic transiting your network, it is possible to identify traffic which should never be allowed to target your 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 ACL example shown below should be included as part of the deployed infrastructure access-list which will protect all devices with IP addresses in the infrastructure IP address range.

A sample access list for devices running Cisco IOS is below:


!--- Permit TCP services from trust hosts destined
!--- to infrastructure addresses.


access-list 150 permit tcp TRUSTED_HOSTS MASK INFRASTRUCTURE_ADDRESSES MASK


!--- Deny TCP packets from all other sources destined to infrastructure addresses.


access-list 150 deny tcp any INFRASTRUCTURE_ADDRESSES MASK


!--- Permit all other traffic to transit the device.


access-list 150 permit IP any any

interface serial 2/0
ip access-group 150 inThe white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection access lists. This white paper can be obtained here: http://www.cisco.com/warp/public/707/iacl.html

Receive ACLs (rACL)
For distributed platforms, Receive ACLs may be an option starting in Cisco IOS Software Versions 12.0(21)S2 for the 12000 (GSR), 12.0(24)S for the 7500, and 12.0(31)S for the 10720. The Receive ACL protects the device from harmful traffic before the traffic can impact the route processor. Receive ACLs are designed to only protect the device on which it is configured. On the 12000, transit traffic is never affected by a receive ACL. Because of this, the destination IP address "any" used in the example ACL entries below only refer to the router's own physical or virtual IP addresses. On the 7500 and 10720, transit traffic with IP options set will be subject to the Receive ACL and permitted or denied accordingly. Receive 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 entitled "GSR: Receive Access Control Lists" will help you identify and allow legitimate traffic to your device and deny all unwanted packets: http://www.cisco.com/warp/public/707/racl.html

The following is the receive path ACL written to permit this type of traffic from trusted hosts:


!--- Permit tcp services from trusted hosts allowed to the RP.


access-list 151 permit tcp TRUSTED_ADDRESSES MASK any


!--- Deny tcp services from all other sources to the RP.


access-list 151 deny tcp any any


!--- Permit all other traffic to the RP.


access-list 151 permit ip any any


!--- Apply this access list to the 'receive' path.


ip receive access-list 151Control Plane Policing (CoPP)
The Control Plane Policing (CoPP) feature may be used to mitigate this vulnerability. In the following example, only TCP traffic from trusted hosts and with 'receive' destination IP addresses is permitted to reach the route processor (RP). All other 'transit' IP traffic is unaffected.

It should be noted that dropping traffic from unknown or untrusted IP addresses may affect hosts with dynamically assigned IP addresses from connecting to the Cisco IOS device.

access-list 152 deny tcp TRUSTED_ADDRESSES MASK any
access-list 152 permit tcp any any
access-list 152 deny ip any any
!
class-map match-all permit-tcp-class
match access-group 152
!
!
policy-map permit-tcp-policy
class permit-tcp-class
drop
!
control-plane
service-policy input permit-tcp-policyIn the above CoPP example, the ACL entries that match the 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 are not affected by the policy-map drop function.

Please note that in the 12.2S and 12.0S Cisco IOS trains the policy-map syntax is different:

policy-map permit-tcp-policy
class class permit-tcp-class
police 32000 1500 1500 conform-action drop exceed-action dropCoPP is available in Cisco IOS release trains 12.0S, 12.2SX, 12.2S, 12.3T, 12.4, and 12.4T.

Additional information on the configuration and use of the CoPP feature can be found at the following URL:

http://www.cisco.com/en/US/products/ps6642/products_white_paper0900aecd804fa16a.shtml

Anti-spoofing
The Unicast Reverse Path Forwarding (Unicast RPF or uRPF) feature helps to mitigate problems that are caused by spoofed IP source addresses. It is available on Cisco routers and firewalls. For further details, please refer to:

http://www.cisco.com/en/US/partner/products/ps6441/products_command_reference_chapter09186a00804ae49f.html#wp1229984

By enabling Unicast Reverse Path Forwarding (uRPF), all spoofed packets will be dropped at the first device. To enable uRPF, use the following commands.

router(config)# ip cef
router(config)# interface interface #
router(config-if)# ip verify unicast source reachable-via rx
BGP and BTSH/GTSM
Depending on your release of software, it may be possible to protect your BGP sessions from this memory leak. With the introduction of CSCee73956 ( registered customers only) , Cisco IOS has improved support for BTSH (BGP TTL Security Hack) to reduce, if not eliminate a risk of a memory leak due to this vulnerability. This functionality is also known as GTSM (Generalized TTL Security Mechanism) and documented in RFC 3682. This section refers to GTSM as applied to eBGP sessions only.

Releases of Cisco IOS that contain CSCee73956 are protected from this attack against the BGP port (TCP port 179) only. Other ports should be protected accordingly.

BTSH is not supported for iBGP sessions. BTSH was first introduced in Cisco IOS in 12.0(27)S, 12.3(7)T and 12.2(25)S. Note that the BTSH feature prior to CSCee73956 will not protect against this vulnerability.

For more information on BTSH, please see: http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_7/gt_btsh.htm

Obtaining Fixed Software
Cisco will make free software available to address this vulnerability for affected customers. This advisory will be updated as fixed software becomes available. 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 http://www.cisco.com/public/sw-license-agreement.html , or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Do not contact either "psirt@cisco.com" or "security-alert@cisco.com" for software upgrades.

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 http://www.cisco.com.

Customers using Third Party Support Organizations
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 guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.

Customers without Service Contracts
Customers who purchase direct 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 get their 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@cisco.com

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.

Refer to 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.

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 discovered by Cisco during our internal testing process.

Status of this Notice:FINAL
THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

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.

Distribution
This advisory is posted on Cisco's worldwide website at: http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml

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@cisco.com

first-teams@first.org

bugtraq@securityfocus.com

vulnwatch@vulnwatch.org

cisco@spot.colorado.edu

cisco-nsp@puck.nether.net

full-disclosure@lists.grok.org.uk

comp.dcom.sys.cisco@newsgate.cisco.com