INFORMATION BULLETIN

L-053: Cisco IOS Software TCP Initial Sequence Number Improvements

March 2, 2001 16:00 GMT


PROBLEM:	Cisco IOS software contains a flaw that permits the successful prediction of TCP Initial Sequence Numbers.
PLATFORM:	All released versions of Cisco IOS software running on Cisco routers and switches. Reference the Cisco Security Advisory for more details.
DAMAGE:	Forged packets can be injected into a network from a location outside its boundary so that they are trusted as authentic by the receiving host, thus resulting in a failure of integrity. Such packets could be crafted to gain access or make some other modification to the receiving system in order to attain some goal, such as gaining unauthorized interactive access to a system or compromising stored data.
SOLUTION:	To remove the vulnerability, Cisco is offering free software upgrades for all affected platforms.

VULNERABILITY ASSESSMENT:		The risk is HIGH. The vulnerability may allow unauthorized access to a machine.

[****** Start of Cisco Security Advisory ******]

Cisco Security Advisory: Cisco IOS Software TCP Initial Sequence Number

Randomization Improvements

Revision 1.0: INTERIM

For Public Release 2001 February 28 18:00 US/Pacific (UTC+0800)

------------------------------------------------------------------------

Summary

Cisco IOS software contains a flaw that permits the successful prediction

of TCP Initial Sequence Numbers.

This vulnerability is present in all released versions of Cisco IOS

software running on Cisco routers and switches. It only affects the

security of TCP connections that originate or terminate on the affected

Cisco device itself; it does not apply to TCP traffic forwarded through the

affected device in transit between two other hosts.

To remove the vulnerability, Cisco is offering free software upgrades for

all affected platforms. The defect is described in DDTS record CSCds04747.

Workarounds are available that limit or deny successful exploitation of the

vulnerability by filtering traffic containing forged IP source addresses at

the perimeter of a network or directly on individual devices.

This notice will be posted

at http://www.cisco.com/warp/public/707/ios-tcp-isn-random-pub.shtml.

Affected Products

The vulnerability is present in all Cisco routers and switches running

affected releases of Cisco IOS Software.

To determine the software running on a Cisco product, log in to the device

and issue the command "show version" to display the system banner. Cisco

IOS software will identify itself as "Internetwork Operating System

Software" or simply "IOS (tm)". 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 IOS release

12.0(3) with an installed image name of C2500-IS-L:

Cisco Internetwork Operating System Software IOS (tm)

2500 Software (C2500-IS-L), Version 12.0(3), RELEASE SOFTWARE

Cisco devices that may be running an affected IOS software release include,

but are not limited to:

* 800, 1000, 1005, 1400, 1600, 1700, 2500, 2600, 3600, MC3810, 4000,

4500, 4700, 6200, 6400 NRP, 6400 NSP series Cisco routers.

* ubr900 and ubr920 universal broadband routers.

* Catalyst 2900 ATM, 2900XL, 2948g, 3500XL, 4232, 4840g, 5000 RSFC

series switches.

* 5200, 5300, 5800 series access servers.

* Catalyst 6000 MSM, 6000 Hybrid Mode, 6000 Native Mode, 6000 Supervisor

Module, Catalyst ATM Blade.

* RSM, 7000, 7010, 7100, 7200, ubr7200, 7500, 10000 ESR, and 12000 GSR

series Cisco routers.

* DistributedDirector.

* Catalyst 8510CSR, 8510MSR, 8540CSR, 8540MSR series switches.

Cisco products that do not run Cisco IOS software and are not affected by

the vulnerabilities described in this notice include, but are not limited

to:

* Cisco PIX firewall.

* Cisco 600 family of routers running CBOS.

* Host-based network management or access management products.

* Cisco IP Telephony and telephony management software (except those

that are hosted on a vulnerable IOS platform).

* Voice gateways and convergence products (except those that are hosted

on a vulnerable IOS platform).

Details

To provide reliable delivery in the Internet, the Transmission Control

Protocol (TCP) makes use of a sequence number in each packet to provide

orderly reassembly of data after arrival, and to notify the sending host of

the successful arrival of the data in each packet.

TCP sequence numbers are 32-bit integers in the circular range of 0 to

4,294,967,295. The host devices at both ends of a TCP connection exchange

an Initial Sequence Number (ISN) selected at random from that range as part

of the setup of a new TCP connection. After the session is established and

data transfer begins, the sequence number is regularly augmented by the

number of octets transferred, and transmitted to the other host. To prevent

the receipt and reassembly of duplicate or late packets in a TCP stream,

each host maintains a "window", a range of values close to the expected

sequence number, in which the sequence number in an arriving packet must

fall if it is to be accepted. Assuming a packet arrives with the correct

source and destination IP addresses, source and destination port numbers,

and a sequence number within the allowable window, the receiving host will

accept the packet as genuine.

This method provides reasonably good protection against accidental receipt

of unintended data. However, to guard against malicious use, it should not

be possible for an attacker to infer a particular number in the sequence.

If the initial sequence number is not chosen randomly or if it is

incremented in a non-random manner between the initialization of subsequent

TCP sessions, then it is possible, with varying degrees of success, to

forge one half of a TCP connection with another host in order to gain

access to that host, or hijack an existing connection between two hosts in

order to compromise the contents of the TCP connection. To guard against

such compromises, ISNs should be generated as randomly as possible.

This defect, documented as DDTS CSCds04747, has been corrected by providing

an improved method for generating TCP Initial Sequence Numbers.

Impact

Forged packets can be injected into a network from a location outside its

boundary so that they are trusted as authentic by the receiving host, thus

resulting in a failure of integrity. Such packets could be crafted to gain

access or make some other modification to the receiving system in order to

attain some goal, such as gaining unauthorized interactive access to a

system or compromising stored data.

- From a position within the network where it is possible to receive the

return traffic (but not necessarily in a position that is directly in the

traffic path), a greater range of violations is possible. For example, the

contents of a message could be diverted, modified, and then returned to the

traffic flow again, causing a failure of integrity and a possible failure

of confidentiality.

NOTE: Any compromise using this vulnerability is only possible for TCP

sessions that originate or terminate on the affected Cisco device itself.

It does not apply to TCP traffic that is merely forwarded through the

device.

Software Versions and Fixes

The following table summarizes the IOS software releases that are known to

be affected, and the earliest estimated dates of availability for the

recommended fixed versions. Dates are always tentative and subject to

change.

Each row of the table 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 and the

anticipated date of availability for each are listed in the "Rebuild",

"Interim", and "Maintenance" columns. A device running any release in the

given train that is earlier the release in a specific column (less than the

earliest fixed release) is known to be vulnerable, and it should be

upgraded at least to the indicated release or a later version (greater than

the earliest fixed release label).

When selecting a release, keep in mind the following definitions:

Maintenance

Most heavily tested and highly recommended release of any label

in a given row of the table.

Rebuild

Constructed from the previous maintenance or major release in the

same train, it contains the fix for a specific defect. Although

it receives less testing, it contains only the minimal changes

necessary to effect the repair.

Interim

Built at regular intervals between maintenance releases and

receive less testing. Interims should be selected only if there

is no other suitable release that addresses the vulnerability,

and interim images should be upgraded to the next available

maintenance release as soon as possible. Interim releases are not

available via manufacturing, and usually they are not available

for customer download from CCO without prior arrangement with the

Cisco TAC.

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 TAC for

assistance as shown later in this notice.

More information on IOS release names and abbreviations is available at

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

+===========================================================================+

Train Description of Availability of Fixed Releases*

Image or Platform

+===========================================================================+

11.0-based Releases Rebuild Interim** Maintenance

+===========================================================================+

11.0(22a)

11.0 Major GD release

for all platforms 2001-Mar-08

+===========================================================================+

11.1-based Releases Rebuild Interim** Maintenance

+===========================================================================+

11.1(24a)

11.1 Major release for

all platforms 2001-Mar-08

+----------+-----------------+---------------+-----------+------------------+

ED release for Unavailable

11.1AA access servers: Upgrade recommended to 12.1(7), available

1600, 3200, and

5200 series. 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Platform-specific 11.1(36)CA1

11.1CA support for 7500,

7200, 7000, and

RSP 2001-Mar-02

+----------+-----------------+---------------+-----------+------------------+

ISP train: added

support for FIB, 11.1(36)CC1

11.1CC CEF, and NetFlow

on 7500, 7200, 2001-Mar-02

7000, and RSP

+----------+-----------------+---------------+-----------+------------------+

Added support for 12.0(11)ST2

11.1CT Tag Switching on

7500, 7200, 7000,

and RSP 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

11.1(28a)IA1

11.1IA Distributed

Director only 2001-Feb-26

+===========================================================================+

11.2-based Releases Rebuild Interim** Maintenance

+===========================================================================+

Major release, 11.2(25a) 11.2(25)

11.2 general

deployment 2001-Mar-05 Available

+----------+-----------------+---------------+-----------+------------------+

Platform-specific Unavailable

support for IBM

11.2BC networking, CIP,

and TN3270 on Upgrade recommended to 12.1(7), available

7500, 7000, and 2001-Feb-26

RSP

+----------+-----------------+---------------+-----------+------------------+

Unavailable

11.2F Feature train for

all platforms Upgrade recommended

+----------+-----------------+---------------+-----------+------------------+

Early deployment Unavailable

11.2GS release to Upgrade recommended to 12.0(15)S1,

support 12000 GSR available 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

11.2(25a)P 11.2(25)P

11.2P New platform

support 2001-Mar-05 Available

+----------+-----------------+---------------+-----------+------------------+

Unavailable

11.2SA Catalyst 2900XL Upgrade recommended to 12.1WC, available

switch only

2001-Apr-12

+----------+-----------------+---------------+-----------+------------------+

Unavailable

11.2WA3 LightStream 1010 Upgrade recommended to 12.0(10)W5(20,

ATM switch

available 2001-Feb-28

+----------+-----------------+---------------+-----------+------------------+

Initial release 11.2(25a)P 11.2(25)P

11.2(4)XA for the 1600 and

3600 2001-Mar-05 Available

+----------+-----------------+---------------+-----------+------------------+

Initial release

for the 5300 and 11.2(25a)P 11.2(25)P

11.2(9)XA digital modem

support for the 2001-Mar-05 Available

3600

+===========================================================================+

11.3-based Releases Rebuild Interim** Maintenance

+===========================================================================+

11.3(11b)

11.3 Major release for

all platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

ED for dial

platforms and 11.3(11a)AA

11.3AA access servers:

5800, 5200, 5300, 2001-Mar-05

7200

+----------+-----------------+---------------+-----------+------------------+

Early deployment Unavailable

11.3DA train for ISP Upgrade recommended to 12.1(5)DA1,

DSLAM 6200

platform available 2001-Mar-19

+----------+-----------------+---------------+-----------+------------------+

Early deployment

train for Unavailable

ISP/Telco/PTT

11.3DB xDSL broadband

concentrator Upgrade recommended to 12.1(4)DB1,

platform, (NRP) available 2001-Feb-28

for 6400

+----------+-----------------+---------------+-----------+------------------+

Short-lived ED

11.3HA release for ISR Vulnerable

3300 (SONET/SDH

router)

+----------+-----------------+---------------+-----------+------------------+

MC3810 11.3(1)MA8

11.3MA functionality

only 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Voice over IP, Unavailable

11.3NA media Upgrade recommended to 12.1(7), available

convergence,

various platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early deployment 11.3(11b)T1

11.3T major release,

feature-rich for

early adopters 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Multilayer

Switching and Unavailable

Multiprotocol

over ATM

11.3WA4 functionality for

Catalyst 5000 Upgrade recommended to 12.0(14)W5(20),

RSM, 4500, 4700, available 2001-Feb-28

7200, 7500,

LightStream 1010

+----------+-----------------+---------------+-----------+------------------+

11.3(11b)T1

11.3(2)XA Introduction of

ubr7246 and 2600 2001-Mar-05

+===========================================================================+

12.0-based Releases Rebuild Interim** Maintenance

+===========================================================================+

General 12.0(15)

12.0 deployment

release for all

platforms Available

+----------+-----------------+---------------+-----------+------------------+

Unavailable

12.0DA xDSL support: Upgrade recommended to 12.1(5)DA1,

6100, 6200

available 2001-Mar-19

+----------+-----------------+---------------+-----------+------------------+

General Unavailable

12.0DB deployment Upgrade recommended to 12.1(4)DB1,

release for all

platforms available 2001-Feb-28

+----------+-----------------+---------------+-----------+------------------+

General Unavailable

12.0DC deployment Upgrade recommended to 12.1(4)DC2,

release for all

platforms available 2001-Feb-28

+----------+-----------------+---------------+-----------+------------------+

12.0(14)S1 12.0(14.6)S

12.0S Core/ISP support:

GSR, RSP, c7200 Available Available

+----------+-----------------+---------------+-----------+------------------+

12.0(15)SC1

12.0SC Cable/broadband

ISP: ubr7200 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

12.0(14)SL1

12.0SL 10000 ESR: c10k

2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

General 12.0(11)ST2

12.0ST deployment

release for all

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

12.0(5c)E8

12.0SX Early Deployment

(ED) 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Unavailable

Deployment(ED):

12.0T VPN, Distributed

Director, various Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Catalyst

switches:

cat8510c, 12.0(14)W5(20)

cat8540c, c6msm,

ls1010, cat8510m,

12.0W5 cat8540m, c5atm,

c5atm, c3620,

c3640, c4500,

c5rsfc, c5rsm, 2001-Feb-28

c7200, rsp,

cat2948g, cat4232

+----------+-----------------+---------------+-----------+------------------+

General 12.0(13)WT6(1)

12.0WT deployment

release for all

platforms 2001-Feb-20

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XA (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Short-lived early Unavailable

12.0XB deployment Upgrade recommended to 12.1(7), available

release 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XC (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XD (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XE (ED): limited Upgrade recommended to 12.1(5)E8,

platforms available 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XF (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XG (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.0(4)XH5

12.0XH (ED): limited

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XI (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XJ (ED): limited Upgrade recommended to 12.1(7), available

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.0(7)XK4

12.0XK (ED): limited

platforms 2001-Mar-19

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.0(4)XH5

12.0XL (ED): limited 12.1(7)

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Short-lived early 12.0(5)XM1

12.0XM deployment

release 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment

12.0XN (ED): limited

platforms

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XP (ED): limited Upgrade recommended to 12.1WC, available

platforms 2001-Apr-12

+----------+-----------------+---------------+-----------+------------------+

Short-lived early Unavailable

12.0XQ deployment Upgrade recommended to 12.1(7), available

release 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

Short-lived early Unavailable

12.0XR deployment Upgrade recommended to 12.1(5)T5,

release available 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Short-lived early Unavailable

12.0XS deployment Upgrade recommended to 12.1(5)E8,

release available 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment Unavailable

12.0XU (ED): limited Upgrade recommended to 12.1WC, available

platforms 2001-Apr-12

+----------+-----------------+---------------+-----------+------------------+

Short-lived early Unavailable

12.0XV deployment Upgrade recommended to 12.1(5)T5,

release available 2001-Mar-05

+===========================================================================+

12.1-based and Later

Releases Rebuild Interim** Maintenance

+===========================================================================+

General 12.1(7)

12.1 deployment

release for all

platforms Available

+----------+-----------------+---------------+-----------+------------------+

12.1(7)AA

12.1AA Dial support

2001-Mar-12

+----------+-----------------+---------------+-----------+------------------+

12.1(5)DA1 12.1(6)DA

12.1DA xDSL support:

6100, 6200 2001-Feb-28 Available

+----------+-----------------+---------------+-----------+------------------+

12.1(4)CX

12.1CX Core/ISP support:

GSR, RSP, c7200 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

General 12.1(4)DB1

12.1DB deployment

release for all

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

General 12.1(4)DC2

12.1DC deployment

release for all

platforms 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

12.1(5c)E8 12.1(5.6)E

12.1E Core/ISP support:

GSR, RSP, c7200 2001-Mar-5

+----------+-----------------+---------------+-----------+------------------+

12.1(5)EC1 12.1(4.5)EC

12.1EC Core/ISP support:

GSR, RSP, c7200 2001-Feb-26

+----------+-----------------+---------------+-----------+------------------+

12.1(5c)EX

12.1EX Core/ISP support:

GSR, RSP, c7200 2001-Mar-5

+----------+-----------------+---------------+-----------+------------------+

Early

Deployment(ED): 12.1(5)T5

12.1T VPN, Distributed

Director, various 2001-Mar-05

platforms

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.1(5)T5

12.1XA (ED): limited

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.1(5)T5

12.1XB (ED): limited

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.1(5)T5

12.1XC (ED): limited

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment 12.1(5)T5

12.1XD (ED): limited

platforms 2001-Mar-05

+----------+-----------------+---------------+-----------+------------------+

Early Deployment