Cisco Security Advisory: Cisco IOS Software TCP Initial Sequence Number Randomization Improvements

Document ID: 13631

Advisory ID: cisco-sa-20010301-ios-tcp-isn-random

http://www.cisco.com/warp/public/707/cisco-sa-20010301-ios-tcp-isn-random.shtml

Revision 1.3

Last Updated 2004 January 07 0830 UTC (GMT)

For Public Release 2001 March 01 0200 UTC (GMT)

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Contents

Summary
Affected Products
Details
Impact
Software Versions and Fixes
Workarounds
Obtaining Fixed Software
Exploitation and Public Announcements
Status of This Notice: INTERIM
Distribution
Revision History
Cisco Security Procedures

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/cisco-sa-20010301-ios-tcp-isn-random.shtml.

Affected Products

This section provides details on affected products.

Vulnerable 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.
  

Products Confirmed Not Vulnerable

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).
  

No other Cisco products are currently known to be affected by these vulnerabilities.

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.

Workarounds

There is no specific configurable workaround to directly address the possibility of predicting a TCP Initial Sequence Number. To prevent malicious use of this vulnerability from inside the network, ensure that transport that makes interception and modification detectable, if not altogether preventable, is in use as appropriate. Examples include using IPSEC or SSH to the Cisco device for interactive session, MD5 authentication to protect BGP sessions, strong authentication for access control, and so on.

Malicious use of this vulnerability from a position outside the administrative boundaries of the network can be mitigated, if not prevented entirely, by using access control lists to prevent the injection of packets with forged source or destination IP addresses.

Obtaining Fixed Software

Cisco has made free software available to address this vulnerability for affected customers. 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 general case of this vulnerability in TCP is well-known to the information system security community. Details specific to TCP connections to or from Cisco products do not appear to be widely known and the topic does not appear to have been widely discussed.

Cisco is not aware of instances in which this vulnerability has been used maliciously. However, there are numerous off-the-shelf programs and scripts available which can demonstrate the vulnerability and which could be modified to exploit it with malicious intent. Various security scanning programs have been known to provide positive test results for this vulnerability on Cisco devices.

This vulnerability was discovered internally. Two customers reported the vulnerability while a fix was still in progress.

Status of This Notice: INTERIM

Distribution

This notice will be posted at http://www.cisco.com/warp/public/707/cisco-sa-20010301-ios-tcp-isn-random.shtml.

In addition to Worldwide Web posting, a text version of this notice will be clear-signed with the Cisco PSIRT PGP key and will be posted to the following e-mail and Usenet news recipients:

• cust-security-announce@cisco.com
  
• bugtraq@securityfocus.com
  
• first-teams@first.org (includes CERT/CC)
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• comp.dcom.sys.cisco
  
• Various internal Cisco mailing lists
  

Future updates of this notice, if any, will be placed on Cisco's Worldwide Web server, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the URL given above for any updates.

Revision History

Cisco Security Procedures

The page at http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html contains instructions for reporting security vulnerabilities in Cisco products, obtaining assistance with customer security incidents, registering to receive security information from Cisco, and making press inquiries regarding Cisco Security Advisories. This document is Cisco's complete public statement regarding this product security vulnerability.

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