Cisco Security Advisory: TCP Vulnerabilities in Multiple Non-IOS-Based Cisco Products

Type securityvulns
Reporter Securityvulns
Modified 2004-04-21T00:00:00



Cisco Security Advisory: TCP Vulnerabilities in Multiple Non-IOS Cisco Products

Revision 1.0

For Public Release 2004 April 20 21:00 UTC (GMT)


A vulnerability in the Transmission Control Protocol (TCP) specification (RFC793) has been discovered by an external researcher. The successful exploitation enables an adversary to reset any established TCP connection in a much shorter time than was previously discussed publicly. Depending on the application, the connection may get automatically re-established. In other cases, a user will have to repeat the action (for example, open a new Telnet or SSH session). Depending upon the attacked protocol, a successful attack may have additional consequences beyond terminated connection which must be considered. This attack vector is only applicable to the sessions which are terminating on a device (such as a router, switch, or computer), and not to the sessions that are only passing through the device (for example, transit traffic that is being routed by a router). In addition, the attack vector does not directly compromise data integrity or confidentiality.

All Cisco products which contain a TCP stack are susceptible to this vulnerability.

This advisory is available at, and it describes this vulnerability as it applies to Cisco products that do not run Cisco IOS® software.

A companion advisory that describes this vulnerability for products that run Cisco IOS software is available at

Affected Products

Products which contain a TCP stack are susceptible to this vulnerability. All Cisco products and models are affected. The severity of the exposure depends upon the protocols and applications that utilize TCP.

The nonexhaustive list of vulnerable non-IOS based Cisco products is as follows:

  • Access Registrar

  • BPX, IGX, MGX WAN switches, and the Service Expansion Shelf

  • BR340, WGB340, AP340, AP350, BR350 Cisco/Aironet wireless products

  • Cache Engine 505 and 570

  • CallManager

  • Catalyst 1200, 1900, 28xx, 29xx, 3000, 3900, 4000, 5000, 6000

  • Cisco 8110 Broadband Network Termination Unit

  • Cisco Element Management Framework

  • Cisco Info Center

  • Cisco Intelligent Contact Management

  • Cisco MDS 9000

  • Cisco ONS 15190/15194 IP Transport Concentrator

  • Cisco ONS 15327 Metro Edge Optical Transport Platform

  • Cisco ONS 15454 Optical Transport Platform

  • Cisco ONS 15531/15532 T31 OMDS Metro WDM System

  • Cisco ONS 15800/15801/15808 Dense Wave Division Multiplexing Platform

  • Cisco ONS 15830 T30 Optical Amplification System

  • Cisco ONS 15831/15832 T31 DWDM System

  • Cisco ONS 15863 T31 Submarine WDM System

  • Content Router 4430 and Content Delivery Manager 4630 and 4650

  • Cisco Secure Intrusion Detection System (NetRanger) appliance and IDS Module

  • Cisco Secure PIX firewall

  • Cisco ws-x6608 and ws-x6624 IP Telephony Modules

  • CiscoWorks Windows

  • Content Engine 507, 560, 590, and 7320

  • CSS11000 (Arrowpoint) Content Services Switch

  • Hosting Solution Engine

  • User Registration Tool VLAN Policy Server

  • Cisco FastHub 300 and 400

  • CR-4430-B

  • Device Fault Manager

  • Internet CDN Content Engine 590 and 7320, Content Distribution Manager 4670, and Content Router 4450

  • IP Phone (all models including ATA and VG248)

  • IP/TV

  • LightStream 1010

  • LightStream 100 ATM Switches

  • LocalDirector

  • ME1100 series

  • MicroHub 1500,MicroSwitch 1538/1548

  • Voice Manager

  • RTM

  • SN5400 series storage routers

  • Switch Probe

  • Unity Server

  • VG248 Analog Phone Gateway

  • Traffic Director

  • WAN Manager

Products Confirmed Not Vulnerable

The following products are not vulnerable:

  • Cisco VPN 3000 Series Concentrators

  • Cisco Firewall Services Module for Cisco Catalyst 6500 Series and Cisco 7600 Series (FWSM)


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 packets are reassembled by the receiving TCP implementation only if their sequence numbers fall within a range of the acknowledgement number (called a "window"). The acknowledgement number is not used in a packet with the reset (RST) flag set because a reset does not expect a packet in return. The full specification of the TCP protocol can be found at .

According to the RFC793 specification, it is possible to reset an established TCP connection by sending a packet with the RST or synchronize (SYN) flag set. In order for this to occur, the 4-tuple must be known or guessed (source and destination IP address and ports) together with a sequence number. However, the sequence number does not have to be an exact match; it is sufficient to fall within the advertised window. This significantly decreases the effort required by an adversary: the larger the window, the easier it is to reset the connection. While source and destination IP addresses may be relatively easy to determine, the source TCP port must be guessed. The destination TCP port is usually known for all standard services (for example, 23 for Telnet, 80 for HTTP). Many operating systems (OSs) use predictable ephemeral ports for known services with a predictable increment (the next port which will be used for a subsequent connection). These values, while constant for a particular OS and protocol, do vary from one OS release to another.

Here is an example of a normal termination of a TCP session:

                Host(1)                       Host(2)
                  |                             |
                  |                             |
                  |  ACK ack=1001, window=5000  |
                  |                             |

              Host(1) is

         closing the session

                  |        RST seq=1001         |
                  |                             |

                                            Host(2) is

                                        closing the session

In addition, the following scenario is also permitted:

                Host(1)                       Host(2)
                  |                             |
                  |                             |
                  |  ACK ack=1001, window=5000  |
                  |                             |

              Host(1) is

         closing the session

                  |        RST seq=4321         |
                  |                             |

                                            Host(2) is

                                        closing the session

Note how the RST packet was able to terminate the session although the sequence number was not the next expected one (which is 1001). It was sufficient for the sequence number to fall within the advertised "window". In this example, Host(2) was accepting sequence numbers from 1001 to 6001 and 4321 is clearly within the acceptable range.

As a general rule, all protocols where a TCP connection stays established for longer than one minute should be considered exposed.


The impact is different for each specific protocol. While, in the majority of cases, a TCP connection will be automatically re-established, in some specific protocols a second order of consequences may have a larger impact than tearing down the connection itself. The Cisco PSIRT has analyzed multiple TCP-based protocols, as they are used within our offering, and we believe that this vulnerability does not have a significant impact on them. We will present our analysis for a few protocols which have the potential for higher impact due to the long lived connections.

Voice signaling H.225, H.245 (part of H.323 suite)

H.225 and H.245 protocols are used in voice signaling. Their purpose is to negotiate parameters for content transfer (voice or video). The established sessions persist for the duration of a call. Any call in progress is terminated when the signaling session is broken. A new signaling session will be established immediately for the new call, but terminated calls cannot be re-established.

Each call from an IP telephone or softphone will result in the creation of a single signaling session. Terminating that signaling session affects only a single user. It is possible that a single signaling session is responsible for multiple calls, but that setup is used deeper within the Service Provider's network. Determining all necessary parameters for mounting an attack is deemed a non-trivial task if the network is designed according to the current best practices.

Network Storage (iSCSI, FCIP)

Network Storage products use two TCP-based protocols: SCSI over IP (iSCSI) and Fiber Channel over IP (FCIP).

  • SCSI over IP (iSCSI)

    iSCSI is used in a client/server environment. The client is your computer and it is only the client that initiates a connection. This connection is not shared with any other users. A separate session is established for each virtual device used. Terminating the session will not have any adverse consequences if people are using current drivers from Microsoft for Windows and from Cisco for Linux. These drivers will re-establish the session and continue transfer from the point where it was disconnected. Drivers from other vendors may behave differently.

    The user may notice that access to a virtual device is slightly slower than usual.

  • Fiber Channel over IP (FCIP)

    FCIP is a peer-to-peer protocol. It is used for mirroring data between switches. Each peer can initiate the session. Switches can, and should be in practice, configured in a mesh. Bringing one link down will cause traffic to be re-routed over other link(s). If an adversary can manage to terminate the session multiple times in a row, the user's application may terminate with a "Device unreachable" or similar error message. This does not have any influence on the switch itself and the user can retry the operation.

    The user may notice that access to a virtual device is slightly slower than usual. An occasional error message is possible.

Transport Layer Security/Secure Socket Layer (TLS/SSL)

Since this vulnerability operates on a TCP layer, encryption does not provide any protection. SSL/TLS connections can be used to encapsulate various kinds of traffic and these sessions can be long lived. A successful exploitation does not impact confidentiality of the data. An encrypted session can be attacked either on the originating or terminating host or on the firewalls in front of them (if they exist).

Software Versions and Fixes

+----------------+------------+--------------+ | Product | Defect ID | Intended | | | | First Fixed | | | | Release | +----------------+------------+--------------+ | LAN Switching | +----------------+------------+--------------+ | Catalyst 1200, | CSCed32349 | No software | | 1900, 28xx, | ( | availability | | 29xx, 3000, | registered | date has | | 3900, 4000, | customers | been | | 5000, 6000 | only) | determined | | | | yet. | +----------------+------------+--------------+ | Catalyst 1900 | | 9.00.07 | | and 2820 | | Available on | | | | 2004-Apr-27 | +----------------+------------+--------------+ | Network Storage | +----------------+------------+--------------+ | Cisco MDS 9000 | CSCed45453 | 1.3(3.8), | | Family | ( | 2.0(0.51) | | | registered | | | | customers | | | | only) | | +----------------+------------+--------------+ | Voice Products | +----------------+------------+--------------+ | WS-6624 analog | CSCee22691 | No software | | station | ( | availability | | gateway module | registered | date has | | for the | customers | been | | Catalyst 6500 | only) | determined | | | | yet. | +----------------+------------+--------------+ | Wireless Products | +----------------+------------+--------------+ | Cisco Aironet | CSCee22526 | No software | | Access Point | ( | availability | | 340, 350, 1200 | registered | date has | | Series (only | customers | been | | VxWorks-based) | only) | determined | | | | yet. | | | | Customers | | | | are | | | | encouraged | | | | to migrate | | | | to IOS. | +----------------+------------+--------------+ | Security Products | +----------------+------------+--------------+ | Cisco PIX | CSCed91445 |, | | Firewall | ( |, | | | registered | and | | | customers | | | | only) | availability | | | | estimate: | | | | 2004-Apr-21 | +----------------+------------+--------------+ | Optical Products | +----------------+------------+--------------+ | Cisco ONS | CSCed73026 | 4.62, 4.14, | | 15327, 15454, | ( | 2.25, | | 15454SDH and | registered | Available | | 15600 Optical | customers | 2004-Apr-27 | | Transport | only) | | | Platform | | | +----------------+------------+--------------+

Obtaining Fixed 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 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 assistance with the upgrade, which should be free of charge.

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:

See for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

Please 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.

Please do not contact either "" or "" for software upgrades.


The effectiveness of any workaround 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 is the most appropriate for use in the intended network before it is deployed.

There are no workarounds available to mitigate the effects of this vulnerability.

It is possible to mitigate the exposure on this vulnerability by applying anti-spoofing measures on the edge of the network.

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)#ip verify unicast reverse-path

Please consult products_configuration_guide_chapter09186a00800ca7d4.html and ftp:// for further descriptions of how uRPF works and how to configure it in various scenarios. This is especially important if you are using asymmetric routing.

Access control lists (ACLs) should also be deployed as close to the edge as possible. Unlike uRPF, you must specify the exact IP range that is permitted. Specifying which addresses should be blocked is not the optimal solution because it tends to be harder to maintain.

Caution: In order for anti-spoofing measures to be effective, they must be deployed at least one hop away from the devices which are being protected. Ideally, they will be deployed at the network edge.

Exploitation and Public Announcements

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

The exploitation of the vulnerability with packets having RST flag set (reset packets) was discovered by Paul (Tony) Watson of The extension of the attack vector to packets with SYN flag set and data injection was discovered by the vendors cooperating on the resolution of this issue.

Status of This Notice: INTERIM

This is a INTERIM advisory. Although Cisco cannot guarantee the accuracy of all statements in this advisory, all of the facts have been checked to the best of our ability. Cisco does not anticipate issuing updated versions of this advisory unless there is some material change in the facts. Should there be a significant change in the facts, Cisco may update this advisory.

A stand-alone copy or Paraphrase of the text of this Security Advisory that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.


This advisory will be 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:


  • (includes CERT/CC)







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 | 2004-Apr-20 | Initial public | | 1.0 | | 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 sec_incident_response.shtml. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at

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