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Cisco Security Advisory

Cisco IOS SSL VPN Vulnerability

Advisory ID: cisco-sa-20100922-sslvpn

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100922-sslvpn

Revision 1.1

Last Updated  2012 September 21 19:17  UTC (GMT)

For Public Release 2010 September 22 16:00  UTC (GMT)

Related Resources:

View related Alert

Summary

Cisco IOS® Software contains a vulnerability when the Cisco IOS SSL VPN feature is configured with an HTTP redirect. Exploitation could allow a remote, unauthenticated user to cause a memory leak on the affected devices, that could result in a memory exhaustion condition that may cause device reloads, the inability to service new TCP connections, and other denial of service (DoS) conditions.

Cisco has released free software updates that address this vulnerability. There is a workaround to mitigate this vulnerability.

This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100922-sslvpn.

Note: The September 22, 2010, Cisco IOS Software Security Advisory bundled publication includes six Cisco Security Advisories. Five of the advisories address vulnerabilities in Cisco IOS Software, and one advisory addresses vulnerabilities in Cisco Unified Communications Manager. Each advisory lists the releases that correct the vulnerability or vulnerabilities detailed in the advisory. The table at the following URL lists releases that correct all Cisco IOS Software vulnerabilities that have been published on September 22, 2010, or earlier:

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100922-bundle

Individual publication links are in "Cisco Event Response: Semiannual Cisco IOS Software Security Advisory Bundled Publication" at the following link:

http://www.cisco.com/web/about/security/intelligence/Cisco_ERP_sep10.html

Affected Products

Vulnerable Products

Devices running affected versions of Cisco IOS Software are vulnerable if configured with SSL VPN and HTTP port redirection.

The following methods may be used to confirm if the device is configured for Cisco IOS SSL VPNs and is vulnerable:

If the output from show running-config | include webvpn contains "webvpn gateway <word>" then the device is supporting the Cisco IOS SSL VPN feature. A device is vulnerable if it has the inservice command in at least one of the "webvpn gateway" sections and is configured for HTTP port redirection. The following example shows a vulnerable device configured with Cisco IOS SSL VPN:

Router#show running | section webvpn
webvpn gateway Gateway
 ip address 10.1.1.1 port 443 
 http-redirect port 80 
 ssl trustpoint Gateway-TP
 inservice
 !
Router#

A device that supports the Cisco IOS SSL VPN is not vulnerable if "webvpn gateway" is not configured.

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 12.4(20)T with an installed image name of C2800NM-ADVSECURITYK9-M:

Router#show version
Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version 12.4(20)T, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by Cisco Systems, Inc.
Compiled Thu 10-Jul-08 22:00 by prod_rel_team


! --- output truncated

Additional information about Cisco IOS Software release naming conventions is available in "White Paper: Cisco IOS Reference Guide" at the following link: http://www.cisco.com/warp/public/620/1.html

Products Confirmed Not Vulnerable

The following products are not affected by this vulnerability:

  • Cisco ASA 5500 Series Adaptive Security Appliances
  • Cisco IOS XR Software
  • Cisco IOS XE Software

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

Details

The Cisco IOS SSL VPN feature provides remote access to enterprise sites to users anywhere on the Internet. The SSL VPN provides users with secure access to specific enterprise applications, such as e-mail and web browsing, without requiring them to have VPN client software installed on their end-user devices.

Further information about Cisco IOS SSL VPN is available in the "Cisco IOS Software Release 12.4T SSL VPN feature guide" at the following link: http://www.cisco.com/en/US/docs/ios/12_4t/12_4t11/htwebvpn.html

A device configured for SSL VPN with HTTP port redirection may leak transmission control blocks (TCBs) when processing an abnormally disconnected SSL session. Continued exploitation may cause the device to deplete memory resources, which could result in device reloads, the inability to service new TCP connections, and other DoS conditions. Authentication is not required to exploit this vulnerability.

A complete TCP 3-way handshake is required to exploit this vulnerability. The memory leak can be detected by running the command show tcp brief as shown in the following example:

Router#show tcp brief
TCB       Local Address      Foreign Address     (state)
468BBDC0  192.168.0.22.80   192.168.0.33.19794   CLOSEWAIT
482D4730  192.168.0.22.80   192.168.0.33.22092   CLOSEWAIT
482779A4  192.168.0.22.80   192.168.0.33.16978   CLOSEWAIT
4693DEBC  192.168.0.22.80   192.168.0.33.21580   CLOSEWAIT
482D3418  192.168.0.22.80   192.168.0.33.17244   CLOSEWAIT
482B8ACC  192.168.0.22.80   192.168.0.33.16564   CLOSEWAIT
46954EB0  192.168.0.22.80   192.168.0.33.19532   CLOSEWAIT
468BA9B8  192.168.0.22.80   192.168.0.33.15781   CLOSEWAIT
482908C4  192.168.0.22.80   192.168.0.33.19275   CLOSEWAIT
4829D66C  192.168.0.22.80   192.168.0.33.19314   CLOSEWAIT
468A2D94  192.168.0.22.80   192.168.0.33.14736   CLOSEWAIT
4688F590  192.168.0.22.80   192.168.0.33.18786   CLOSEWAIT
4693CBA4  192.168.0.22.80   192.168.0.33.12176   CLOSEWAIT
4829ABC4  192.168.0.22.80   192.168.0.33.39629   CLOSEWAIT
4691206C  192.168.0.22.80   192.168.0.33.17818   CLOSEWAIT
46868224  192.168.0.22.80   192.168.0.33.16774   CLOSEWAIT
4832BFAC  192.168.0.22.80   192.168.0.33.39883   CLOSEWAIT
482D10CC  192.168.0.22.80   192.168.0.33.13677   CLOSEWAIT
4829B120  192.168.0.22.80   192.168.0.33.20870   CLOSEWAIT
482862FC  192.168.0.22.80   192.168.0.33.17035   CLOSEWAIT
482EC13C  192.168.0.22.80   192.168.0.33.16053   CLOSEWAIT
482901D8  192.168.0.22.80   192.168.0.33.16200   CLOSEWAIT

In the output above, the Transmission Control Blocks (TCBs) in the state CLOSEWAIT will not transition and represent a memory leak. Note that only TCP connections with a local TCP port of 80 (the well-known port for HTTP), as evidenced in the above example by a Local Address of 192.168.0.22.80, are relevant.

This vulnerability is documented in Cisco bug ID CSCtg21685 ( registered customers only) and Common Vulnerabilities and Exposures (CVE) identifier CVE-2010-2836 has been assigned to this vulnerability.

Vulnerability Scoring Details

Cisco has provided scores 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 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 at

http://intellishield.cisco.com/security/alertmanager/cvss .

CSCtg21685 - SSLVPN : TCP remains stuck in closewait state

Calculate the environmental score of CSCtg21685

CVSS Base Score - 7.8

Access Vector

Access Complexity

Authentication

Confidentiality Impact

Integrity Impact

Availability Impact

Network

Low

None

None

None

Complete

CVSS Temporal Score - 6.4

Exploitability

Remediation Level

Report Confidence

Functional

Official-Fix

Confirmed

Impact

Successful exploitation of the vulnerability may result in a lack of available memory resources on the affected device, which could affect new connections to the device such as SSH and Telnet connections. Depletion of memory resources may also result in failing of routing protocols and other services.

Software Versions 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 following Cisco IOS Software table corresponds to a Cisco IOS Software train. If a particular train is vulnerable, the earliest releases that contain the fix are listed in the First Fixed Release For This Advisory column. The First Fixed Release for All Advisories in the September 2010 Bundle Publication column lists the earliest possible releases that correct all the published vulnerabilities in the Cisco IOS Software Security Advisory bundled publication. Cisco recommends upgrading to the latest available release, where possible.

Major Release

     Availability of Repaired Releases

Affected 12.0-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

There are no affected 12.0 based releases

Affected 12.1-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

There are no affected 12.1 based releases

Affected 12.2-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

There are no affected 12.2 based releases

Affected 12.3-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

There are no affected 12.3 based releases

Affected 12.4-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

12.4

Not Vulnerable

12.4(25d)

12.4GC

Not Vulnerable

12.4(24)GC2

12.4JA

Not Vulnerable

Not Vulnerable

12.4JDA

Not Vulnerable

Not Vulnerable

12.4JDC

Not Vulnerable

Not Vulnerable

12.4JDD

Not Vulnerable

Not Vulnerable

12.4JHA

Not Vulnerable

Not Vulnerable

12.4JHB

Not Vulnerable

Not Vulnerable

12.4JK

Not Vulnerable

Not Vulnerable

12.4JL

Not Vulnerable

Not Vulnerable

12.4JMA

Not Vulnerable

Not Vulnerable

12.4JMB

Not Vulnerable

Not Vulnerable

12.4JX

Not Vulnerable

Not Vulnerable

12.4JY

Not Vulnerable

Not Vulnerable

12.4MD

Not Vulnerable

12.4(24)MD2

12.4MDA

Not Vulnerable

12.4(22)MDA4

12.4(24)MDA1

12.4MR

Not Vulnerable

Vulnerable; first fixed in 12.4MRA

12.4MRA

Not Vulnerable

12.4(20)MRA1

12.4SW

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4T

Releases Prior to 12.4(15)T13 are not vulnerable. First fixed 12.4(15)T14

Releases Prior to 12.4(20)T5 are not vulnerable. First fixed 12.4(20)T6

Releases Prior to 12.4(24)T2 are not vulnerable. First fixed 12.4(24)T4

12.4(15)T14

12.4(20)T6

12.4(24)T4

12.4XA

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XB

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XC

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XD

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XE

Not Vulnerable

Releases prior to 12.4(6)XE5 are vulnerable, release 12.4(6)XE5 and later are not vulnerable; first fixed in 12.4T

12.4XF

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XG

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XJ

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XK

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XL

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4XM

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XN

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4XP

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4XQ

Not Vulnerable

12.4(15)XQ6; Available on 22-SEP-10

12.4XR

Not Vulnerable

12.4(15)XR9

12.4(22)XR7

12.4XT

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XV

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4XW

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XY

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4XZ

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4YA

Not Vulnerable

Vulnerable; first fixed in 12.4T

12.4YB

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4YD

Not Vulnerable

Vulnerable; Contact your support organization per the instructions in Obtaining Fixed Software section of this advisory

12.4YE

Not Vulnerable

12.4(24)YE1

12.4YG

Not Vulnerable

12.4(24)YG3

Affected 15.0-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

15.0M

15.0(1)M3

15.0(1)M3

15.0S

Cisco 7600 and 10000 Series routers: Not vulnerable

Please see Cisco IOS-XE Software Availability

Cisco 7600 and 10000 Series routers: 15.0(1)S1 (available early October 2010)

Please see Cisco IOS-XE Software Availability

15.0XA

Not Vulnerable

Vulnerable; first fixed in 15.1T

15.0XO

Not Vulnerable

Not Vulnerable

Affected 15.1-Based Releases

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

15.1T

15.1(1)T1

15.1(2)T0a

15.1(2)T1

15.1XB

Vulnerability limited to 15.1(1)XB1.

Vulnerable; first fixed in 15.1T

Cisco IOS XE Software

Cisco IOS XE Release

First Fixed Release for This Advisory

First Fixed Release for All Advisories in the September 2010 Bundle Publication

2.1.x

Not Vulnerable

Not Vulnerable

2.2.x

Not Vulnerable

Not Vulnerable

2.3.x

Not Vulnerable

Not Vulnerable

2.4.x

Not Vulnerable

Not Vulnerable

2.5.x

Not Vulnerable

Vulnerable; migrate to 2.6.2 or later

2.6.x

Not Vulnerable

2.6.2

3.1.xS

Not Vulnerable

Not Vulnerable

For mapping of Cisco IOS XE Software to Cisco IOS Software releases, please refer to the Cisco IOS XE 2 and Cisco IOS XE 3S Release Notes.

Cisco IOS XR System Software

Cisco IOS XR Software is not affected by the vulnerabilities disclosed in the September 22, 2010, Cisco IOS Software Security Advisory bundled publication.


Workarounds

Disabling HTTP redirection for SSL VPN connections can be used as a workaround for this vulnerability. HTTP redirection for SSL VPN connections is disabled by executing the command no http-redirect port in webvpn gateway configuration mode.

In addition, manually clearing the hung TCBs with the command clear tcp tcb * will transition the TCBs into a CLOSED state. After a time they will clear the CLOSED state and the memory will be released.

Note:  Clearing the TCB will clear both legitimate and hung connections, including remote connections to the device such as Telnet and SSH connections.

The Cisco Applied Mitigation Bulletin (AMB) "Identifying and Mitigating Exploitation of the TCP State Manipulation Denial of Service Vulnerabilities in Multiple Cisco Products", available at http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20090908-tcp24, contains two mitigations (EEM scripts and SNMP) that can be used to detect and clear hung TCP connections.

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: http://forums.cisco.com/eforum/servlet/EEM?page=eem&fn=script&scriptId=2041. A sample device 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 CLOSEWAIT

    
!
    !-- Register the script as a Cisco EEM policy
    !


    event manager policy monitor-sockets.tcl

    !

For more details, refer to the sections "EEM Detecting And Clearing Hung TCP Connection" and "Identification: Detecting and Clearing Hung TCP Connection Using SNMP" of this AMB at the following link: http://tools.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20090908-tcp24.

Obtaining Fixed Software

Cisco has released free software updates that address these vulnerabilities. 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/en/US/docs/general/warranty/English/EU1KEN_.html , or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml .

Do not contact 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 agreements 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 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@cisco.com

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 http://www.cisco.com/en/US/support/tsd_cisco_worldwide_contacts.html for additional TAC contact information, including 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 found during the troubleshooting of a customer service request.

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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20100922-sslvpn

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-bulletins@lists.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

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-September-22

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 http://www.cisco.com/web/about/security/psirt/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.


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