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Cisco IOS Software Crafted Encryption Packet Denial of Service Vulnerability

Related Vulnerabilities: CVE-2009-2871  

Cisco IOS® Software contains a vulnerability that could allow an attacker to cause a Cisco IOS device to reload by remotely sending a crafted encryption packet. Cisco has released software updates that address this vulnerability. This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090923-tls. Note: The September 23, 2009, Cisco IOS Security Advisory bundled publication includes eleven Security Advisories. Ten of the advisories address vulnerabilities in Cisco IOS Software, and one advisory addresses a vulnerability in Cisco Unified Communications Manager. Each advisory lists the releases that correct the vulnerability or vulnerabilities detailed in the advisory. Individual publication links are in "Cisco Event Response: Semiannual Cisco IOS Software Advisory Bundled Publication" at the following link: http://www.cisco.com/web/about/security/intelligence/Cisco_ERP_ sep09.html

Source

Summary

  • Cisco IOS® Software contains a vulnerability that could allow an attacker to cause a Cisco IOS device to reload by remotely sending a crafted encryption packet.

    Cisco has released software updates that address this vulnerability.

    This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20090923-tls.

    Note: The September 23, 2009, Cisco IOS Security Advisory bundled publication includes eleven Security Advisories. Ten of the advisories address vulnerabilities in Cisco IOS Software, and one advisory addresses a vulnerability in Cisco Unified Communications Manager. Each advisory lists the releases that correct the vulnerability or vulnerabilities detailed in the advisory.

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

    http://www.cisco.com/web/about/security/intelligence/Cisco_ERP_ sep09.html

Affected Products

  • Vulnerable Products

    Devices running affected versions of Cisco IOS Software are susceptible if configured with any of the following features:

    • Secure Socket Layer (SSL) Virtual Private Network (VPN)
    • Secure Shell (SSH)
    • Internet Key Exchange (IKE) Encrypted Nonces

    Note: Other SSL/HTTPS related features than WebVPN and SSL VPN are not affected by this vulnerability.

    To determine whether SSLVPN is enabled on a device, log in to the device and issue the command-line interface (CLI) command show running-config | include webvpn . If the device returns any output then SSLVPN is configured and the device may be vulnerable. Vulnerable configurations vary depending on whether the device is supporting Cisco IOS WebVPN (introduced in Release 12.3(14)T) or Cisco IOS SSLVPNs (introduced in Release 12.4(6)T). The following methods describe how to confirm if the device is vulnerable:

    If the output from "show running-config | include webvpn" contains "webvpn enable" then the device is configured with the original Cisco IOS WebVPN. The only way to determine whether the device is vulnerable is to examine the output of "show running-config" to confirm that webvpn is enabled via the command "webvpn enable" and that a "ssl trustpoint" has been configured. The following example shows a vulnerable device configured with Cisco IOS WebVPN:

    webvpn enable
!
webvpn
 ssl trustpoint TP-self-signed-29742012

    If the output from "show running-config | include webvpn" contains "webvpn gateway " then the device is supporting the Cisco IOS SSLVPN feature. A device is vulnerable if it has the "inservice" command in at least one of the "webvpn gateway" sections. The following example shows a vulnerable device configured with Cisco IOS SSLVPN:

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

    A device that supports the Cisco IOS SSLVPN is not vulnerable if it has no "webvpn gateways" configured or all the configured "webvpn gateways" contain the "no inservice" webvpn gateway command.

    To determine if SSH is enabled use the show ip ssh command, as shown in the following example:

    Router#show ip ssh 
SSH Enabled - version 1.99
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 1024 bits

    To determine if the IKE encrypted nonces feature is enabled, use the show running-config | include rsa-encr command as follows: 

    Router#show running-config | inc rsa-encr
 authentication rsa-encr

    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 12.3(26) with an installed image name of C2500-IS-L:

    Router#show version
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-IS-L), Version 12.3(26), RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by cisco Systems, Inc.
Compiled Mon 17-Mar-08 14:39 by dchih


!--- output truncated

    The following example identifies a Cisco product that is running Cisco IOS Software Release 12.4(20)T with an installed image name of C1841-ADVENTERPRISEK9-M:

    Router#show version
Cisco IOS Software, 1841 Software (C1841-ADVENTERPRISEK9-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 20:25 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 Cisco ASA 5500 Series Adaptive Security Appliances are not affected by this vulnerability.

    Cisco IOS XR Software is not affected by this vulnerability.

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

Details

  • A Cisco IOS device that is configured for SSLVPN or SSH may reload when it receives a specially crafted TCP packet on TCP port 443 (SSLVPN) or TCP port 22 (SSH). Completion of the three-way handshake to the associated TCP port number of these features is required for the vulnerability to be successfully exploited; however, authentication is not required. A Cisco IOS device that is configured for IKE encrypted nonces may reload when it receives a specially crafted UDP packet on port 500 or 4500 (if configured for NAT Traversal (NAT-T)).

    This vulnerability is documented in Cisco bug ID CSCsq24002 ( registered customers only) and has been assigned the Common Vulnerabilities and Exposures (CVE) identifier CVE-2009-2871.

Workarounds

  • There are no available workarounds other than disabling the affected features and protecting SSH access with the use of VTY access control lists.

    Use the no webvpn enable command to disable SSL VPN use.

    For Cisco IOS the SSH server can be disabled by applying the command crypto key zeroize rsa while in configuration mode. The SSH server is enabled automatically upon generating an RSA key pair. Zeroing the RSA keys is the only way to completely disable the SSH server.

    Access to the SSH server on Cisco IOS Software may also be disabled by removing SSH as a valid transport protocol. This action can be done by reapplying the transport input command with 'ssh' removed from the list of permitted transports on vty lines while in configuration mode. For example: 

    line vty 0 4                                                                    
      transport input telnet                                                        
    end

    If SSH server functionality is desired, access to the server can be restricted to specific source IP addresses or blocked entirely through the use of Access Control Lists (ACLs) on the vty lines as shown in the following URL: http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_9_ea1/configuration/guide/swacl.html#xtocid14

    More information on configuring ACLs can be found on Cisco's public website:

    http://www.cisco.com/en/US/products/sw/secursw/ps1018/products_tech_note09186a00800a5b9a.shtml

    The following is an example of a vty access-list:

    access-list 2 permit 10.1.1.0 0.0.0.255
access-list 2 deny any

line vty 0 4
 access-class 2 in

    In the previous example, only the 10.1.1.0/24 network is allowed to SSH to the Cisco IOS device.

    To disable IKE encrypted nonces use the no authentication rsa-encr command under an ISAKMP policy, as shown in the following example: 

    crypto isakmp policy
 no authentication rsa-encr

Fixed Software

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 internal testing.

Cisco Security Vulnerability Policy

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Related to This Advisory

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Revision History

  • Revision 1.0

    2009-September-23

    Initial public release

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