Summary

• A vulnerability in the Voice Telephony Service Provider (VTSP) service of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to bypass configured destination patterns and dial arbitrary numbers.

  This vulnerability is due to insufficient validation of dial strings at Foreign Exchange Office (FXO) interfaces. An attacker could exploit this vulnerability by sending a malformed dial string to an affected device via either the ISDN protocol or SIP. A successful exploit could allow the attacker to conduct toll fraud, resulting in unexpected financial impact to affected customers.

  Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-fxo-pattern-bypass-jUXgygYv

  This advisory is part of the September 2021 release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: September 2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco devices if they are running a vulnerable release of Cisco IOS or IOS XE Software and both of the following are true:

  • There is a destination pattern with at least one wildcard configured for an FXO interface.
  • The device is enabled to support incoming calls via ISDN or SIP.

  For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.

  Determine the FXO Interface Configuration

  To determine the FXO interface configuration, first use the show inventory | include FXO command in the device CLI to determine whether at least one FXO interface card is installed in the device. It this command returns empty output, no FXO interface card is installed and the device is not vulnerable.

  The following example shows the output of the show inventory | include FXO command on a device that has an FXO interface card installed in subslot 0/3:

  ios#show inventory | include FXO
  NAME: "NIM subslot 0/3", DESCR: "NIM-4FXO Voice Analog Module"
  PID: NIM-4FXO          , VID: V02  , SN: XXXXXXXXXXX

  If the preceding command does return output, note the (sub-)slot number(s) from the command output.

  Then use the show running-config | section dial-peer voice command and look for an output section where all the following are true:

  • The output starts with dial-peer voice tag pots.
  • The entry has a destination-pattern pattern where pattern contains at least one wildcard character.
  • The same entry indicates port x/y/z where x/y/z is a port on an FXO interface card that correlates to the output from the earlier show inventory | include FXO command.

  The following example illustrates show running-config | section dial-peer voice output where the entry has a destination pattern with the dot (.) wildcard character and the dial peer with tag 35 is linked to the FXO interface card in subslot 0/3.

  ios#show running-config | section dial-peer voice
  .
  .
  .
  dial-peer voice 35 pots
   destination-pattern 123.
   port 0/3/5
   forward-digits all
  .
  .
  .

  Determine the ISDN Interface Configuration

  To determine whether a device has an ISDN interface configured, use the show running config | include isdn switch-type command in the CLI. If a value is returned, an ISDN interface is enabled. The following example shows the output of the command on a device that has an ISDN interface:

  ios#show running-config | include isdn switch-type
   isdn switch-type primary-net5

  Note: The exact switch type does not affect this vulnerability.

  Determine the SIP Configuration

  The default dial-peer voice tag voip, which is not visible in the output of show running-config or show running-config all, supports incoming SIP calls. Therefore, no specific configuration is required.

  Products Confirmed Not Vulnerable

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • IOS XR Software
  • Meraki products
  • NX-OS Software

Workarounds

• There are no workarounds that address this vulnerability.

Fixed Software

• Cisco has released free software updates that address the vulnerability described in this advisory. Customers may only install and expect support for software versions and feature sets for which they have purchased a license. By installing, downloading, accessing, or otherwise using such software upgrades, customers agree to follow the terms of the Cisco software license:
  https://www.cisco.com/c/en/us/products/end-user-license-agreement.html

  Additionally, customers may only download software for which they have a valid license, procured from Cisco directly, or through a Cisco authorized reseller or partner. In most cases this will be a maintenance upgrade to software that was previously purchased. Free security software updates do not entitle customers to a new software license, additional software feature sets, or major revision upgrades.

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

  Customers Without Service Contracts

  Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html

  Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade.

  Cisco IOS and IOS XE Software

  To help customers determine their exposure to vulnerabilities in Cisco IOS and IOS XE Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific software release and the earliest release that fixes the vulnerabilities described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities described in all the advisories identified (“Combined First Fixed”).

  Customers can use the Cisco Software Checker to search advisories in the following ways:

  • Choose the software and one or more releases
  • Upload a .txt file that includes a list of specific releases
  • Enter the output of the show version command

  After initiating a search, customers can customize the search to include all Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication.

  Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE Software release—for example, 15.1(4)M2 or 3.13.8S:

  By default, the Cisco Software Checker includes results only for vulnerabilities that have a Critical or High Security Impact Rating (SIR). To include results for Medium SIR vulnerabilities, customers can use the Cisco Software Checker on Cisco.com and check the Medium check box in the drop-down list under Impact Rating when customizing a search.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

Source

• Cisco would like to thank Austin Martinetti for reporting this vulnerability.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-fxo-pattern-bypass-jUXgygYv

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2021-SEP-22

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