Summary

• A vulnerability in auxiliary asynchronous port (AUX) functions of Cisco IOS XE Software could allow an authenticated, local attacker to cause an affected device to reload or stop responding.

  This vulnerability is due to the incorrect handling of specific ingress traffic when flow control hardware is enabled on the AUX port. An attacker could exploit this vulnerability by reverse telnetting to the AUX port and sending specific data after connecting. A successful exploit could allow the attacker to cause the device to reset or stop responding, resulting in a denial of service (DoS) condition.

  Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-aux-333WBz8f

  This advisory is part of the March 2024 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: March 2024 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected Cisco platforms if they were running a vulnerable release of Cisco IOS XE Software and the following conditions were true:

  • The device has an AUX port.
  • The AUX port is configured with flowcontrol hardware.
  • There is no hardware attached to the AUX port or the attached modem has signaled for flow control to be off and does not turn it on again.

  For information about which Cisco software releases were vulnerable at the time of publication, see the Fixed Software section of this advisory.

  Determine the AUX Port Configuration

  To determine whether the device has an AUX port and whether flowcontrol hardware is enabled, log in to the system and use the show running-config | section aux 0 CLI command to check for the presence of the flowcontrol hardware and transport input all|telnet commands. If these are present, as shown in the following example, the device is considered vulnerable if either no hardware is attached or if the attached modem has signaled for flow control to be off and does not turn it back on:

  Router#show running-config | section line aux 0
  line aux 0
   transport input all
   transport output none
   flowcontrol hardware
  Router#

  If the output does not contain flowcontrol hardware or is blank, as shown in the following example, the device is not affected by this vulnerability:

  Switch#show running-config | section line aux 0
  Switch#

  
  

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

Workarounds

• There is a workaround that addresses this vulnerability.

  If the AUX port is not being used, it should be disabled, as shown in the following example:

  Router# show running-config | section line aux 0
  line aux 0
   transport input none
   transport output none
   no exec
  Router#

  While this workaround has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

Fixed Software

• 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.

  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. This tool identifies any Cisco security advisories that impact a specific software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).

  To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to determine whether a release is affected by any Cisco Security Advisory. To use the form, follow these steps:

  1. Choose which advisories the tool will search-only this advisory, only advisories with a Critical or High Security Impact Rating (SIR), or all advisories.
  2. Enter a release number-for example, 15.9(3)M2 or 17.3.3.
  3. Click Check.

  
  

  Only this advisory All Critical and High advisories All advisories

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

• This vulnerability was found during internal security testing.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-aux-333WBz8f

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2024-MAR-27

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