Summary

• A vulnerability in the DHCP version 4 (DHCPv4) server feature of Cisco IOS XR Software could allow an unauthenticated, remote attacker to trigger a crash of the dhcpd process, resulting in a denial of service (DoS) condition.

  This vulnerability exists because certain DHCPv4 messages are improperly validated when they are processed by an affected device. An attacker could exploit this vulnerability by sending a malformed DHCPv4 message to an affected device. A successful exploit could allow the attacker to cause a NULL pointer dereference, resulting in a crash of the dhcpd process. While the dhcpd process is restarting, which may take up to approximately two minutes, DHCPv4 server services are unavailable on the affected device. This could temporarily prevent network access to clients that join the network during that time period.

  Note: Only the dhcpd process crashes and eventually restarts automatically. The router does not reload.

  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-iosxr-dhcp-dos-pjPVReLU

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

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected the following Cisco products if they were running Cisco IOS XR Software releases 6.7.2 and later, 7.1.2 and later, or 7.2.1 and later but earlier than Release 7.3.2 or earlier than Release 7.4.1 and had the DHCPv4 server feature or the DHCPv4 proxy feature enabled:

  • ASR 9000 Series Aggregation Services Routers
  • IOS XRv 9000 Routers
  • Network Convergence System (NCS) 540 Series Routers
  • NCS 560 Series Routers
  • NCS 5000 Series Routers
  • NCS 5500 Series Routers

  See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Determine the DHCPv4 Configuration

  To determine whether the DHCPv4 server feature or the DHCPv4 proxy feature is enabled on a device, run the show running-config dhcp ipv4 command in privileged EXEC mode on the device CLI and check for a server profile that is either directly or indirectly (through a base profile) bound to at least one interface.

  The following example shows the output of the show running-config dhcp ipv4 command on a device that has the DHCPv4 server profile TEST directly bound to interface GigabitEthernet0/0/0/0:

  dhcp ipv4
   profile TEST server
    .
    .
    .
   !
   interface GigabitEthernet0/0/0/0 server profile TEST

  The following example shows the output of the show running-config dhcp ipv4 command on a device that has the DHCPv4 server profiles DHCP_SERVER and DEFAULT_PROFILE indirectly bound to interface GigabitEthernet0/0/0/0 through base profile DHCP_BASE:

  dhcp ipv4
   profile DHCP_BASE base
    match option 60 41424355 profile DHCP_SERVER server
    default profile DEFAULT_PROFILE server
    .
    .
    .
   !
   profile DHCP_SERVER server
    .
    .
    .
   !
   profile DEFAULT_PROFILE server
    .
    .
    .
   !
   interface GigabitEthernet0/0/0/0 base profile DHCP_BASE

  The following example shows the output of the show running-config dhcp ipv4 command on a device that has the DHCPv4 proxy profile PROXY directly bound to interface GigabitEthernet0/0/0/0:

  dhcp ipv4
   profile PROXY proxy
    helper-address vrf default 192.168.23.7 giaddr 192.168.23.11
   !
   interface GigabitEthernet0/0/0/0 proxy profile PROXY

   

  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 XE Software
  • NX-OS Software

Workarounds

• There are no workarounds that address this vulnerability.

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.

  Fixed Releases

  At the time of publication, the following Cisco IOS XR Software releases contained the fix for this vulnerability:

  • 7.3.2 and later
  • 7.4.1 and later

  Cisco has released the following SMU to address this vulnerability. Customers who require SMUs for platforms or releases that are not listed are advised to contact their support organization.

  Cisco IOS XR Software Release	Platform	SMU Name
  7.1.3	ASR9K-X64	asr9k-x64-7.1.3.CSCvw95930

  See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information, including SMU availability.

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 the resolution of a Cisco TAC support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxr-dhcp-dos-pjPVReLU

Revision History

• Version	Description	Section	Status	Date
  1.1	Clarified vulnerable releases.	Vulnerable Products	Final	2021-OCT-20
  1.0	Initial public release.	—	Final	2021-SEP-08

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