Summary

• A vulnerability in the UDP broadcast forwarding function of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition on the affected device.
  
  The vulnerability is due to improper handling of UDP broadcast packets that are forwarded to an IPv4 helper address. An attacker could exploit this vulnerability by sending multiple UDP broadcast packets to the affected device. An exploit could allow the attacker to cause a buffer leak on the affected device, eventually resulting in a DoS condition requiring manual intervention to recover.
  
  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-20180418-iosxr

Affected Products

• Vulnerable Products

  This vulnerability affects all Cisco IOS XR platforms running 6.3.1, 6.2.3, or earlier releases of Cisco IOS XR Software when at least one IPv4 helper address is configured on an interface of the device.
  
  By default, no IPv4 helper address is enabled in Cisco IOS XR Software.
  
  DHCP Relay Profile helper addresses and DHCPv6 Relay Service configurations are not affected.
  
  Determining if the Device Has an IPv4 Helper Address Configured
  
  This vulnerability can be exploited only if the affected device has at least one interface configured with an IPv4 helper address.
  
  The following example shows the output of the show running-config | include ipv4 helper-address CLI command on a device with an IPv4 helper address configured:
  

  RP/0/RSP0/CPU0:Router# show running-config | include ipv4 helper-address
  Building configuration...
  ipv4 helper-address vrf default 192.0.2.1 

  
  Determining the Cisco IOS XR Software Release
  
  To determine which Cisco IOS XR Software release is running on a device and the name of the device on which it is running, administrators can log in to the device and use the show version command in the CLI. If the device is running Cisco IOS XR Software, Cisco IOS XR Software or similar text appears in the system banner. The location and name of the system image file that is currently running on the device appears next to the System image file is text. The name of the hardware product appears on the line after the name of the system image file.
  
  The following example shows the output of the show version command on a device that is running Cisco IOS XR Software release 5.3.4:
  

  RP/0/RSP0/CPU0:ASR9001#show version
  Wed Jan 24 01:32:32.751 EST
  
  Cisco IOS XR Software, Version 5.3.4[Default]
  Copyright (c) 2017 by Cisco Systems, Inc.
  
  ROM: System Bootstrap, Version 2.04(20140227:092320) [ASR9K ROMMON],  
  
  ASR9001 uptime is 6 hours, 17 minutes
  System image file is "bootflash:disk0/asr9k-os-mbi-5.3.4.sp4-1.0.0/0x100000/mbiasr9k-rp.vm"
  
  cisco ASR9K Series (P4040) processor with 8388608K bytes of memory.
  P4040 processor at 1500MHz, Revision 2.0
  ASR-9001 Chassis
  
  2 Management Ethernet
  8 TenGigE
  20 GigabitEthernet
  8 DWDM controller(s)
  8 WANPHY controller(s)
  44 GigabitEthernet/IEEE 802.3 interface(s)
  219k bytes of non-volatile configuration memory.
  2880M bytes of hard disk.
  3932144k bytes of disk0: (Sector size 512 bytes).
  
  Configuration register on node 0/RSP0/CPU0 is 0x2102
  

  
  

  Products Confirmed Not Vulnerable

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

Details

• An attacker could exploit this vulnerability by sending multiple UDP broadcast packets to an interface that is configured with an IPv4 helper address. This would cause a buffer leak in the Software Packet Path (SPP) on the active Route Processor (RP) of the affected device.
  
  Consumption of all available SPP buffers on the active RP would cause the device to start dropping control plane traffic, including routing protocol packets. While experiencing this problem, traffic that is destined to the device and, depending on the information learned from the routing protocols, pass-through traffic could be dropped, resulting in a DoS condition. A manual reload of the device would be needed to restore operations.
  
  SPP buffers utilization on the active RP can be monitored using the show spp buffers location CLI command. The active RP can be identified with the show redundancy CLI command.
  
  The following example shows the output of the show redundancy CLI command on a device with 0/RSP0/CPU0 in the active RP role:
  

  RP/0/RSP0/CPU0:Router#show redundancy
  
  Redundancy information for node 0/RSP0/CPU0:
  ==========================================
  Node 0/RSP0/CPU0 is in ACTIVE role
  .
  .
  .

  The following example shows the output of the show spp buffer location 0/RSP0/CPU0 CLI command on an affected device:

  RP/0/RSP0/CPU0:Router#show spp buffer location 0/RSP0/CPU0
  
  SPP Buffer Analysis
  
  PID Buffers
  0 15065
  225413 41108
  
  Total Buffers in use: 49138
  Owned by clients/spp (of which plugins): 41108/8030 (0)
  .
  .
  .

  A continuous increase of the Owned by clients buffers could be an indication of a buffer leak.

  The number of free buffers can be calculated by subtracting the number of Owned by SPP buffers from the number of buffers associated with PID 0. When the number of free buffers is reduced to 8192 or fewer, the affected device might be starting to drop control plane packets. In the previous example, the number of buffers associated with PID 0 is 15065 and the number of Owned by SPP buffers is 8030, so the number of free buffers is 15065 - 8030 = 7035.

Indicators of Compromise

• Exploitation of this vulnerability could cause the affected device to consume all available SPP buffers and generate error messages similar to the following:
  

  RP/0/RSP0/CPU0:Feb 23 17:25:11.697 : netio[340]: %PKT_INFRA-spp-4-PKT_ALLOC_FAIL : Failed to allocate 1 packets for sending 

  For a final confirmation on whether this message does indicate exploitation of this issue, please contact your support organization.
  

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 and Alerts 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.

  Fixed Releases

  This vulnerability has been corrected in Cisco IOS XR Software Release 6.3.2 and later.
  
  Software maintenance upgrades (SMUs) for Cisco Aggregation Services Router (ASR) 9000 Series have also been made available that address this vulnerability.

  IOS XR Release	SMU ID	ASR 9000 SMU Name
  5.3.4	AA14136	asr9k-px-5.3.4.CSCvi35625.tar 1
  6.1.4	AA14137	asr9k-px-6.1.4.CSCvi35625.tar
  6.2.3	AA14138	asr9k-px-6.2.3.CSCvi35625.tar

  
  ¹ Note: An SMU including a partial fix named "asr9k-px-5.3.4.CSCvh49341.tar" was made available for a short time for Cisco IOS XR Software Release 5.3.4. After additional testing, this was verified to be an incomplete fix. Cisco recommends that customers who might have installed this SMU uninstall it and install the new SMU containing the complete fix referenced in the table.

  Customers using a Cisco IOS XR platform other than ASR 9000 Series Routers, or running a Cisco IOS XR Software Release for which no SMU is listed in the previous table, are advised to upgrade the IOS XR Software or open a support case to evaluate upgrade options.

  Cisco IOS XR Software can be downloaded from the Software Center on Cisco.com by navigating to Downloads Home > Products > Routers > Service Provider Edge Routers or Service Provider Core Routers, selecting the appropriate product, and selecting IOS XR Software.

  The SMUs for Cisco ASR 9000 Series Routers can be downloaded from the Software Center on Cisco.com by navigating to Downloads Home > Products > Routers > Service Provider Edge Routers > ASR 9000 Series Aggregation Services Routers > ASR 9xxx Router > Software on Chassis > IOS XR Software Maintenance Upgrades (SMU).

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-20180418-iosxr

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2018-April-18

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