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Cisco Security Advisory

Cisco IOS XR Software for ASR 9000 Series Routers Bidirectional Forwarding Detection Denial of Service Vulnerability

High
Advisory ID:
cisco-sa-bfd-XmRescbT
First Published:
2023 March 8 16:00 GMT
Version 1.0:
Final
Workarounds:
Yes
Cisco Bug IDs:
CSCwc39336
CVE-2023-20049
CWE-805
CVSS Score:
Base 8.6Click Icon to Copy Verbose Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
CVE-2023-20049
CWE-805

Summary

  • A vulnerability in the bidirectional forwarding detection (BFD) hardware offload feature of Cisco IOS XR Software for Cisco ASR 9000 Series Aggregation Services Routers, ASR 9902 Compact High-Performance Routers, and ASR 9903 Compact High-Performance Routers could allow an unauthenticated, remote attacker to cause a line card to reset, resulting in a denial of service (DoS) condition.

    This vulnerability is due to the incorrect handling of malformed BFD packets that are received on line cards where the BFD hardware offload feature is enabled. An attacker could exploit this vulnerability by sending a crafted IPv4 BFD packet to an affected device. A successful exploit could allow the attacker to cause line card exceptions or a hard reset, resulting in loss of traffic over that line card while the line card reloads.

    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-bfd-XmRescbT

    This advisory is part of the March 2023 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: March 2023 Semiannual Cisco IOS XR Software Security Advisory Bundled Publication.

Affected Products

  • Vulnerable Products

    This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco IOS XR 64-bit Software and have BFD hardware offload enabled for any of the installed line cards:

    • ASR 9000 Series Aggregation Services Routers only if they have a Lightspeed or Lightspeed-Plus-based line card installed
    • ASR 9902 Compact High-Performance Routers
    • ASR 9903 Compact High-Performance Routers

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

    Determine Which Line Cards are Installed

    To determine which line cards are installed in the device, use the show platform CLI command.

    Cisco ASR 9902 and 9903 Compact High-Performance Routers integrate a Lightspeed-Plus-based line card.

    The following line cards are Lightspeed-based:

    • A9K-16X100GE-TR
    • A99-16X100GE-X-SE
    • A99-32X100GE-TR

    The following line cards are Lightspeed-Plus-based:

    • A9K-4HG-FLEX-SE
    • A9K-4HG-FLEX-TR
    • A9K-8HG-FLEX-SE
    • A9K-8HG-FLEX-TR
    • A9K-20HG-FLEX-SE
    • A9K-20HG-FLEX-TR
    • A99-4HG-FLEX-SE
    • A99-4HG-FLEX-TR
    • A99-10X400GE-X-SE
    • A99-10X400GE-X-TR
    • A99-32X100GE-X-SE
    • A99-32X100GE-X-TR

    For more information about line card type identification, see ASR 9000 Series Line Card Types.

    Note: The Cisco Lightspeed-Plus list of product identifiers was accurate at the time of publication. For specific questions and further clarification about a product identifier, contact the Cisco Technical Assistance Center (TAC).

    Determine Whether BFD Hardware Offload is Enabled

    To determine which line cards have BFD hardware offload enabled, use the show bfd hw-offload state CLI command. If the configuration and operational states are enabled on one of the previously mentioned line cards, the device is affected by this vulnerability. The following example shows a device in which BFD hardware offload is not enabled:

    RP/0/RSP0/CPU0:ASR9006#show bfd hw-offload state
Wed Mar 8 16:00:00.000 UTC
BFD HW OFFLOAD Feature state:
0/2/CPU0
-----------------
Configuration State:   Disabled
Operational   State:   Disabled
RP/0/RSP0/CPU0:ASR9006#

    The following example shows a device in which BFD hardware offload is enabled:

    RP/0/RSP0/CPU0:ASR9006#show bfd hw-offload state
Wed Mar 8 16:00:00.000 UTC

BFD HW OFFLOAD Feature state:
0/2/CPU0
-----------------
Configuration State:  Enabled
Operational   State:  Enabled
RP/0/RSP0/CPU0:ASR9006#

    Not all BFD sessions are handled in hardware even if BFD hardware offload is enabled. At least one BFD session has to indicate H/W as Yes, as shown in the following example:

    RP/0/RSP0/CPU0:ASR9006#show bfd all session
Wed Mar 8 16:00:00.000 UTC
IPv4:
-----
Interface  Dest Addr         Local det time(int*mult)      State
                               Echo         Async   H/W   NPU
---------- -------------   ------------ ----------------   ----------
ti100      10.0.0.1        0s(0s*0)     900ms(300ms*3)     UP
                                                  No    n/a
    Hu0/2/0/15 192.168.100.100 0s(0s*0)     900ms(300ms*3)     UP
                                                  Yes   0/2/CPU0
RP/0/RSP0/CPU0:ASR9006#

    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
    • IOS XR Platforms not listed in the Vulnerable Products section of this advisory
    • NX-OS Software

Details

  • This vulnerability cannot be exploited with IPv6 BFD packets. However, if an IPv6 BFD session also has an IPv4 address configured on the attached interface, an attacker could exploit this vulnerability by sending a crafted IPv4 BFD packet to the configured IPv4 address on the interface. If interface Te0/1/0/10/0 was also configured with IPv4 address 192.168.1.1, the attacker could exploit the IPv6 BFD session by sending a crafted IPv4 BFD packet to 192.168.1.1, as shown in the following example:

    RP/0/RSP0/CPU0:ASR9006#show bfd all session
Wed Mar 8 16:00:00.000 UT
IPv4:
-----


IPv6:
-----
Interface           Dest Addr
                                       Local det time(int*mult)      State
H/W                 NPU             Echo             Async
------------------- --------------- ---------------- ---------------- ----------
Te0/1/0/10/0        fe80::36f8:e7ff:fee0:947
Yes                 0/1/CPU0        0s(0s*0)         900ms(300ms*3)   UP

Workarounds

  • There is a workaround and a mitigation that address this vulnerability.

    The only workaround that fully mitigates this vulnerability is disabling BFD hardware offload. Creating Infrastructure Access Control Lists (iACLs) is a mitigation that only limits the attack surface.

    Disable BFD Hardware Offload

    To disable BFD hardware offload, remove any of the hw-module bfw-hw-offload enable commands and reset the assigned line card, as shown in the following example:

    RP/0/RSP0/CPU0:ASR9006#config terminal
    Wed Mar 8 16:00:00.000 UTC
    RP/0/RSP0/CPU0:ASR9006(config)#no hw-module bfd-hw-offload enable location 0/2/CPU0
    RP/0/RSP0/CPU0:ASR9006(config)#commit
    RP/0/RSP0/CPU0:ASR9006(config)#end
    RP/0/RSP0/CPU0:ASR9006#hw-module subslot 0/2/CPU0 reload

    Create Infrastructure Access Control Lists

    While iACLs limit the attack surface, they do not prevent exploitation from allowed peers and are subject to spoofing.

    The following example shows an iACL that allows only infrastructure BFD peers (where 192.0.2.x/24 is the infrastructure address space):

    RP/0/RSP0/CPU0:ASR9006# show running-config ipv4 access-list
    ipv4 access-list BFD_DROP
      5 remark * Mark sure to Allow Legitimate BFD peers *
     10 permit udp 192.0.2.0 0.0.0.255 192.0.2.0 0.0.0.255 eq bfd
     11 remark * Depending on BFD deployment may need *
     12 permit udp 192.0.2.0 0.0.0.255 192.0.2.0 0.0.0.255 eq 4784
     13 permit udp 192.0.2.0 0.0.0.255 192.0.2.0 0.0.0.255 eq 6784
     15 remark * Drop all other attempts to the infrastructure address space *
     20 deny udp any 192.0.2.0 0.0.0.255 eq bfd
     30 permit ipv4 any any
    !
    RP/0/RSP0/CPU0:ASR9006#

    Apply this iACL to all exposed interfaces.

    The following example shows an iACL that allows only BFD packets with a Time to Live (TTL) of 255 (expected for single hop sessions): 

    RP/0/RSP0/CPU0:ASR9006# show running-config ipv4 access-list
    ipv4 access-list BFD_DROP_TTL
     5 remark * Drop based purely on TTL. Allow our Single Hop BFD sessions * 
    10 permit udp any 192.0.2.0 0.0.0.255 eq bfd ttl eq 255
    11 remark * Depending on BFD deployment may need *
    12 permit udp any 192.0.2.0 0.0.0.255 eq 4784 ttl eq 255
    13 permit udp any 192.0.2.0 0.0.0.255 eq 6784 ttl eq 255
    14 remark * You would need to tune the above two lines for multi-hop sessions *
    15 remark * Deny anything else for BFD * 
    20 deny udp any any eq bfd ttl lt 255
    21 remark * You would need to tune the above line for multi-hop sessions *
    30 permit ipv4 any any
    !
    RP/0/RSP0/CPU0:ASR9006#

    Apply this iACL to all exposed interfaces.

    While this workaround and mitigation have been deployed and were 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

  • Cisco has released free software updates that address the vulnerability described in this advisory. Customers with service contracts that entitle them to regular software updates should obtain security fixes through their usual update channels.

    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.

    The Cisco Support and Downloads page on Cisco.com provides information about licensing and downloads. This page can also display customer device support coverage for customers who use the My Devices tool.

    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.

    Fixed Releases

    In the following table, the left column lists Cisco software releases. The right column indicates whether a release is affected by the vulnerability that is described in this advisory and the first release that includes the fix for this vulnerability. Customers are advised to upgrade to an appropriate fixed software release as indicated in this section.

    Cisco IOS XR Software Release First Fixed Release
    6.5 Migrate to a fixed release.
    6.6 Migrate to a fixed release.
    7.0 Migrate to a fixed release.
    7.1 Migrate to a fixed release.
    7.3 Migrate to a fixed release.
    7.4 Migrate to a fixed release.
    7.5 7.5.3
    7.6 7.6.2
    7.7 and later 7.7.1

    Cisco has released the following SMUs to address this vulnerability.

    Note: Customers who require SMUs for releases that are not listed in the following table 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.CSCwc39336
    7.3.2 ASR9K-X64 asr9k-x64-7.3.2.CSCwc39336
    7.5.2 ASR9K-X64 asr9k-x64-7.5.2.CSCwc39336

    The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

Exploitation and Public Announcements

  • The Cisco 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.

Cisco Security Vulnerability Policy

  • To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.

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Revision History

  • Version Description Section Status Date
    1.0 Initial public release. - Final 2023-MAR-08
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Cisco Security Vulnerability Policy

  • To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.

Subscribe to Cisco Security Notifications

Related to This Advisory