Cisco Security Advisory
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CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
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A vulnerability in the implementation of Multiprotocol Border Gateway Protocol (MP-BGP) for the Layer 2 VPN (L2VPN) Ethernet VPN (EVPN) address family in Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.
The vulnerability is due to incorrect processing of Border Gateway Protocol (BGP) update messages that contain crafted EVPN attributes. An attacker could exploit this vulnerability by sending BGP update messages with specific, malformed attributes to an affected device. A successful exploit could allow the attacker to cause an affected device to crash, resulting in a DoS condition.
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-ios-bgp-evpn-dos-LNfYJxfF
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Vulnerable Products
At the time of publication, this vulnerability affected Cisco devices if they were running a vulnerable release of Cisco IOS or IOS XE Software and were configured for BGP on a device with the L2VPN EVPN address family.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine the Device Configuration
To determine whether the device has a vulnerable configuration, assess the BGP routing configuration and then the L2VPN EVPN address family status (if necessary).
Determine Whether the Device Is Configured for BGP Routing
To determine whether the device is configured for BGP routing, use the show running-config | section router bgp EXEC command in the CLI. If the router is configured for BGP, this command will return output. The following example shows the partial output of the show running-config | section router bgp command on a device that is configured for BGP routing:
router#show running-config | section router bgp
router bgp <AS-Number>
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.
.If the command returns no output, or the command returns an error, the device is not affected by this vulnerability. If the device returns output, proceed to the next step.
Determine Whether the Device Is Configured for the L2VPN EVPN Address Family
To determine whether the device is configured for the BGP EVPN address family, use the show running-config | section router bgp | address-family l2vpn evpn EXEC command in the CLI. If the router is configured for the BGP EVPN address family, this command will return output. The following example shows the output of the show running-config | section router bgp | address-family l2vpn evpn command on a device with the BGP EVPN address family configured:
router#show running-config | section router bgp | address-family l2vpn evpn
address-family l2vpn evpn
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.If the command returns no output, or the command returns an error, the device is not affected by this vulnerability.
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 Cisco IOS XR Software or Cisco NX-OS Software.
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The Cisco implementation of MP-BGP accepts incoming BGP traffic only from explicitly defined peers. To exploit this vulnerability, the malicious BGP update message must originate from one of the following:
- A configured, valid BGP peer
- The attacker’s injection into the victim’s BGP network on an existing, valid TCP connection to a BGP peer
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There are no workarounds that address this vulnerability.
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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 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.
For a mapping of Cisco IOS XE Software releases to Cisco IOS Software releases, refer to the Cisco IOS XE 2 Release Notes, Cisco IOS XE 3S Release Notes, or Cisco IOS XE 3SG Release Notes, depending on the Cisco IOS XE Software release.
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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.
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This vulnerability was found during the resolution of a Cisco TAC support case.
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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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Show LessVersion Description Section Status Date 1.0 Initial public release. — Final 2020-SEP-24
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