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AUSCERT External Security Bulletin Redistribution
ESB-2020.3302
Cisco IOS XE Software for Cisco cBR-8 Converged Broadband
Routers DHCP Denial of Service Vulnerability
25 September 2020
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AusCERT Security Bulletin Summary
---------------------------------
Product: Cisco IOS XE Software
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Denial of Service -- Remote/Unauthenticated
Resolution: Patch/Upgrade
CVE Names: CVE-2020-3509
Original Bulletin:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxe-dhcp-dos-JSCKX43h
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Cisco IOS XE Software for Cisco cBR-8 Converged Broadband Routers DHCP Denial
of Service Vulnerability
Priority: High
Advisory ID: cisco-sa-iosxe-dhcp-dos-JSCKX43h
First Published: 2020 September 24 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds availableCisco Bug IDs: CSCvr70940
CVE-2020-3509
CWE-388
CVSS Score:
8.6 AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
Summary
o A vulnerability in the DHCP message handler of Cisco IOS XE Software for
Cisco cBR-8 Converged Broadband Routers could allow an unauthenticated,
remote attacker to cause the supervisor to crash, which could result in a
denial of service (DoS) condition.
The vulnerability is due to insufficient error handling when DHCP version 4
(DHCPv4) messages are parsed. An attacker could exploit this vulnerability
by sending a malicious DHCPv4 message to or through a WAN interface of an
affected device. A successful exploit could allow the attacker to cause a
reload of the affected device.
Note: On Cisco cBR-8 Converged Broadband Routers, all of the following are
considered WAN interfaces:
10 Gbps Ethernet interfaces
100 Gbps Ethernet interfaces
Port channel interfaces that include multiple 10 and/or 100 Gbps
Ethernet interfaces
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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-iosxe-dhcp-dos-JSCKX43h
This advisory is part of the September 24, 2020, release of the Cisco IOS
and IOS XE Software Security Advisory Bundled Publication, which includes
25 Cisco Security Advisories that describe 34 vulnerabilities. For a
complete list of the advisories and links to them, see Cisco Event
Response: September 2020 Semiannual Cisco IOS and IOS XE Software Security
Advisory Bundled Publication .
Affected Products
o Vulnerable Products
This vulnerability affects Cisco cBR-8 Converged Broadband Routers if they
are running a vulnerable release of Cisco IOS XE Software and have a WAN
interface connected. On these devices, the DHCP relay feature is enabled by
default on all WAN interfaces and cannot be disabled.
For information about which Cisco software releases are vulnerable, see the
Fixed Software section of this advisory.
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
NX-OS Software
Workarounds
o There are no workarounds that address this vulnerability.
To reduce the attack surface, customers can apply infrastructure access
control lists (iACLs) that permit DHCP messages from only trusted DHCP
servers to all WAN interfaces. Using iACLs is a network security best
practice and a long-term addition to good network security, as well as a
partial mitigation for this vulnerability. To help protect all devices with
IP addresses in the infrastructure IP address range, customers are advised
to include the following iACL example as part of the deployed iACL:
!---
!--- Define trusted DHCP servers
!---
object-group network Trusted_Dhcp_Servers
host TRUSTED_DHCP_SERVER_1
host TRUSTED_DHCP_SERVER_2
!---
!--- Define infrastructure addresses
!---
object-group network Infrastructure_Addresses
NETWORK_1 WILDCARD_1
NETWORK_2 WILDCARD_2
!---
!--- Permit traffic from trusted DHCP servers
!---
ip access-list extended WAN-IN
10 permit udp object-group Trusted_Dhcp_Servers eq bootps object-group Infrastructure_Addresses eq bootpc
!---
!--- Deny traffic to DHCP clients from all other sources
!---
ip access-list extended WAN-IN
20 deny udp any any eq bootpc
!---
!--- Permit/deny all other Layer 3 and Layer 4 traffic in
!--- accordance with existing security policies and
!--- configurations. Permit all other traffic to transit the
!--- device.
!---
ip access-list extended WAN-IN
30 permit ip any object-group Infrastructure_Addresses
!---
!--- Apply access-list to all WAN interfaces (only one example
!--- shown)
!---
interface Port-channel1
ip access-group WAN-IN in
For further guidelines and recommendations for deployment techniques for
iACLs, see the white paper Protecting Your Core: Infrastructure Protection
Access Control Lists and the Cisco Guide to Harden Cisco IOS Devices .
Fixed Software
o 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 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.
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 :
[ ] [Check]
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.
Exploitation and Public Announcements
o 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
o This vulnerability was found during the resolution of a Cisco TAC support
case.
Cisco Security Vulnerability Policy
o 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.
Related to This Advisory
o Cisco Event Response: September 2020 Semiannual Cisco IOS and IOS XE
Software Security Advisory Bundled Publication
URL
o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
cisco-sa-iosxe-dhcp-dos-JSCKX43h
Revision History
o +----------+---------------------------+----------+--------+--------------+
| Version | Description | Section | Status | Date |
+----------+---------------------------+----------+--------+--------------+
| 1.0 | Initial public release. | - | Final | 2020-SEP-24 |
+----------+---------------------------+----------+--------+--------------+
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NOTE: Third Party Rights
This security bulletin is provided as a service to AusCERT's members. As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content. The decision to follow or act on information or advice
contained in this security bulletin is the responsibility of each user or
organisation, and should be considered in accordance with your organisation's
site policies and procedures. AusCERT takes no responsibility for consequences
which may arise from following or acting on information or advice contained in
this security bulletin.
NOTE: This is only the original release of the security bulletin. It may
not be updated when updates to the original are made. If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the author's website to ensure that the information is still current.
Contact information for the authors of the original document is included
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information, please contact them directly.
Previous advisories and external security bulletins can be retrieved from:
https://www.auscert.org.au/bulletins/
===========================================================================
Australian Computer Emergency Response Team
The University of Queensland
Brisbane
Qld 4072
Internet Email: auscert@auscert.org.au
Facsimile: (07) 3365 7031
Telephone: (07) 3365 4417 (International: +61 7 3365 4417)
AusCERT personnel answer during Queensland business hours
which are GMT+10:00 (AEST).
On call after hours for member emergencies only.
===========================================================================
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