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AUSCERT External Security Bulletin Redistribution
ESB-2020.4491
BIG-IP (LTM/CGNAT) - CGNAT LSN vulnerability CVE-2020-27720
18 December 2020
===========================================================================
AusCERT Security Bulletin Summary
---------------------------------
Product: BIG-IP (LTM/CGNAT)
Publisher: F5 Networks
Operating System: Network Appliance
Impact/Access: Denial of Service -- Remote/Unauthenticated
Resolution: Patch/Upgrade
CVE Names: CVE-2020-27720
Original Bulletin:
https://support.f5.com/csp/article/K04048104
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K04048104: CGNAT LSN vulnerability CVE-2020-27720
Original Publication Date: 17 Dec, 2020
Security Advisory Description
When processing NAT66 traffic with Port Block Allocation (PBA) mode and SP-DAG
enabled, and dag-ipv6-prefix-len configured with a value less than the default
of 128, an undisclosed traffic pattern may cause the Traffic Management
Microkernel (TMM) to restart. (CVE-2020-27720)
PBA mode provides the ability to log only the allocation and release of port
blocks for a subscriber, instead of separately logging each network address
translation (NAT) session as a separate translation event, as with network
address and port translation (NAPT), thus reducing the number of log entries
while maintaining legal mapping and reverse mapping requirements.
The SP-DAG uses a hash of source IP address (from client) and destination IP
address (from server).
Impact
Traffic processing is disrupted while TMM restarts. If the affected BIG-IP
system is configured as part of a device group, the system triggers a failover
to the peer device.
Security Advisory Status
F5 Product Development has assigned ID 935029 (BIG-IP) to this vulnerability.
To determine if your product and version have been evaluated for this
vulnerability, refer to the Applies to (see versions) box. To determine if your
release is known to be vulnerable, the components or features that are affected
by the vulnerability, and for information about releases, point releases, or
hotfixes that address the vulnerability, refer to the following table. For more
information about security advisory versioning, refer to K51812227:
Understanding security advisory versioning.
+-------------------+------+----------+----------+----------+------+----------+
| | |Versions |Fixes | |CVSSv3|Vulnerable|
|Product |Branch|known to |introduced|Severity |score^|component |
| | |be |in | |1 |or feature|
| | |vulnerable| | | | |
+-------------------+------+----------+----------+----------+------+----------+
| |16.x |16.0.0 |16.0.1 | | | |
| +------+----------+----------+ | | |
| |15.x |15.1.0 |15.1.1 | | | |
| +------+----------+----------+ | | |
| |14.x |14.1.0 - |14.1.3.1 | | | |
| | |14.1.3 | | | | |
| +------+----------+----------+ | | |
|BIG-IP (LTM/CGNAT) |13.x |13.1.0 - |None |High |7.5 |CGNAT LSN |
| | |13.1.3 | | | | |
| +------+----------+----------+ | | |
| |12.x |None |Not | | | |
| | | |applicable| | | |
| +------+----------+----------+ | | |
| |11.x |None |Not | | | |
| | | |applicable| | | |
+-------------------+------+----------+----------+----------+------+----------+
| |16.x |None |Not | | | |
| | | |applicable| | | |
| +------+----------+----------+ | | |
| |15.x |None |Not | | | |
| | | |applicable| | | |
|BIG-IP (AAM, +------+----------+----------+ | | |
|Advanced WAF, AFM, |14.x |None |Not | | | |
|Analytics, APM, | | |applicable|Not | | |
|ASM, DDHD, DNS, +------+----------+----------+vulnerable|None |None |
|FPS, GTM, Link |13.x |None |Not | | | |
|Controller, PEM, | | |applicable| | | |
|SSLO) +------+----------+----------+ | | |
| |12.x |None |Not | | | |
| | | |applicable| | | |
| +------+----------+----------+ | | |
| |11.x |None |Not | | | |
| | | |applicable| | | |
+-------------------+------+----------+----------+----------+------+----------+
| |7.x |None |Not | | | |
| | | |applicable| | | |
| +------+----------+----------+ | | |
|BIG-IQ Centralized |6.x |None |Not |Not |None |None |
|Management | | |applicable|vulnerable| | |
| +------+----------+----------+ | | |
| |5.x |None |Not | | | |
| | | |applicable| | | |
+-------------------+------+----------+----------+----------+------+----------+
|Traffix SDC |5.x |None |Not |Not |None |None |
| | | |applicable|vulnerable| | |
+-------------------+------+----------+----------+----------+------+----------+
^1The CVSSv3 score link takes you to a resource outside of AskF5, and it is
possible that the document may be removed without our knowledge.
Security Advisory Recommended Actions
If you are running a version listed in the Versions known to be vulnerable
column, you can eliminate this vulnerability by upgrading to a version listed
in the Fixes introduced in column. If the table lists only an older version
than what you are currently running, or does not list a non-vulnerable version,
then no upgrade candidate currently exists.
Mitigation
To mitigate this vulnerability, you can set the value of dag-ipv6-prefix-len of
the global disaggregation setting to its default value of 128. To do so,
perform the following procedure:
Impact of action: Changing the IPv6 prefix length for DAG changes the
disaggregation of IPv6 packets to TMM, resulting in a different distribution
that may affect your specific application environment requirements.
1. Log in to the TMOS Shell (tmsh) by entering the following command:
tmsh
2. Change the value of the dag-ipv6-prefix-len setting to 128 by entering the
following command:
modify /net dag-globals dag-ipv6-prefix-len 128
3. Save the change by entering the following command:
save /sys config
Supplemental Information
o K41942608: Overview of security advisory articles
o K4602: Overview of the F5 security vulnerability response policy
o K4918: Overview of the F5 critical issue hotfix policy
o K9502: BIG-IP hotfix and point release matrix
o K13123: Managing BIG-IP product hotfixes (11.x - 16.x)
o K167: Downloading software and firmware from F5
o K9970: Subscribing to email notifications regarding F5 products
o K9957: Creating a custom RSS feed to view new and updated documents
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