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             AUSCERT External Security Bulletin Redistribution

  BIG-IP virtual server TCP sequence numbers vulnerability CVE-2020-5947
                             19 November 2020


        AusCERT Security Bulletin Summary

Product:           BIG-IP Products
Publisher:         F5 Networks
Operating System:  Network Appliance
Impact/Access:     Reduced Security -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2020-5947  

Original Bulletin: 

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K64571774: BIG-IP virtual server TCP sequence numbers vulnerability

Original Publication Date: 19 Nov, 2020

Security Advisory Description

On specific BIG-IP platforms, attackers may be able to obtain TCP sequence
numbers from the BIG-IP system that can be reused in future connections with
the same source and destination port and IP numbers. (CVE-2020-5947)


Attackers may be able to spoof TCP packets to be used by a future connection,
resulting in a TCP sequence prediction attack.

The following BIG-IP platforms do not contain the high speed bus (HSBe2) chip
and do not have hardware-based SYN cookie capability, and are therefore

  o BIG-IP 2000 series (C112)
  o BIG-IP 4000 series (C113)
  o BIG-IP i2000 series (C117)
  o BIG-IP i4000 series (C115)
  o BIG-IP Virtual Edition (VE)

Security Advisory Status

F5 Product Development has assigned ID 919989 (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  |15.0.0 -  |15.1.2    |          |      |          |
|                   |      |15.1.1    |          |          |      |          |
|BIG-IP (LTM, AAM,  +------+----------+----------+          |      |          |
|Advanced WAF, AFM, |14.x  |None      |Not       |          |      |          |
|Analytics, APM,    |      |          |applicable|          |      |BIG-IP    |
|ASM, DDHD, DNS,    +------+----------+----------+Medium    |6.3   |virtual   |
|FPS, GTM, Link     |13.x  |None      |Not       |          |      |server    |
|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.


To mitigate this vulnerability, you can set the value for the system
database variable connection.syncookies.algorithm to software. For more
information on the connection.syncookies.algorithm database variable, refer to 
K16500: Overview of the connection.syncookies.algorithm database key.

Set the 'connection.syncookies.algorithm' database variable to 'software'

Impact of action: This procedure may increase the CPU usage of the BIG-IP

 1. Using the command line, log in to the TMOS Shell (tmsh) by entering the
    following command:


 2. Enter the following command:

    modify /sys db connection.syncookies.algorithm value software


F5 would like to acknowledge Ben Miles of ZX Security for bringing this issue
to our attention and following the highest standards of coordinated disclosure.

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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