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             AUSCERT External Security Bulletin Redistribution

BIG-IP Advanced WAF and ASM Brute Force Protection feature may not properly
              support the Post-Redirect-Get application flow
                               29 April 2021


        AusCERT Security Bulletin Summary

Product:           BIG-IP (Advanced WAF, ASM)
Publisher:         F5 Networks
Operating System:  Network Appliance
Impact/Access:     Reduced Security -- Existing Account
Resolution:        Patch/Upgrade

Original Bulletin: 

- --------------------------BEGIN INCLUDED TEXT--------------------

K91414704: BIG-IP Advanced WAF and ASM Brute Force Protection feature may not
properly support the Post-Redirect-Get application flow

Original Publication Date: 29 Apr, 2021

Security Advisory Description

The Advanced WAF and BIG-IP ASM systems may not properly support the
Post-Redirect-Get (PRG) application flow implemented on a back-end web server.

This issue occurs when all of the following conditions are met:

  o You enabled brute force protection in your security policy.
  o You configure your security policy to use Client Side Integrity (CSI) or
    CAPTCHA challenge for brute force protection.
  o You associated the security policy with the virtual server to protect the
    back-end web server with the PRG application flow.

The PRG flow is a common design for login URLs. After users log out, PRG
prevents users from clicking the Back button twice to re-post sensitive
information and log in again. When the brute force protection challenge
receives a response, the flow becomes Post-200OK-Post-Redirect-Get, resulting
in a broken flow.


After users log out of an application or site, they may log in again by
clicking the browser Back button a multiple times.


As a result of this issue, you may encounter the following symptom:

  o After logging out, users log in to an application or site again by
    clicking the Back button multiple times.

Security Advisory Status

F5 Product Development has assigned ID 941621 to this issue. F5 has confirmed
that this issue exists in the products listed in the Applies to (see versions)
box, located in the upper-right corner of this article. For information about
releases, point releases, or hotfixes that resolve this issue, refer to the
following table.

Note: F5 evaluates only software versions that have not yet reached the End of
Technical Support (EoTS) phase of their lifecycle. For more information, refer
to the Security hotfixes section of K4602: Overview of the F5 security
vulnerability response policy.

|Type of fix       |Fixes introduced |Related articles                        |
|                  |in               |                                        |
|                  |15.1.3           |K2200: Most recent versions of F5       |
|Release           |14.1.4           |software                                |
|                  |13.1.4           |                                        |
|Point release/    |         |K9502: BIG-IP hotfix and point release  |
|hotfix            |                 |matrix                                  |

Recommended Actions


To work around this issue, you can disable the CSI or CAPTCHA challenge in your
security policy that is currently protecting your PRG application flow URI from
brute force attack. Then, you can enable the DoS Protection or Bot Defense
feature instead.

You also must add the URI of your PRG application flow to two
database variables: dosl7.prg_iframe_urls and dosl7.cs_qualified_urls. Doing so
allows the Bot Defense feature to send out the CSI challenge to support your

To add your PRG URI to the database variables, perform the following procedure:

Impact of workaround: Performing the following procedure should not have a
negative impact on your system.

 1. Log in to TMOS Shell (tmsh).
 2. Enter the following command syntax to add a PRG URI to a database variable:

    modify /sys db <db variable> value <PRG URI>

    Note: The dosl7.prg_iframe_urls database variable does not currently
    support sending out the CAPTCHA challenge for PRG URIs. Additionally, it is
    not available in BIG-IP 11.6.x and earlier versions.

    For example, you use the DoS Protection or Bot Defense feature to protect
    your PRG URI, and the URI is /user_login.php. You enter the following

    modify /sys db dosl7.cs_qualified_urls value /user_login.php
    modify /sys db dosl7.prg_iframe_urls value /user_login.php

    Note: The database variable accepts a comma-separated list of URIs. When
    you add more URIs to the database variable, you must include the existing
    URIs and the new URIs in the following format: <old URI>,<new URI>.

 3. Enter the following command to verify the PRG URI you entered for
    database variable:

    list /sys db <db variable>

    For example, you enter the following command to list the URIs you added to 

    list /sys db dosl7.prg_iframe_urls

 4. For BIG-IP 14.1.x and later, if the affected virtual server is not
    associated with a Bot Defense profile, you must configure one using the
    following settings.

    If the affected virtual server is associated with an existing Bot Defense
    profile, you must ensure that it you configured it using the following

       Go to Security > Bot Defense > Bot Defense Profiles > <profile name>.
           Select Browsers, and then in Browser Verification, select Verify
            Before Access.
           Select General Settings, and then in Enforcement Mode, select 

            If you do not want to set Enforcement Mode to Blocking, do the

            Select Browsers, and then in Verification and Device-ID Challenges
            in Transparent Mode, select Enabled.

    Note: Other configurations of the Bot Defense Profile are also possible, as
    long as you set Browser Verification to Verify Before Access for the
    challenged page.


This issue was discovered internally by F5.

Supplemental Information

o K51812227: Understanding security advisory versioning
  o K41942608: Overview of AskF5 security advisory articles
  o K4602: Overview of the F5 security vulnerability response policy
  o K4918: Overview of the F5 critical issue hotfix policy
  o K8986: F5 software lifecycle 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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