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AUSCERT External Security Bulletin Redistribution
ESB-2021.1452
BIG-IP Advanced WAF and ASM Brute Force Protection feature may not properly
support the Post-Redirect-Get application flow
29 April 2021
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AusCERT Security Bulletin Summary
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Product: BIG-IP (Advanced WAF, ASM)
Publisher: F5 Networks
Operating System: Network Appliance
Impact/Access: Reduced Security — Existing Account
Resolution: Patch/Upgrade
Original Bulletin:
https://support.f5.com/csp/article/K91414704
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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.
Impact
After users log out of an application or site, they may log in again by
clicking the browser Back button a multiple times.
Symptoms
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/ |16.0.1.1 |K9502: BIG-IP hotfix and point release |
|hotfix | |matrix |
+——————+—————–+—————————————-+
Recommended Actions
Workaround
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
PRG URI.
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 value
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
commands:
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: ,.
3. Enter the following command to verify the PRG URI you entered for
database variable:
list /sys db
For example, you enter the following command to list the URIs you added to
dosl7.prg_iframe_urls:
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
settings:
Go to Security > Bot Defense > Bot Defense Profiles > .
Select Browsers, and then in Browser Verification, select Verify
Before Access.
Select General Settings, and then in Enforcement Mode, select
Blocking.
If you do not want to set Enforcement Mode to Blocking, do the
following:
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.
Acknowledgements
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
– ————————–END INCLUDED TEXT——————–
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you do not know who that is, please send an email to auscert@auscert.org.au
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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
in the Security Bulletin above. If you have any questions or need further
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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