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06 December 2021

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             AUSCERT External Security Bulletin Redistribution

       K50839343: NGINX ModSecurity WAF vulnerability CVE-2021-42717
                              6 December 2021


        AusCERT Security Bulletin Summary

Product:           NGINX ModSecurity WAF
Publisher:         F5 Networks
Operating System:  Network Appliance
Impact/Access:     Denial of Service -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2021-427171  

Original Bulletin: 

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

K50839343: NGINX ModSecurity WAF vulnerability CVE-2021-42717

Original Publication Date: 04 Dec, 2021

Security Advisory Description

** RESERVED ** This candidate has been reserved by an organization or
individual that will use it when announcing a new security problem. When the
candidate has been publicized, the details for this candidate will be provided.


An attacker using specifically formatted JSON messages can cause high resource
utilization and potentially denial-of-service (DoS).

Security Advisory Status

F5 Product Development has assigned ID NWA-1216 (NGINX ModSecurity WAF) to this
vulnerability. This issue has been classified as CWE-674: Uncontrolled
Recursion (4.6).

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 be  |introduced |Severity  |score^|component or |
|            |      |vulnerable^1 |in         |          |2     |feature      |
|NGINX       |R19 - |R25          |R25+1.0.2-3|          |      |ModSecurity  |
|ModSecurity |R25   |R24          |R24+1.0.2-2|Medium    |5.3   |JSON parser  |
|WAF         |      |             |           |          |      |             |
|NGINX Plus  |R19 - |None         |Not        |Not       |None  |None         |
|            |R25   |             |applicable |vulnerable|      |             |
|            |16.x  |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|            +------+-------------+-----------+          |      |             |
|            |15.x  |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|            +------+-------------+-----------+          |      |             |
|            |14.x  |None         |Not        |          |      |             |
|BIG-IP (all |      |             |applicable |Not       |      |             |
|modules)    +------+-------------+-----------+vulnerable|None  |None         |
|            |13.x  |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|            +------+-------------+-----------+          |      |             |
|            |12.x  |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|            +------+-------------+-----------+          |      |             |
|            |11.x  |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|            |8.x   |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|BIG-IQ      +------+-------------+-----------+          |      |             |
|Centralized |7.x   |None         |Not        |Not       |None  |None         |
|Management  |      |             |applicable |vulnerable|      |             |
|            +------+-------------+-----------+          |      |             |
|            |6.x   |None         |Not        |          |      |             |
|            |      |             |applicable |          |      |             |
|F5OS        |1.x   |None         |Not        |Not       |None  |None         |
|            |      |             |applicable |vulnerable|      |             |
|Traffix SDC |5.x   |None         |Not        |Not       |None  |None         |
|            |      |             |applicable |vulnerable|      |             |

^1F5 evaluates only software versions that have not yet reached the End of
Technical Support (EoTS) phase of their lifecycle.

^2The CVSSv3 score link takes you to a resource outside of AskF5, and it is
possible that the document may be removed without our knowledge.

Recommended Actions

If you are running a version listed in the Versions known to be vulnerable
column, you can eliminate this vulnerability by installing a version listed in
the Fixes introduced in column. If the Fixes introduced in column does not list
a version for your branch, then no update candidate currently exists for that
branch and F5 recommends upgrading to a version with the fix (refer to the

If the Fixes introduced in column lists a version prior to the one you are
running, in the same branch, then your version should have the fix.


To mitigate this vulnerability, you can either upgrade to a fixed version or
disable JSON request body processing by disabling any ModSecurity rules that
activate the JSON parser. Use your system package manager to view the current
dynamic module version or to update to the most recent module version from the
NGINX module repository. For more information, refer to Installing the NGINX
ModSecurity WAF.

Impact of action: When JSON parsing rules are disabled, the system does not
parse HTTP request bodies that are in JSON format. Typically that is HTTP
requests with the Content-Type header application/json. Before modifying the
configuration, you should perform changes in a test environment to ensure
compatibility. You should implement modifications to your production
environment during a maintenance window to allow for time to revert changes if

Supplemental Information

o Trustwave SpiderLabs ModSecurity DoS Vulnerability in JSON parsing

    Note: The previous link take you to a resource outside of AskF5. The
    third-party could remove the document without our knowledge.

  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 K8986: F5 software lifecycle policy
  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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