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===========================================================================
             AUSCERT External Security Bulletin Redistribution

                              ESB-2019.2185.4
          F5 Networks: Excess resource consumption due to low MSS
                    values vulnerability CVE-2019-11479
                             25 September 2019

===========================================================================

        AusCERT Security Bulletin Summary
        ---------------------------------

Product:           F5 products
Publisher:         F5 Networks
Operating System:  Network Appliance
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Mitigation
CVE Names:         CVE-2019-11479  

Reference:         ESB-2019.2171
                   ESB-2019.2151
                   ESB-2019.2132.3

Original Bulletin: 
   https://support.f5.com/csp/article/K35421172

Revision History:  September 25 2019: An update is available for the 14.x.x branch.
                   July      12 2019: Vendor updated Impact and Mitigation details
                   June      25 2019: updates addressing the vulnerability
                                      have been issued for Enterprise Manager,
                                      BIG-IQ Centralized Management and iWorkflow
                   June      19 2019: Initial Release

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K35421172:Excess resource consumption due to low MSS values vulnerability CVE-2019-11479

Security Advisory

Original Publication Date: 19 Jun, 2019

Latest   Publication Date: 25 Sep, 2019

Security Advisory Description

Jonathan Looney discovered that the Linux kernel default MSS is hard-coded to
48 bytes. This allows a remote peer to fragment TCP resend queues significantly
more than if a larger MSS were enforced. A remote attacker could use this to
cause a denial of service. This has been fixed in stable kernel releases
4.4.182, 4.9.182, 4.14.127, 4.19.52, 5.1.11, and is fixed in commits
967c05aee439e6e5d7d805e195b3a20ef5c433d6 and
5f3e2bf008c2221478101ee72f5cb4654b9fc363. (CVE-2019-11479)

The Linux kernel is vulnerable to a flaw that allows attackers sending crafted
packets with low maximum segment size (MSS) values to trigger excessive
resource consumption.

Impact

BIG-IP

The BIG-IP system has no exposure to this vulnerability within the Traffic
Management Microkernel (TMM), including virtual servers and virtual IP
addresses (also known as the data plane). However, the BIG-IP system is
vulnerable via the self IP addresses and the management interface (also known
as the control plane). A remote attacker can exploit this vulnerability to
cause a denial of service (DoS) by sending a sequence of specially crafted TCP
packets. 

Backend systems accessed via a FastL4 virtual server

By its nature as a full-proxy, the BIG-IP system protects backend systems
accessed through a standard virtual server, as any attacker's TCP connection
would be terminated at the BIG-IP system. However, backend systems accessed via
a FastL4 virtual server (a virtual server configured with a FastL4 profile) are
exposed by default as the attack traffic is forwarded as-is to the backend
system.  

Traffix SDC

A remote attacker can exploit this vulnerability to cause a DoS by sending a
sequence of specially crafted TCP packets.

Security Advisory Status

F5 Product Development has assigned ID 795197 (BIG-IP, BIG-IQ, Enterprise
Manager, and F5 iWorkflow) and CPF-25102 (Traffix SDC) to this vulnerability.
Additionally, F5 iHealth may list Heuristic H35421172 on the Diagnostics >
Identified > Medium page.

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|          |        |      |           |
+--------------------+------+----------+----------+--------+------+-----------+
|                    |15.x  |15.0.0 -  |None      |        |      |Self IP    |
|                    |      |15.0.1    |          |        |      |addresses  |
|                    +------+----------+----------+        |      |Control    |
|                    |14.x  |14.0.0 -  |14.1.2.1  |        |      |plane      |
|BIG-IP (LTM, AAM,   |      |14.1.2    |          |        |      |(management|
|AFM, Analytics, APM,+------+----------+----------+        |      |interface) |
|ASM, DNS, Edge      |13.x  |13.1.0 -  |None      |Medium  |5.3   |           |
|Gateway, FPS, GTM,  |      |13.1.3    |          |        |      |Backend    |
|Link Controller,    +------+----------+----------+        |      |systems    |
|PEM, WebAccelerator)|12.x  |12.1.0 -  |None      |        |      |accessed   |
|                    |      |12.1.5    |          |        |      |via a      |
|                    +------+----------+----------+        |      |FastL4     |
|                    |11.x  |11.5.2 -  |None      |        |      |virtual    |
|                    |      |11.6.5    |          |        |      |server     |
+--------------------+------+----------+----------+--------+------+-----------+
|                    |      |          |          |        |      |Self IP    |
|                    |      |          |          |        |      |addresses  |
|Enterprise Manager  |3.x   |3.1.1     |None      |Medium  |5.3   |Control    |
|                    |      |          |          |        |      |plane      |
|                    |      |          |          |        |      |(management|
|                    |      |          |          |        |      |interface) |
+--------------------+------+----------+----------+--------+------+-----------+
|                    |      |6.0.0 -   |          |        |      |Self IP    |
|                    |6.x   |6.1.0     |None      |        |      |addresses  |
|BIG-IQ Centralized  |      |          |          |        |      |Control    |
|Management          +------+----------+----------+Medium  |5.3   |plane      |
|                    |      |5.1.0 -   |          |        |      |(management|
|                    |5.x   |5.4.0     |None      |        |      |interface) |
|                    |      |          |          |        |      |           |
+--------------------+------+----------+----------+--------+------+-----------+
|F5 iWorkflow        |2.x   |2.3.0     |None      |Medium  |5.3   |Linux      |
|                    |      |          |          |        |      |kernel     |
+--------------------+------+----------+----------+--------+------+-----------+
|Traffix SDC         |5.x   |5.0.0 -   |None      |Medium  |5.3   |Linux      |
|                    |      |5.1.0     |          |        |      |kernel     |
+--------------------+------+----------+----------+--------+------+-----------+

^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

BIG-IP, BIG-IQ, and Enterprise Manager

  o Self IP addresses

    To mitigate risk to this vulnerability for self IP addresses, you can
    configure the Port Lockdown setting to Allow None. For information about
    configuring the Port Lockdown setting, refer to K17333: Overview of port
    lockdown behavior (12.x - 15.x).

    Alternatively, if configuring the Port Lockdown setting to Allow None is
    not an option for your specific environment, you can mitigate this
    vulnerability by using iptables to drop packets with a low MSS value.

  o Control plane (management interface)

    To mitigate risk to this vulnerability for the control plane (management
    interface), you should permit management access to F5 products only over a
    secure network and restrict command line access for affected systems to
    trusted users. For more information, refer to K13309: Restricting access to
    the Configuration utility by source IP address (11.x - 15.x) and K13092:
    Overview of securing access to the BIG-IP system.

    Alternatively, you can also mitigate this vulnerability on the control
    plane (management interface) by configuring iptables to drop packets with a
    low MSS value.

    Impact of mitigation: The impact of dropping packets with a low MSS value
    depends on your specific environment. F5 recommends testing any such
    changes during a maintenance window with consideration to the possible
    impact on your specific environment.

    To add iptables rules in the /config/startup file, use the following
    command syntax:

    /usr/sbin/xtables-multi iptables -A INPUT -p tcp -m tcpmss --mss <min>:
    <max> -j DROP

    /usr/sbin/xtables-multi ip6tables -A INPUT -p tcp -m tcpmss --mss <min>:
    <max> -j DROP

    Where <min>:<max> is the MSS value range you want to drop.

    Important: The 500 MSS value used in the following example is an example
    value; use a value appropriate for your specific environment.

    For example, to drop packets with an MSS under 500, use the following
    iptables rules:

    iptables -A INPUT -p tcp -m tcpmss --mss 1:500 -j DROP
    ip6tables -A INPUT -p tcp -m tcpmss --mss 1:500 -j DROP

  o Protecting backend systems accessed via a FastL4 virtual server (BIG-IP)

    To mitigate risk to this vulnerability for a FastL4 virtual server, you can
    use an iRule to drop packets with a low MSS.

    Impact of mitigation: The impact of dropping packets with a low MSS value
    depends on your specific environment. F5 recommends testing any such
    changes during a maintenance window with consideration to the possible
    impact on your specific environment.

    To drop packets with an MSS of 500 or lower, you can configure the FastL4
    virtual server to use an iRule similar to the following:

    Important: The 500 MSS value used in the following iRule is an example
    value; use a value appropriate for your specific environment.

    when CLIENT_ACCEPTED {
    if { [TCP::mss] <= 500 } {
    drop
    }
    }

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