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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 =========================================================================== AUSCERT External Security Bulletin Redistribution ESB-2019.2185.6 F5 Networks: Excess resource consumption due to low MSS values vulnerability CVE-2019-11479 11 February 2020 =========================================================================== AusCERT Security Bulletin Summary --------------------------------- Product: F5 products Publisher: F5 Networks Operating System: Network Appliance Impact/Access: Denial of Service -- Remote/Unauthenticated Resolution: Patch/Upgrade 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: February 11 2020: Updated Security Advisory Status Table November 18 2019: Updated Security Advisory Status Table 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 - --------------------------BEGIN INCLUDED TEXT-------------------- K35421172:Excess resource consumption due to low MSS values vulnerability CVE-2019-11479 Security Advisory Original Publication Date: 19 Jun, 2019 Latest Publication Date: 11 Feb, 2020 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 - |15.1.0 | | |Self IP | | | |15.0.1 |15.0.1.1 | | |addresses | | +------+----------+----------+ | |Control | | |14.x |14.0.0 - |14.1.2.1 | | |plane | |BIG-IP (LTM, AAM, | |14.1.2 |14.0.1.1 | | |(management| |AFM, Analytics, APM,+------+----------+----------+ | |interface) | |ASM, DNS, Edge |13.x |13.1.0 - |13.1.3.2 |Medium |5.3 | | |Gateway, FPS, GTM, | |13.1.3 | | | |Backend | |Link Controller, +------+----------+----------+ | |systems | |PEM, WebAccelerator)|12.x |12.1.0 - |12.1.5.1 | | |accessed | | | |12.1.5 | | | |via a | | +------+----------+----------+ | |FastL4 | | |11.x |11.5.2 - |11.6.5.1 | | |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 } } Supplemental Information o K78234183: Linux SACK Panic vulnerability CVE-2019-11477 o K26618426: Linux SACK Slowness vulnerability CVE-2019-11478 o K75521003: FreeBSD SACK Slowness vulnerability CVE-2019-5599 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 K9970: Subscribing to email notifications regarding F5 products o K9957: Creating a custom RSS feed to view new and updated documents - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. 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