F5 Networks BIG-IP : Linux SACK Slowness vulnerability (K26618426) (SACK Slowness)

high Nessus Plugin ID 129310


The remote device is missing a vendor-supplied security patch.


Jonathan Looney discovered that the TCP retransmission queue implementation in tcp_fragment in the Linux kernel could be fragmented when handling certain TCP Selective Acknowledgment (SACK) sequences. 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 commit f070ef2ac66716357066b683fb0baf55f8191a2e. (CVE-2019-11478)



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 denial of service by sending a sequence of specially crafted TCP SACK packets.


Upgrade to one of the non-vulnerable versions listed in the F5 Solution K26618426.

See Also


Plugin Details

Severity: High

ID: 129310

File Name: f5_bigip_SOL26618426.nasl

Version: 1.5

Type: local

Published: 9/25/2019

Updated: 2/12/2020

Configuration: Enable paranoid mode

Risk Information


Risk Factor: Medium

Score: 5.1


Risk Factor: Medium

Base Score: 5

Temporal Score: 3.7

Vector: AV:N/AC:L/Au:N/C:N/I:N/A:P

Temporal Vector: E:U/RL:OF/RC:C


Risk Factor: High

Base Score: 7.5

Temporal Score: 6.5

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Temporal Vector: E:U/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:f5:big-ip_access_policy_manager, cpe:/a:f5:big-ip_advanced_firewall_manager, cpe:/a:f5:big-ip_application_acceleration_manager, cpe:/a:f5:big-ip_application_security_manager, cpe:/a:f5:big-ip_application_visibility_and_reporting, cpe:/a:f5:big-ip_global_traffic_manager, cpe:/a:f5:big-ip_link_controller, cpe:/a:f5:big-ip_local_traffic_manager, cpe:/a:f5:big-ip_policy_enforcement_manager, cpe:/a:f5:big-ip_webaccelerator, cpe:/h:f5:big-ip

Required KB Items: Host/local_checks_enabled, Host/BIG-IP/hotfix, Host/BIG-IP/modules, Host/BIG-IP/version, Settings/ParanoidReport

Exploit Ease: No known exploits are available

Patch Publication Date: 6/19/2019

Vulnerability Publication Date: 6/19/2019

Reference Information

CVE: CVE-2019-11478