Bug ID 667618: Hardware SYN Cookies may not deactivate after the SYN attack ends and valid TCP traffic starts

Last Modified: Sep 13, 2023

Affected Product(s):
BIG-IP LTM, vCMP(all modules)

Known Affected Versions:
11.5.4, 11.5.5, 11.5.6, 11.5.7, 11.5.8, 11.5.9, 11.5.10, 11.6.0, 11.6.1, 11.6.2, 11.6.3,,,,, 11.6.4, 11.6.5,,,, 12.0.0, 12.0.0 HF1, 12.1.0 HF1, 12.0.0 HF2, 12.1.0 HF2, 12.0.0 HF3, 12.0.0 HF4, 12.1.1 HF1, 12.1.1 HF2, 12.1.2 HF1, 12.1.2 HF2, 12.1.0, 12.1.1, 12.1.2, 12.1.3,,,,,,,, 12.1.4,, 12.1.5,,,, 12.1.6, 13.0.0, 13.0.0 HF1, 13.0.0 HF2, 13.0.0 HF3, 13.0.1, 13.1.0,,,,,,,,, 13.1.1,,,,, 14.0.0,,,,,, 14.0.1,

Fixed In:
14.1.0, 13.1.3

Opened: Jun 02, 2017

Severity: 3-Major


Hardware SYN Cookies activated on a virtual server under a SYN attack may not deactivate after the SYN attack ends and valid TCP traffic starts. The non-supported TCP options under SYN Cookie protection will continue to be unsupported until the machine exit hardware SYN cookies.


This can successfully cause hardware SYN cookies to be activated on the BIG-IP virtual server under attack. However, once the attack subsides and falls below the SYN check threshold, SYN cookies may not immediately deactivate. Because SYN cookie protection is still active, and because under SYN cookie protection some TCP options are not supported, the options will not be taken into account when processing traffic. For example, under SYN cookie protection, MSS is fixed to a few sizes. For traffic that arrives with a different MSS size, the system uses a supported size instead. Note that if no good traffic hits the virtual server, syncookies will also fail to deactivate, but will do so once both good traffic has been seen, and the attack has ended.


A SYN flood attack or similar SYN attack where SYNs are flooded into the BIG-IP system.


Restarting TMM or rebooting the device will clear the HSB issue

Fix Information

Hardware SYN Cookies now immediately deactivate after the SYN attack ends and valid TCP traffic starts.

Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips