Bug ID 672480: WebSSO plugin process may become unresponsive in rare situations for Kerberos SSO

Last Modified: May 14, 2019

Bug Tracker

Affected Product:  See more info
BIG-IP APM(all modules)

Known Affected Versions:
11.5.1, 11.5.1 HF1, 11.5.1 HF10, 11.5.1 HF11, 11.5.1 HF2, 11.5.1 HF3, 11.5.1 HF4, 11.5.1 HF5, 11.5.1 HF6, 11.5.1 HF7, 11.5.1 HF8, 11.5.1 HF9, 11.5.2, 11.5.2 HF1, 11.5.3, 11.5.3 HF1, 11.5.3 HF2, 11.5.4, 11.5.4 HF1, 11.5.4 HF2, 11.5.4 HF3, 11.5.4 HF4, 11.5.5, 11.5.6, 11.6.0, 11.6.0 HF1, 11.6.0 HF2, 11.6.0 HF3, 11.6.0 HF4, 11.6.0 HF5, 11.6.0 HF6, 11.6.0 HF7, 11.6.0 HF8, 11.6.1, 11.6.1 HF1, 11.6.1 HF2, 11.6.2, 11.6.2 HF1, 11.6.3,

Fixed In:, 11.5.7

Opened: Jul 05, 2017
Severity: 2-Critical
Related AskF5 Article:


HTTP requests that are being processed by Kerberos SSO never leaves APM, and connections simply time out.


A deadlock occurs within the Kerberos SSO. Eventually there will be a global deadlock, which causes this particular WebSSO process to be completely unresponsive for Kerberos SSO functionality. APM end users cannot access the backend.


There is an issue in MIT krb5 library for calculating wait time for responses from KDC, which ends up with a negative value. This translates to infinite timeout by poll() syscall. At the same time, if all Kerberos requests to KDC are dropped (e.g., by a misconfigured firewall), Kerberos SSO never receives the responses, and Kerberos SSO never gives up on waiting for the KDC response (this is an issue in the library).


For this issue to have a real impact, there must be an unresponded-to Kerberos request. To eliminate this possibility, make sure there is no firewall blockage, incorrect routing, etc., so that WebSSO always receives responses, even negative ones. Note: WebSSO will never use infinite timeout when waiting for Kerberos responses, so even if a firewall blocks the Kerberos request, although Kerberos SSO does not function, it does not cause global unresponsiveness from the WebSSO process.

Fix Information


Behavior Change