Question:
RCA: mpx reboot generating core file on 11.1 build 55.13

Answer:
A NetScaler appliance might crash, if a particular sequence of white space and CR-LF characters is sent to an HTTP or SSL virtual server instead of a valid HTTP request- Known Issue BUG ID: 683512

Known issue in the existing firmware build 11.1 build 55.13, it has been fixed in 11.1 build 56.15, upgrading the firmware resolves the issue.

Explanation:
The first packet with only whitespaces that arrives is added to the http incomplete queue, the second packet then starts with whitspaces with \n\n in the middle, the http parser treats this as the end of headers and passes the NetScaler buffer(NSB) chain that includes the first and second packet. Since HINFO CR-LF flag is marked, the http parser drops this NSB chain, with the http in INT state the third packet arrives, the http parsing is skipped causing the load balancing to be skipped, then a series of function calls leads to the panic function being triggered ultimately causing the NS to crash.

To Mitigate the issue, the NetScaler firmware has to be upgraded to 11.1 latest build