Lately I have seen this in my error log (1 per day, and I have 40k visitors per day):
[22-Sep-2009 21:13:52] PHP Warning: session_start() [function.session-start]: The session id contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in /var/my_files/class.session.php on line 67
[22-Sep-2009 21:13:52] PHP Warning: Unknown: The session id contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,' in Unknown on line 0
[22-Sep-2009 21:13:52] PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct () in Unknown on line 0
This is not a config issue because it is working for everybody.
I already modified php.ini to have this:
session.use_only_cookies = 1
session.use_trans_sid = 0
I suspect a session hijacking or a kind of attack I am not aware of (I am parano ;) ).
Do you have any idea what it could be? What can I do to improve the security and avoid this?