If the issue can be caused by inserting null bytes into the session, then it sounds like the signed cookie verification might have stopped at the first null byte, allowing the attacker to place other data after the null byte which the application would continue to read and use.

It looks like they used the null byte as a separator with the assumption that users couldn't submit a null byte. So attacker could submit a value like foo\x00user:admin and it would be serialised as user:bad\x00key:foo\x00user:admin and deserialized as key:foo, user:admin. that's my takeaway from the patch. I haven't played with it so I could be off track.