What panel you using?
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Well I was about to start a big long rant about this until I saw these two posts below - just take a look at them, and yes the session that is being set contains the hashed password (I'm not stupid).
Okay, now get ready for the [poop] to hit the fan....
In order for this to be a flaw, the user would have to create a session called "rp_username" that contained the malicious code. Problem here is that sessions are all server side therefore the user has no real control over them.
So, on a closing note, if you have the latest security updates to the panel then there is absolutely nothing [that I know of or that anyone has told me of] that would make the panel vulnerable to be hacked three different times.