The GPL does not prohibit anyone from using a piece of software. It exclusively limits the actions of bad faith users. If all people engaged with FOSS in good faith, we wouldn't need licenses, because all most FOSS licenses require of the acceptors is to do a couple of small, free activities that any decent person would do anyway. Thank/give credit to the authors who so graciously allowed you to use their work, and if you make any fixes or improvements, share alike.

Security issues like this are a prime example of why all FOSS software should be at least LGPLed. If a security bug is found in FOSS library, who's the more motivated to fix it? The dude who hacked the thing together and gave it away, or the actual users? Requesting that those users share their fixes is farrr from unreasonable, given that they have clearly found great utility in the software.