Security could be thought of as a subset of "features", hence it is caught up in the eternal feature vs. stability compromises (more on that here: viewtopic.php?f=61&t=17101
) It's also largely the responsibility of the app developer/maintainer - they have to release a security bug-fix update in order for it to be distributed to all users. In the case you mention, I bet it's one of the following situations:
1) The vulnerability is unknown, or known but unpatched
2) The vulnerability is known, but has only been patched in their newer main version, which introduces its own instability issues
3) The vulnerability is known, and has been patched in a bugfix to the version Mint uses, but it hasn't made it through the stability checks yet
In the case of (1) feel free to try to create a patch if you can, and/or disable the app in the meantime.
In the tricky case of (2), you could try to backport the patch yourself, or you could compromise on stability in favour of security and install the newer version.
In the case of (3), you could try to incorporate the patch yourself or you could just wait for it to make it through. If the app developer made a clean patch for the security issue, then it will pass the stability checks.
It's all about choice, in this case you obviously value security extremely highly. If enabling debian repos will solve your problem, then feel free, but be aware it will compromise your system's stability. Can you give an example of an unpatched security vulnerability in a Mint app, or is this just hypothetical?
EDIT: I see you've replied - how old is old?
If you have a question that has been answered and solved, then please edit your original post and put a [SOLVED] at the end of your subject header
Hint - use a google search including the search term site:forums.linuxmint.com