If you are the maintainer of a package which is contained in the Debian archive please consider looking over the code yourself.
The availability of source code auditing tools can ease this process significantly, even if you don't have the time to do a thorough audit yourself you can find areas which are potentially problematic.
If you require assistance consider joining the debian-audit mailing list and asking for a volunteer to look over your package.
As part of being a responsive maintainer you should also be keeping an eye upon new releases of your package upstream. If the changelog mentions a security problem you should attempt to see if you have a version of the code in the stable distribution which is vulnerable.
If you do have a vulnerable version available in the stable distribution then please contact the security team - as described in the security team FAQ.