If you find a security flaw, please follow the following steps in order:
1) Give details to upstream developers to develop or merge a fix
2) Notify downstream developers/distributors of the risk and how to patch it (not the details) so they can ship it ASAP
3) Give people at least a week to upgrade on their own
4) Notify the public that a vulnerability in the old version exists (no details)
5) After you can reasonably hope everyone has updated (months or years later), publish details and/or proof-of-concept
There may be circumstances (eg, active exploitation in the wild) that justify deviation from this, but generally, this is a good approach.
However, starting with #5 is black-hat behaviour.