*At least not to these guys, because they might sue you under german or austrian law**:

Modern Solution GmbH & Co. KG
Federal Ministry of Labour, Social Affairs, Health and Consumer Protection of Austria

**This is not legal advice. Just be careful what you are doing out there!

Why not take a look at open source software? Usually it is easier to find and exploit vulnerabilities if you can look at the code! Also, open source communities are by nature more welcoming towards meaningful contributions.

If you did find a vulnerability in a proprietary software or library, an option would be to disclose it without revealing your identity. The Chaos Computer Club can help you with that: