This is a very legalistic question, and I am not a lawyer. This answer does not constitute legal advice in any way.
DMCA is intended to protect copyright holders from piracy, but there is a provision in the statute that allows for copyright holders to pursue action against anyone who aids in the circumvention of copyright protections.
The “act” prohibition, set out in section 1201(a)(1), prohibits the
act of circumventing a technological measure used by copyright owners
to control access to their works (“access controls”). So, for example,
this provision makes it unlawful to defeat the encryption system used
on DVD movies.
By offering information about vulnerabilities on 3rd party websites, you are opening yourself up to these types of complaints, and you may have legal exposure for damages incurred.
The burden of proof is on the complainant. To enforce any DMCA claim, the complainant would need to file a lawsuit against you (they could do it in their jurisdiction or yours) and prove in court that their copyright was harmed by your actions.
Could that happen? Maybe. They can definitely definitely take you to court if they have the resources to do so. If you don't show up or send a lawyer, then you will lose. Remember that the case may be somewhere you don't live.
It's up you to decide how you will proceed.