There is this interesting part: > Furthermore, the attackers attempted to exploit an unpatched vulnerability in an open source software that is widely used by

There is this interesting part: > Furthermore, the attackers attempted to exploit an unpatched vulnerability in an open source software that is widely used by email providers in an effort to bypass spam and abuse filters. We were previously aware of this vulnerability and have already been watching it for some time, but we will not disclose it here because the software in question is not developed by ProtonMail, and it has not yet been patched by the software maintainers. This vulnerability, however, is not widely known and indicates a higher level of sophistication on the part of the attackers. So there's a yet unfixed vulnerability, but some email provider knows about it and also probably some russian spies. Yet protonmail doesn't want to disclose it. Why do they know about it in the first place? Who else knows about it?