[Security Issue]

Anybody knows how to report a security issue in arduino site?

The GITHUB repo is the best place.

There is also the CONTACT US link at the bottom of the page.
What appears to be the issue ?

Typically best practices would be to first notify them privately, then make a public disclosure if the stated amount of time has passed without the issue being addressed. The webmaster's email is:
webmaster (at) arduino.cc.
I haven't found him to be terribly responsive but if this is a serious matter then I'd hope that will not be the case.

If it's a problem specific to the Arduino forum then the appropriate place to submit a public issue report is here:

If it's a problem with the website in general then appropriate place to submit a public issue report is here: