Did you Bitcoin Core had an alert system where developers could send messages to users?

This was added by Satoshi after the inflation bug in August 2010

They could send a 256 bytes message to your node, like in the picture.

Satoshi, Gavin and theymos were the 3 known people to have access to the trigerring key.

It was speculated Mark Karpeles(Mt. Gox CEO) had one.

The only one to trigger these alerts was Gavin.

Even though it could only send a text message that was sent to your node, it was still dangerous.

Some rogue actor could ask people to update to a bad client, and claim a vulnerability.

Because of this it was completely removed in 2016, and the private key was made public.

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