In 2012, Google introduced Certificate Transparency, an internet-wide tripwire system designed to catch cryptographic "certificate authorities" who abused their position to produce counterfeit credentials that would allow criminals, governments and police to spy on and tamper with secure internet connections.
Since then, CT has caught some pretty egregious offenders, most notably Symantec, whose certificates were nearly phased out of the web earlier this year after being caught in repeated cheats and malpractice.
Now Google — with help from Mozilla — is giving the internet death penalty to WoSign and its subsidiary StartCom, Chinese Certificate Authorities who have been caught forging multiple certificates, including fake Github credentials.
Update: Symantec's certificates were not purged from the web in the end; after a substantial wrangle in which Symantec made promises to reform its processes and in which browser vendors reduced their trust levels in pre-reform certificates, a reprieve was sought and won for Symantec's internet trust death-penalty.
The move to begin blacklisting the CA authority occurred last year. In August 2016, WoSign was caught issuing fake HTTPS certificates for GitHub domains, which are a severe security risk as attackers could use the certificate to impersonate GitHub domains to compromise user communications.
Google and Mozilla then teamed up to investigate the CA and managed to uncover other instances of unauthorized certificates being issued.
Chrome version 56 was the first to disallow certificates issued by WoSign and StartCom, after "technical limitations and concerns" raised concerns that the CA was not complying with Chrome's Certificate Transparency policy.
Mozilla also announced plans to ban new certificates signed off by WoSign and StartCom through Firefox 51, released in January.
Google guillotine falls on certificate authorities WoSign, StartCom