Last August, after being alerted by GitHub's security team that the certificate authority WoSign had errantly issued a certificate for a GitHub domain to someone other than GitHub, Google began an investigation in collaboration with the Mozilla Foundation and a group of security professionals into the company's certificate issuance practices. The investigation uncovered a pattern of bad practices at WoSign and its subsidiary StartCom dating back to the spring of 2015. As a result, Google moved last October to begin distrusting new certificates issued by the two companies, stating "Google has determined that two CAs, WoSign and StartCom, have not maintained the high standards expected of CAs and will no longer be trusted by Google Chrome."

WoSign (based in Shenzen, China) and StartCom (based in Eliat, Israel) are among the few low-cost certificate providers who've offered wildcard certificates. StartCom's StartSSL offers free Class 1 certificates, and $60-per-year wildcard certificates—allowing the use of a single certificate on multiple subdomains with a single confirmation. This made the service wildly popular. But bugs in WoSign's software allowed a number of misregistrations of certificates. One bug allowed someone with control of a subdomain to claim control of the whole root domain for certificates. The investigation also found that WoSign was backdating the SSL certificates it issued to get around the deadline set for certificate authorities to stop issuing SHA-1 SSL certificates by January 1, 2016. WoSign continued to issue the less secure SHA-1 SSL certificates well into 2016.

Initially, Google only revoked trust for certificates issued after October 21, 2016. But over the past six months, Google has walked that revocation back further, only whitelisting certificates for domains from a list based on Alexa's top one million sites. But today, Google announced that it would phase out trust for all WoSign and StartCom certificates with the release of Chrome 61. That release, about to be released for beta testing, will be fully released in September.

Mozilla has not yet announced a termination for support of older WoSign and StartCom certificates. However, as most of the certificates issued by the companies have a one-year expiration, many of the whitelisted certificates have already begun to expire.