In a severe rebuke of one of the biggest suppliers of HTTPS credentials, Google Chrome developers announced plans to drastically restrict transport layer security certificates sold by Symantec-owned issuers following the discovery they have allegedly mis-issued more than 30,000 certificates.

Effective immediately, Chrome plans to stop recognizing the extended validation status of all certificates issued by Symantec-owned certificate authorities, Ryan Sleevi, a software engineer on the Google Chrome team, said Thursday in an online forum. Extended validation certificates are supposed to provide enhanced assurances of a site's authenticity by showing the name of the validated domain name holder in the address bar. Under the move announced by Sleevi, Chrome will immediately stop displaying that information for a period of at least a year. In effect, the certificates will be downgraded to less-secure domain-validated certificates.

More gradually, Google plans to update Chrome to effectively nullify all currently valid certificates issued by Symantec-owned CAs. With Symantec certificates representing more than 30 percent of the Internet's valid certificates by volume in 2015, the move has the potential to prevent millions of Chrome users from being able to access large numbers of sites. What's more, Sleevi cited Firefox data that showed Symantec-issued certificates are responsible for 42 percent of all certificate validations. To minimize the chances of disruption, Chrome will stagger the mass nullification in a way that requires they be replaced over time. To do this, Chrome will gradually decrease the "maximum age" of Symantec-issued certificates over a series of releases. Chrome 59 will limit the expiration to no more than 33 months after they were issued. By Chrome 64, validity would be limited to nine months.

Thursday's announcement is only the latest development in Google's 18-month critique of practices by Symantec issuers. In October 2015, Symantec fired an undisclosed number of employees responsible for issuing test certificates for third-party domains without the permission of the domain holders. One of the extended-validation certificates covered google.com and www.google.com and would have given the person possessing it the ability to cryptographically impersonate those two addresses. A month later, Google pressured Symantec into performing a costly audit of its certificate issuance process after finding the mis-issuances went well beyond what Symantec had first revealed.

In January, an independent security researcher unearthed evidence that Symantec improperly issued 108 new certificates. Thursday's announcement came after Google's investigation revealed that over a span of years, Symantec CAs have improperly issued more than 30,000 certificates. Such mis-issued certificates represent a potentially critical threat to virtually the entire Internet population because they make it possible for the holders to cryptographically impersonate the affected sites and monitor communications sent to and from the legitimate servers. They are a major violation of the so-called baseline requirements that major browser makers impose of CAs as a condition of being trusted by major browsers.

In Thursday's post, Sleevi wrote:

As captured in Chrome's Root Certificate Policy, root certificate authorities are expected to perform a number of critical functions commensurate with the trust granted to them. This includes properly ensuring that domain control validation is performed for server certificates, to audit logs frequently for evidence of unauthorized issuance, and to protect their infrastructure in order to minimize the ability for the issuance of fraudulent certs. On the basis of the details publicly provided by Symantec, we do not believe that they have properly upheld these principles, and as such, have created significant risk for Google Chrome users. Symantec allowed at least four parties access to their infrastructure in a way to cause certificate issuance, did not sufficiently oversee these capabilities as required and expected, and when presented with evidence of these organizations' failure to abide to the appropriate standard of care, failed to disclose such information in a timely manner or to identify the significance of the issues reported to them. These issues, and the corresponding failure of appropriate oversight, spanned a period of several years, and were trivially identifiable from the information publicly available or that Symantec shared. The full disclosure of these issues has taken more than a month. Symantec has failed to provide timely updates to the community regarding these issues. Despite having knowledge of these issues, Symantec has repeatedly failed to proactively disclose them. Further, even after issues have become public, Symantec failed to provide the information that the community required to assess the significance of these issues until they had been specifically questioned. The proposed remediation steps offered by Symantec have involved relying on known-problematic information or using practices insufficient to provide the level of assurance required under the Baseline Requirements and expected by the Chrome Root CA Policy.

In an e-mailed statement, Symantec officials wrote:

As the world’s leading cyber security company and the market leading Certificate Authority, we understand the importance of the trust chain we provide for our customers and everyone who uses the Internet. We learned of Google’s proposal when they posted it on their blog today. Their communication was unexpected and their proposed action is irresponsible. Our SSL/TLS certificate customers and partners need to know that this does not require any action at this time.

Symantec's repeated violations underscore one of the problems Google and others have in enforcing terms of the baseline requirements. When violations are carried out by issuers with a big enough market share they're considered too big to fail. If Google were to nullify all of the Symantec-issued certificates overnight, it might cause widespread outages. The penalties outlined by Sleevi seem to be aimed at minimizing such disruptions while still exacting a meaningful punishment.

The penalties immediately revoke only the status of extended validation certificates issued by Symantec, a move that is likely to be a major annoyance to many Symantec customers and their website visitors, but not make sites unavailable. The untrusting of all Symantec certificates, meanwhile, has a much higher potential of creating Internet-wide problems.

As Sleevi explained it: "By phasing such changes in over a series of releases, we aim to minimize the impact any given release poses, while still continually making progress towards restoring the necessary level of security to ensure Symantec-issued certificates are as trustworthy as certificates from other CAs."

Update 3/24/2017 08:22 PDT: In a blog post published Friday morning, Symantec officials once again criticized the Google post. The officials also disputed the 30,000 certificate figure.

"Google's statements about our issuance practices and the scope of our past mis-issuances are exaggerated and misleading," they wrote. "For example, Google’s claim that we have mis-issued 30,000 SSL/TLS certificates is not true. In the event Google is referring to, 127 certificates—not 30,000—were identified as mis-issued, and they resulted in no consumer harm. We have taken extensive remediation measures to correct this situation, immediately terminated the involved partner’s appointment as a registration authority (RA), and in a move to strengthen the trust of Symantec-issued SSL/TLS certificates, announced the discontinuation of our RA program."

In an e-mail, Google officials wrote: "We appreciate Symantec's response. This remains an ongoing discussion, and we look forward to continuing our conversations with Symantec about this issue. We want to enable an open and transparent assessment of the compatibility and interoperability risks, relative to potential security threats to our users."