Intent to Remove: Support for commonName matching in certificates

[Ryan Sleevi]

Primary eng (and PM) emails
rsl...@chromium.org awha...@chromium.org

Link to "Intent to Deprecate" thread

No deprecate thread; it's been deprecated since HTTPS was first introduced (RFC 2818) in 2000.

Summary

Certificates have two ways to express the domain/IP they're bound to - one which is unstructured and ambiguous (commonName), and one which is well-defined (subjectAltName). In the absence of any subjectAltNames, Chrome currently falls back to comparing the domain against the commonName, if present.

This proposal is to remove that fallback path; in effect, requiring a subjectAltName. Ideally, we would do this for all certificates (publicly trusted and privately trusted), but if there are concerns about compat risk, we can restrict it to publicly trusted certificates.

Motivation

Since 1997 (X.509v3's ratification), certificates have had two ways to express a binding to a domain name - either via the commonName attribute within the certificate's subject, or via the explicitly typed (dNSName or iPAddress) of the SubjectAlternativeName Extension.

Since RFC 2818 (published in 2000, first drafted in January 1998), the use of the commonName field has been considered deprecated, because it's ambiguous and untyped - that is, it might contain a human-readable name or it might be a domain name.

The use of the subjectAlternativeName fields leaves it unambiguous whether a certificate is expressing a binding to an IP address or a domain name, and is fully defined in terms of its interaction with Name Constraints. commonName, however, is ambiguous, and because of this, support for the commonName has been a source of security bugs - in both Chrome and the libraries it uses and within the TLS ecosystem at large.

Compatibility Risk

Low. RFC 2818 has deprecated this for nearly two decades, and the Baseline Requirements (which all publicly trusted CAs must abide by) has required the presence of a subjectAltName since 2012.

Mozilla Firefox already requires the subjectAltName for any newly issued publicly trusted certificates since Firefox 48 ( https://bugzilla.mozilla.org/show_bug.cgi?id=1245280 ).

Usage information

From a metrics perspective, less than .002% of publicly trusted certificate validations would require this behaviour (Net.CertCommonNameFallback minus Net.CertCommonNameFallbackPrivateCA).

As 1.57% of privately-trusted CA certificates rely on this behaviour (or 0.1% of all certificate validations), it may be premature to deprecate it for privately-trusted CAs; alternatively, we could remove it with an enterprise policy to allow it for a limited number of releases. Unfortunately, despite being deprecated for nearly 20 years, it's unlikely we'd be able to drive this number down (and improve the security of the ecosystem) without taking further action.

OWP launch tracking bug

https://bugs.chromium.org/p/chromium/issues/detail?id=308330

Entry on the feature dashboard

https://www.chromestatus.com/feature/4981025180483584 (Apologies if I botched this; not sure how to capture "The standard says this is deprecated, and Mozilla supports deprecating")

[fatm...@gmail.com]

RFC 2818 states: (Section 3.1, para 4)

"If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead."

By removing support for the CN field, surely Chrome becomes RFC non-compliant (regardless of deprecation) because the RFC states that "Common Name field in the Subject field of the certificate MUST be used" if the CN is not present? How can the MUST be obeyed if the certificate contains only a populated CN field and no populated subjectAltName field if the use of CN is deprecated in Chrome?

In any case, from the perspective of private PKIs, this may be challenging from an operational perspective. For example, the default "WebServer" template in Microsoft ADCS does not use subjectAltName. Whilst this default template can be copied and modified to include the subjectAltName field, reissuance of all these already issued Web Server certificates may be a significant undertaking for those organisations with private PKIs using Chrome if this policy is implemented.

[Ryan Sleevi]

On Wed, Feb 1, 2017 at 1:25 AM, <fatm...@gmail.com> wrote:

RFC 2818 states: (Section 3.1, para 4)

"If a subjectAltName extension of type dNSName is present, that MUST
   be used as the identity. Otherwise, the (most specific) Common Name
   field in the Subject field of the certificate MUST be used. Although
   the use of the Common Name is existing practice, it is deprecated and
   Certification Authorities are encouraged to use the dNSName instead."

By removing support for the CN field, surely Chrome becomes RFC non-compliant (regardless of deprecation) because the RFC states that "Common Name field in the Subject field of the certificate MUST be used" if the CN is not present? How can the MUST be obeyed if the certificate contains only a populated CN field and no populated subjectAltName field if the use of CN is deprecated in Chrome?

RFC 6125

 

In any case, from the perspective of private PKIs, this may be challenging from an operational perspective. For example, the default "WebServer" template in Microsoft ADCS does not use subjectAltName. Whilst this default template can  be copied and modified to include the subjectAltName field, reissuance of all these already issued Web Server certificates may be a significant undertaking for those organisations with private PKIs using Chrome if this policy is implemented.

It might be; however, Microsoft ADCS is also AD integrated for deployment of those certificates, so it's also arguable that resolving that issue may not be a significant undertaking, because ADCS is highly automated.

In such situations, we rely on our data, enterprise policy controls, and enterprise policy support team, all of which indicate that this concern is unlikely to actually manifest as a significant customer issue. 

[bhu...@gmail.com]

We use our MS CA for other non AD integrated applications, and we actually have the subjectAltName field disabled for any certificates that are only authenticating one domain, user, device, etc. and only have the Common Name field to verify against.

There may be others in this situation as Microsoft actually recommends disabling the SubectAltName field, see this link https://technet.microsoft.com/en-us/library/ff625722(v=ws.10).aspx

Nathan

[PhistucK]

:S

So who is right? Microsoft sort of says that only using commonName prevents impersonation attacks and this intent says it is sort of the other way around. :(

☆PhistucK

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[Adam Langley]

On Thu, Feb 2, 2017 at 9:37 PM, <bhu...@gmail.com> wrote:

We use our MS CA for other non AD integrated applications, and we actually have the subjectAltName field disabled for any certificates that are only authenticating one domain, user, device, etc. and only have the Common Name field to verify against.

There may be others in this situation as Microsoft actually recommends disabling the SubectAltName field, see this link https://technet.microsoft.com/en-us/library/ff625722(v=ws.10).aspx

The document merely notes that SANs need to be carefully reviewed because they specify who a certificate is being issued for. However, if you only use commonNames then you already have to review those names to the same standard. Like many other extension fields, the SAN is sensitive and MS are right to highlight that, but there's no argument there for continuing to wedge hostnames into CN.

Cheers

AGL

[bhu...@gmail.com]

Any Idea what the time frame would be to make the change for private CA's or when you may know if they will be included/excluded for this update to chrome?
I have verified our publicly signed certificates would not be affected.

However, we would need to modify the registry value indicated in the previous article to enable rather then disable the use of the subjectAltName field on our MS CAs and reissue all affected certificates. Hopefully prior to the implementation of this Chrome change...

[Ryan Sleevi]

I'm exploring this with our Enterprise teams to assess the best path to support enterprises such as yourself that may need additional time to accommodate such changes, while also ensuring the vast majority of Chrome users are safe, and setting an appropriate expectation for the timeframe to remove the code. 

[ravi...@gmail.com]

It looks like this change was pushed with Chrome 58 and certificates issued by private CAs are impacted. The initial post and the comments indicated - it would be premature to push this change for certs issued by private PKIs.

[Ryan Sleevi]

On Wed, Apr 19, 2017 at 8:53 PM, <ravi...@gmail.com> wrote:

It looks like this change was pushed with Chrome 58 and certificates issued by private CAs are impacted. The initial post and the comments indicated - it would be premature to push this change for certs issued by private PKIs.

"As 1.57% of privately-trusted CA certificates rely on this behaviour (or 0.1% of all certificate validations), it may be premature to deprecate it for privately-trusted CAs; alternatively, we could remove it with an enterprise policy to allow it for a limited number of releases. Unfortunately, despite being deprecated for nearly 20 years, it's unlikely we'd be able to drive this number down (and improve the security of the ecosystem) without taking further action."

An enterprise policy was added for a limited number of releases. EnableCommonNameFallbackForLocalAnchors

[Ravi Yalamanchili]

This thread had no activity from early Feb and we were not aware of this change until our browsers were upgraded today. Is there one location where we can follow upcoming changes to Chrome?

[PhistucK]

The Chromium blog should announce this kind of information, however, it seems to be missing this change. :(

https://blog.chromium.org/2017/03/chrome-58-beta-indexeddb-20_21.html

Just using the beta channel will likely be useful in this regard.

☆PhistucK

[Ryan Sleevi]

Yeah, I'm not sure how this didn't end up there. I'll try to figure out where the draft changes went, as awhalley@ and I both tried to make sure it went out as part of the communications for 58's promotions along the way.

[pavel....@gmail.com]

Yes, I agree, absolutely there should be some anouncement ... like changing padding lock icon with information this kind of connection will be deprecated. Upgrading to Chrome 58 just killed our company internal connections without any warning.

Dne čtvrtek 20. dubna 2017 8:05:53 UTC+2 PhistucK napsal(a):

> To unsubscribe from this group and stop receiving emails from it, send an email to security-dev...@chromium.org.

[mathew]

Add me to the list of people not happy about this change being slipped in without prominent announcements months in advance. Now I have to scramble to recertify half a dozen servers before I get users complaining.

mathew

[kingr...@gmail.com]

You guys are killing me with these sudden changes. Don't worry, I didn't already have tons of work to do this week.

[sasc...@gmail.com]

God...there are days I really believe some browser programmers are so far away from their users and breaks or complicate HTTPS/SSL/TLS stuff in the name "of security" to the worse by for example breaking or hindering SSL scanning / MITM in most ways possible as by enforcing HSTS in the browsers or this unnecessary change to ignore the CN field in favor of SAN even if CN never was declared dead AFAIK.

Same with SSL warnings in a browser in general. Long time ago I liked chrome for it´s simple, logic and intuitive Cert warning approach "red warning page for a certificate issue" with a direct accessible "Proceed" Button shown instead a oversized warning page which has to be expanded everytime to continue to a warned website. This definately is not a security improvement, this is only enforcing people to follow many sequential steps to reach a website instead simply go to a "proceed" button on a red warning page.

Independent of various discussions by some security researchers and/or developers "MITM breaks your security" IS MITM / SSL scanning one of the best ways for companies to protect normal users against their internet usage against every common sense. And I keep that position against all RC4, TLS1.x etc. discussions, which definately have their right to be discussed due security concerns (on a high level). Todays real threats normal users a confronted are malware or sensitive documents slipping through SSL/TLS secured connectios uncontrolled. I personally prefer any "insecure" RC4 cipers for my connection over any "sorry, we do not scan SSL...this breaks the intent of SSL".

Really...real concerns aren´t nation state hackers exploiting broken or compromised cipher suites....todays companies are more hurt by all that crap entering a network via encrypted connections.

And even if HSTS should "improve" security....Yes, I like to MITM services as Google Search and other websites to halfway effectively block advertisments, trackers and data mining companies and all other stuff which concerns me more than those proposed, theoretical "security issues"

Sometimes I also get tempted to think some "enhancements" are not done to improve security, but to keep money flowing by protecting the ability to flood people with advertisements and track every virtual movement with tons of trackers, analytics and user behaviour webservices, which btw. as side effect also namely slows down the whole surfing experience besides the related security concerns...

Also why shouldn´t I trust for internal use more my internal PKI used for MITM in proxies than trusted certification authorities ?

Really....why is no one developing technologies which proactively assists intended MITM, but maybe allows to place an overlay in a browser or a warning sign somewhere in the browser telling the user "this connection is MITM´ed, as the original certificate got replaced", but allows the users to do their work instead making life of admins in the companies difficult by maintaining tons of SSL exclusions and intermittently get hit by new SSL behaviours or issues due "security" changes. In 99% of the MITM cases these are most likely legitimate SSL Proxies used in companies to protect their infrastructure and users, and a small percentage are real attacks maybe used in some public places or "live hacker demos" on some security events....

my very personal 5 cents...

[Eran Messeri]

On Wed, Apr 26, 2017 at 2:58 PM, <sasc...@gmail.com> wrote:

God...there are days I really believe some browser programmers are so far away from their users and breaks or complicate HTTPS/SSL/TLS stuff in the name "of security" to the worse by for example breaking or hindering SSL scanning / MITM in most ways possible as by enforcing HSTS in the browsers or this unnecessary change to ignore the CN field in favor of SAN even if CN never was declared dead AFAIK.

It was declared deprecated 17 years ago:

https://tools.ietf.org/html/rfc2818#section-3.1 

[sasc...@gmail.com]

BTW - small addendum: My previous post wasn´t specifically placed due this concrete or specific thread/announcement - I mixed various contents from the past and my considerations to all those "SSL/TLS" enhancements and general discussions in terms of MITM usage. I generally support the idea to assist MITM instead making it worse to use in a way which still keeps the ability to inform users, that they may get MITM´ed / SSL scanned, but helps especially companies to protect their infrastructure and users against crap passing encrypted connections.

[met...@gmail.com]

It turns out that CentOS 6 and CentOS 7 ship with OpenSSL configured to generate certificates with only CN, no SAN.

Mozilla's reference guide to server side TLS configuration talks a lot about algorithms, but doesn't even mention subjectAltName:
https://wiki.mozilla.org/Security/Server_Side_TLS

The official OpenSSL site seems to have nothing relevant in the documentation:
https://www.google.com/search?sitesearch=www.openssl.org&q=subjectAltName

Microsoft presents it as an extension which can *damage* security and "increase the risk of impersonation attack":
https://technet.microsoft.com/en-us/library/ff625722%28v=ws.10%29.aspx?f=255&MSPPError=-2147217396

Given the above and the fact that I'd never seen a single set of instructions for SSL even mention SAN before all our certificates stopped working, I'm betting there are a lot of angry people out there.

Next time, please:
- Evaluate how many people are likely to be affected
- Make prominent public announcements, not on a developer mailing list
- Provide instructions for fixing the problem
- Deprecate, rather than proceeding straight to the "throw up an error page" approach

It should have been handled more like the SHA1 or TLS 1.0 deprecation transitions.

mathew

[Ryan Sleevi]

On Wed, Apr 26, 2017 at 10:18 AM, <met...@gmail.com> wrote:

It turns out that CentOS 6 and CentOS 7 ship with OpenSSL configured to generate certificates with only CN, no SAN.

OpenSSL is not a tool to be used to generate certificates that are expected to 'just work'. For example, an out of the box OpenSSL on these platforms also uses an invalid string encoding. It is meant to provide a tool and framework for folks to configure appropriately.

 

Mozilla's reference guide to server side TLS configuration talks a lot about algorithms, but doesn't even mention subjectAltName:
https://wiki.mozilla.org/Security/Server_Side_TLS

That's because it's not something server operators need to be aware of, because it's been required of CAs since 2012.

 

The official OpenSSL site seems to have nothing relevant in the documentation:
https://www.google.com/search?sitesearch=www.openssl.org&q=subjectAltName

The difficulty of documentation in OpenSSL is unsurprising, especially as they've significantly reorgnized the site and removed a lot of their excellent documentation.

For what it's worth, the page is https://www.openssl.org/docs/manmaster/man5/x509v3_config.html , which is the official manual page for generating X.509 certificates.

 

Microsoft presents it as an extension which can *damage* security and "increase the risk of impersonation attack":
https://technet.microsoft.com/en-us/library/ff625722%28v=ws.10%29.aspx?f=255&MSPPError=-2147217396

To be clear, Microsoft's aptly stating that, on older versions of Windows, it's a security risk to allow clients to automatically enroll (thereby control) the values for the subjectAlternativeName, because the autoenroll system does not validate these names are accurate (unlike what they do for directory names). Subsequent versions of Windows do allow auto-enrollment of SANs, provided they match the FQDN of the enrolling machine. For non-machine enrollment (e.g. end-user), it's also correct that having them generate arbitrary SANs is dangerous - much like allowing the attacker to say "I am google.com". For those cases, they rightfully recommend that you require manual review.

I'm afraid you've misread this documentation, but I can understand that if you're looking to support the view the SAN isn't required, this would seem appealing.

 

Given the above and the fact that I'd never seen a single set of instructions for SSL even mention SAN before all our certificates stopped working, I'm betting there are a lot of angry people out there.

Next time, please:
 - Evaluate how many people are likely to be affected

We did. As you can see on the original announcement, the number of validations affected represent an incredibly small fraction of both users and site operators. It also is exclusively limited to those managing their own PKIs that have not followed the industry standards - as noted, it's been deprecated since HTTPS itself was introduced.

 

 - Make prominent public announcements, not on a developer mailing list

This was tracked on chromestatus.com. It also went out with our Enterprise release notes regularly in advance. There's also the opportunity to run Dev and Beta versions within your organization, to help surface any concerns you might have well before it's launched.

 

 - Provide instructions for fixing the problem

As noted, EnableCommonNameFallbackForLocalAnchors is an enterprise policy to work around this. However, in doing so, you introduce security risks to your organization, and should do so sparingly.

 

 - Deprecate, rather than proceeding straight to the "throw up an error page" approach

What do you believe this means? Particularly, what do you believe this should mean for situations where there is an active security bug?

 

It should have been handled more like the SHA1 or TLS 1.0 deprecation transitions.

Thank you for your feedback. Certainly, I wish we would have phased these out much sooner - as noted on the bug, it was something I've been tracking since 2013 to measure the impact of such a phase out, to make sure we had accurate numbers.

As always, you can help make sure your concerns are surfaced by running earlier versions (Dev, Beta, or Canary), and as an enterprise, opting in to anonymous usage stats to help ensure the impact to your organization is measured. While we always strive to provide ample notice to the ecosystem, it's also worth noting we're not the only one to have deprecated this, and that there are indeed active security issues that arise from continuing to support this. In balacing the risk versus compatibility, the risk (to all Internet users) was much, much greater than the compatibility impact to the limited number of Enterprises, for whom mitigation options were and are provided, and for which communications went out. For enterprise-focused customers, https://enterprise.google.com/chrome/chrome-browser/ can be a one-stop portal for signing up to updates and even support agreements to help measure, evangelize, and minimize any impact to Enterprise-specific use cases.

[Ryan Sleevi]

On Wed, Apr 26, 2017 at 9:58 AM, <sasc...@gmail.com> wrote:

God...there are days I really believe some browser programmers are so far away from their users and breaks or complicate HTTPS/SSL/TLS stuff in the name "of security" to the worse by for example breaking or hindering SSL scanning / MITM in most ways possible as by enforcing HSTS in the browsers

If you're having issues, it's not related to Chrome policies. https://dev.chromium.org/Home/chromium-security/security-faq#TOC-How-does-key-pinning-interact-with-local-proxies-and-filters- documents our policies here that apply to both HSTS and HPKP, but errors are not fatal if you're using a properly configured scanning software legitimately installed on a device operated and owned by the Enterprise organization. As long as its your device, and is properly observing the protocol specifications, we aim to minimize impact.

 

or this unnecessary change to ignore the CN field in favor of SAN even if CN never was declared dead AFAIK.

It was declared dead in RFC 2818. Support for commonName allows for bypassing restrictions on certificates in such a way that it's possible to issue a legitimate certificate for any valid domain undetectably, and without consequence, because it's fully conforming to all the valid specifications.

As I'm sure you can understand, allowing arbitrary parties to Internet-wide intercept communication is something we treat as a Very Serious security issue.

 

Same with SSL warnings in a browser in general. Long time ago I liked chrome for it´s simple, logic and intuitive Cert warning approach "red warning page for a certificate issue" with a direct accessible "Proceed" Button shown instead a oversized warning page which has to be expanded everytime to continue to a warned website. This definately is not a security improvement, this is only enforcing people to follow many sequential steps to reach a website instead simply go to a "proceed" button on a red warning page.

Thank you for your feedback. Have you had the opportunity to review the peer-reviewed scientific research that explains these changes, and the process and methodology for making them? It might help explain how this is, from a quantifiable measured perspective, an improvement overall, even if individual opinions may vary. 

You can find just a sampling of these efforts at:

https://research.google.com/pubs/pub41323.html

https://research.google.com/pubs/pub41927.html

https://research.google.com/pubs/pub43265.html

https://research.google.com/pubs/pub45374.html

https://research.google.com/pubs/pub45366.html

I've refrained from addressing most of your other points, as I think you acknowledge many of them are opinions, and as such, we can reasonably disagree. We see a variety of threats against users, both from inside the organization and from outside, and do our best to provide a browser that can provide security for all users. While understandably, this may occasionally conflict with an individual organizations good, it's a collective health problem in which sometimes the needs to provide a consistent, baseline assurance of security outweighs those of individual organizations who may have the necessary technical skill and sufficiency to operate things in a way that's reasonable for their threat model. That said, I think most of those actively practicing in the field would know the limits of our knowledge and ability, and suggest that this is not realistically possible, but that's simply the result of knowing our limitations, the threats, and the perfect skills required to even get remotely close to that level.

[PhistucK]

>  It also went out with our Enterprise release notes regularly in advance.

Where can I find those "Enterprise release notes"?

I searched for chrome release notes "commonName" and nothing came up.

☆PhistucK

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[Ryan Sleevi]

Sign up at the site I mentioned.

[Ryan Sleevi]

And it was also captured in https://developers.google.com/web/updates/2017/03/chrome-58-deprecations but it looks like there's a JS error on the page at the moment. I let folks know.

[PhistucK]

:( Why is there no page, blog or similar for that as well? In order to keep track of the readers and be able to contact them in critical change cases?

☆PhistucK

[Ryan Sleevi]

There is https://developers.google.com/web/updates/2017/03/chrome-58-deprecations . The Enterprise notes focus on drawing specific attention to Enterprise-oriented concerns, which I'm sure you can understand aren't useful in a broadly reaching communication. But there was the broad-reaching comms at the Developer site. It just didn't show in the Beta notes.

[PhistucK]

I do not mind about it not being broad reaching, but a place that keeps all of the release notes of the past releases makes perfect sense to me. For example, say I signed up after the Chrome 58 Enterprise release notes came out (but before it was released to stable), I am going to miss them forever and will be surprised by the breaking changes.

This is actionable. I can delay the installation of Chrome 58 for a short while until I get the breaking changes sorted in the intranet.

It is 2017, not 1997. Archives are very common nowadays.

☆PhistucK

[Eric Lawrence]

> Archives are very common nowadays.

Am I missing something?

https://developers.google.com/web/updates/2017/03/chrome-58-deprecations

https://developers.google.com/web/updates/2017/02/chrome-57-deprecations

https://developers.google.com/web/updates/2016/#api_deprecations_and_removals_in_chrome_56

https://developers.google.com/web/updates/2016/#api_deprecations_and_removals_in_chrome_55

https://developers.google.com/web/updates/2016/#api_deprecations_and_removals_in_chrome_54

https://developers.google.com/web/updates/2016/#api_deprecations_and_removals_in_chrome_53

https://developers.google.com/web/updates/2016/#api_deprecations_and_removals_in_chrome_52

...

-Eric