Tag Archives: Security

How did the Terminal Services Licensing PKI effect you?

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The other day I did a post on the age of the Microsoft PKI that was used for Terminal Services Licensing, today I thought I would talk about what that age meant in the context of the vulnerabilities it introduced.

The oldest certificate I have been able to find from the same hierarchy is from April 1999 (that’s the issuance date for the Microsoft Enforced Licensing PCA).

Based on the post from the Microsoft Security Research & Defense blog  we know that the reason the attacker had to do the MD5 collision was that as of Vista there was a change in the way critical extensions were handled.

This change made it so that Vista clients would fail when they saw a certificate that contained an unknown critical extension in a signing certificate making it an ineffective attack vector for those clients.

But what does this mean for the period of time before that? Well Windows Vista was released in November of 2006, that’s nearly 8 years in which any enterprise with a terminal services deployment could have owned a Windows PC “As Microsoft” or potentially attack a PKI based system with “trusted” but fraudulent certificate.

But did it really get better in with the release of VISTA? According to StatCounter Windows Vista received its maximum market share of 23% in October of 2009. Yes, two years after the release of Vista 77% of the Windows clients on the Internet were still vulnerable as a result of the design of the terminal services licensing solution.

Things didn’t really start to get better until XP SP3 which was released in April of 2008 as it contained the same certificate validation engine that was found in Vista.

While I do not have any public statistics I can share I can say that this service pack was picked up faster than any other service pack up until that point which says a lot since it was not a “forced” update.

If we are optimistic and say that it took only one year to get 100% penetration and we believe stat counters statistics for XPs Market share of 71% in April 2009 that it took till 2009 to get to 95% patched.

Now these numbers are just for clients on the Internet and not servers. This “fixed” chain validation engine wouldn’t have even found its way into the Windows Server code base until Windows Server 2008 which was released in February 2008 but took some time to get broadly used.

While Windows Servers are not terribly common on the Internet they are extremely common in the Enterprise, especially in 2009 where it had 73.9% market share. Again I don’t have numbers but antidotal we know that Enterprises are notoriously slow to upgrade or patch.

So where does this leave us in 2012? Still vulnerable that’s where.

The other day Microsoft released a patch that in-essence revokes the PKI in question and today WSUS announced their patch that introduces additional pinning.

You need to apply both to secure your systems.

Ryan

How one typically verifies code comes from Microsoft

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Microsoft signs all code that ships from it (that is except for the case when it doesn’t); so how does one verify that the code came from them vs. anyone with a fraudulent certificate claiming to be them?

Well the answer is pinning, but the pinning for the most part does not happen at the signing certificate level as those keys change to frequently to be pinned. The pinning happens at the certificate issuer level, much like has been proposed by CAA.

So to understand how this happens in code you need to understand code signing in Windows a little. There are two types of signatures those that are “embedded” and those that are “attached”.

Authenticode would be an example of an embedded signature; Catalog Files on the other hand are detached signatures. There are other examples of both of these but in general these other formats have the same abstract properties.

So when do you use one vs. the other? Well if a user will download your binary directly (ActiveX, Setup, etc.) you would typically embed a signature.

If that’s not the case you would use a detached signature because a single signature can be placed on a list of binaries without increasing the package size (it’s one signature on n hashes). You produce these signatures with SignerSignEx.

For the two signature formats I mentioned you do your signature verifications using an API called WinVerifyTrust. You can also use this API to get information out of a signature (who signed it, when, etc.).

With that information you do a certificate validation that is accomplished with the CertGetCertificateChain API, that gives you a CERT_CHAIN_CONTEXT which you can use to call CertVerifyCertificateChainPolicy.

This API is used to do application specific verifications, for example in the case of TLS the CERT_CHAIN_POLICY_SSL provider is used – this is where the name and EKU verification logic for Windows exists.

The providers we care about for our story today are:

  • CERT_CHAIN_POLICY_AUTHENTICODE – Is this certificate appropriate for code-signing?
  • CERT_CHAIN_POLICY_AUTHENTICODE_TS – Is this certificate appropriate for time stamping code?
  • CERT_CHAIN_POLICY_MICROSOFT_ROOT – Is this certificate ultimately issued by a Microsoft Product Root?

The last one is the one in has an array of hashes in it, I don’t recall if they are key hashes or thumbprints of the certificate. In essence what it does is look at the top of the chain to see if the Root matches one of these hashes if it does it passes, if it doesn’t it fails.

Applications may choose to do additional “pinning” also, for example in the case of Windows Update Services a manifest is exchanged between the client and server, that manifest comes down over SSL they may choose to also do pinning to a set of certificates for the SSL session or apply digital signatures to the manifest and do pinning for the keys that are used to do signing.

I do not know personally for a fact that any such pinning takes place but I have been told by several people I respect that it does.

It looks as if Windows Update was not doing pinning but as of this week it will, this is of course  a best practice and I am glad to see them doing it now.

So now you know how to validate that code is coming from Microsoft, or at least how it’s supposed to work.

Issues like represented in MSRC 2718704 / Flame put some of these assumptions in question, it’s clear that pinning is an important concept though and one that should be supported more broadly.

Ryan

The tale of two (Microsoft) PKIs

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As you know I used to work at Microsoft on areas surrounding cryptography, certificates, protocols and other such things.

While I can’t talk about the internal workings of things I can explain the high level roles the two sets of PKIs Microsoft maintains.

The first PKI is the Microsoft “Product Roots”; these are the keys and certificates that are managed with the sole purpose of being used for product scenarios. You can see the names and serial numbers of these CAs here.

The next is the “IT Root”, that is the one folks see associated with a S/MIME email from a Microsoft employee, some of the corporate websites have SSL certificates from this hierarchy and if you were to inspect the badges of an employee you would see a certificate from here that is used for smart card login and other related scenarios.

The “Product Roots” are trusted only by Windows and applications that have either built on the Windows CryptoAPI or simply imported the roots from the Microsoft Root Program.

The “IT Root” is trusted by third-party browsers because it has been cross signed with a public CA, currently that CA is Verizon the certificate is labeled “GTE CyberTrust”.

I hope that helps clarify for people,

Ryan

MSRC 2718704 and the age of the rogue certificates

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One of the things I love about security is how security researchers are so passionate about their jobs; they for the most part respect their peers and enjoy learning about attacks so they are willing to share information.

As a result of this dynamic over the past few days I have had a chance to look at a number of details about the #Flame malware as well as the rogue certificates it used as part of the its Windows Update attack.

With that said the raw data having already been published here quite a long time ago I think it’s OK to go into details on this now (Another interesting data point is that this link is discussing how to bypass the Terminal Services license mechanism).

This certificate chain gives us some insights into the age of the solution Microsoft was using to license Terminal Server Clients, for example one certificate chain I received shows us the following:

   
   
   
   
 

 

From this we can tell a few things, including:

  1. The licensing solution has been in use since at least 1999 – Its issuance date is ‎Thursday, ‎April ‎15, ‎1999 12:00:00 AM, we can assume this is the case if we assume the “Microsoft Enforced Licensing Intermediate PCA” is in a Microsoft vault (very safe to assume).
  2. The “Licensing Server” was built to include unverified information in each license originally – We can assume this since the other certificates we saw included no organization details in the subject name while this one has a DN of  “CN = Terminal Services LS, L = Stradella, S = PVC =  IT, E = ater**@libero.it”
  3. The PKI was rebuilt in around 2004 – In the first certificates we saw the Microsoft operated CA that issued the “licensing server” (Microsoft Enforced Licensing Registration Authority CA) expired in 2009, in the most recent certificate the chain is a one leaf shorter but same logical entity expires in ‎2004.
  4. The format of the certificates changed around 2004 – In the first certificates we saw only a single proprietary extension, in this one we see four. This suggests that there was a code change in how licenses were validated around this time. This also approximately aligns with when one might expect the Windows Server development to begin for the next version (at the time development cycles were 3-4 years).
  5. Attackers were using this in combination with other attacks even then – In 2003 there was a security defect in the way Windows handled Basic Constraints (see MS 02-050). This allowed certificates that were not CAs to be trusted to issue CA certificates.

In this most recent certificate chain we can see that a Terminal Services LS (a license) has issued another certificate, the validity of this certificate was based on a combination of two factors:

    • The root CA being trusted by the entity validating the certificate.
    • The license certificate being able to issue other certificates.

This combination allowed the attacker to simply have access to a CAL, not even the License Server and produce a certificate that could be used to attack another system – I will do a post about this later.

  1. Terminal Services doesn’t do full certificate validation on its licenses – The post I got this certificate from states it works on machines as recent as 2008 but the “Terminal Services LS” expires in in 2004, the certificate that was minted bellow it (the evil “PORTATILE” one) is good until 2038. This would not pass the CryptoAPI certificate validation logic on those platforms due to the BasicConstraints restrictions on the certificate and the date issues alone.

So in summary, this issue has been around a very long time, much longer than Flame or Stuxnet. There are a few very interesting conclusions I come to as I write this:

  1. Even without the Basic Constraints issue no collision attack would have been needed to make use of this original Licensing PKI .
  2. Despite the changes in the client behavior in Windows Vista in how Critical Extensions are handled, the weak cryptography made it practical to continue to use this PKI maliciously.
  3. Those who knew about this tried to keep it a secret as long as possible as it was very valuable.
  4. Despite developing a world-class security program over the last decade this issue managed to fly under the radar until now.

MSRC 2718704 and Terminal Services License Certificates

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When looking at the certificates the Flame authors used to sign the malware in their Windows Update module one has to wonder what was different between a normal terminal services license certificate and the one they used.

Well getting at one of these certificates is a little awkward as they are kept in the registry vs. in the traditional certificate stores, thank goodness with a little due diligence you can find one.

   
   

 

So from this what can we learn? There are a few things:

  1. Every subordinate CA has the same name “Microsoft LSRA PA”
  2. Every terminal services license has the same name “Terminal Services LS”
  3. The license appears to have a random serial number (even the CA certs did not!)
  4. The license has a 512bit RSA key.
  5. The license is signed with MD5.
  6. No restrictions have been explicitly placed on this certificate.
  7. This license is good for 257 days.
  8. The CRL url is invalid.
  9. The Certificate Issuer url is invalid.
  10. It contains a proprietary extension (1.3.6.1.311.18)

As far as best practices go there are clearly a lot of problems here:

  1. Same name for every CA – the point of a CA is to identify an entity, if every CA has the same name this can’t really happen.
  2. Same name for every subject – while the certificates here are for authenticating entitlement to make a connection and not to authenticate an identity the subject should still be unique.
  3. Over entitled– The certificate contains no Extended Key Usage or other restrictions, as a result this certificate is only as restricted as its issuers issuer is, namely it is good for:
    • Code Signing (1.3.6.1.5.5.7.3.3)
    • License Server Verification (1.3.6.1.4.1.311.10.6.2)
  4. Weak keys – RSA 512 bit was factored in 1999, there should be no CA issuing 512 bit keys period especially in a PKI as powerful as this, especially for one that is good for code signing.
  5. Weak hash algorithm – MD5 has been known to be weak since 1993 and has had increasingly effective attacks published against it ever since — There should be no CA issuing MD5 signed certificates in 2012.
  6. Invalid URLs – This indicates that the CA was not actively managed, it also included internal only file references in the certificate also a no-no and a indicator of the same.
  7. Poorly thought out proprietary extension – The extension is not well thought out, the OID (1.3.6.1.311.18) is an arc for Hydra (Terminal Services) and not a unique identifier (or shouldn’t be at least) for a specific extension. This isn’t harmful but a signal the design didn’t go through much review.

The other items are odd, like the 257 day validity window and the proprietary extension but they are not problematic – at least on the surface.

Well that’s what we know so far.

 

MSRC 2718704 and Nested EKU enforcement

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There are a number of technical constraints a Certificate Authority can put into place on a subordinate Certificate Authority; the general concept is referred to as Qualified Subordination.

One of the most important ways to constrain a certificate is through by restricting what it can be good for.

The foundation for such a constraint is provided by PKIX in the Extended Key Usage extension (RFC 5280), this extension can be put into a certificate to restrict what it is trusted for – for example a certificate might be OK for SSL Server Authentication but not for S/MIME.

The problem is the RFC provides no practical guidance on how to act when this certificate is encountered in a CA certificate, all it says is:

In general, this extension will appear only in end entity certificates.

One can interpret this to mean that its semantics are the same in issuer or subscriber certificates, this makes sense but isn’t very useful as a CA is not very likely to ever perform “application tasks” like S/MIME or SSL Server authentication with its signing key, so why would you put it in a CA certificate?

Also if you look back at the history this extension was really one of the first that was introduced, it came into existence in a time where PKIs were only one level deep – the absence of guidance on how to handle this could easily be seen as an omission.

Microsoft saw it this way and decided to have their implementation treat this extension as a constraint, in other words if no EKU is present in the chain then the chain is considered good for all usages. But once a single EKU is added into the path nothing bellow it can be considered good for a non-listed EKU.

In Windows applications validate certificates using the CertGetCertificateChain API takes a number of control parameters via the PCERT_CHAIN_PARA structure, one can specify what EKUs they want to make sure a certificate is good for via the RequestedUsage parameter.

This logic (frankly almost all of the certificate validation) is all wrapped into this one call.

So what does this have to do with MSRC 2718704? Well it has reduced the risk of this mess up in a meaningful way I thought I would explain but before I do let me explain that I am not trying to downplay the significance of this issue I am just trying to clarify where the risks are.

As we know now the “licensing solution” deployed for terminal services has put a signing CA that is trusted for Code Signing in ever enterprise that uses the product.   But how is it restricted to just Code Signing, that’s really what this post is about.

Let’s look at the EKUs included in the offending “MS” certificate, in that chain we see:

  • Microsoft Root Authority
    • No EKUs
  • Microsoft Enforced Licensing Intermediate PCA
    • EKUs = Code Signing, Key Pack Licenses, License Server Verification
    • Effective EKUs = Code Signing, Key Pack Licenses, License Server Verification
  • Microsoft Enforced Licensing Certificate Authority CA
    • EKUs = Code Signing, License Server Verification
    • Effective EKUs = Code Signing, License Server Verification
  • Microsoft LSRA PA
    • EKUs = None
    • Effective EKUs = Code Signing, License Server Verification
  • MS
    • EKUs = None
    • Effective EKUs = Code Signing, License Server Verification

You will notice that the “Microsoft LSRA PA” certificate lists no EKUs but the Effective EKUs are listed as “Code Signing” and “License Server Verification”, this is because of the Nested EKU behavior I describe above.

The same thing happens in the end “MS” certificate; even though it has no EKUs listed I can only be used to validate licenses and sign-code because that’s all it’s issuers are entitled to bestow onto its subordinates.

OK so what does all of this mean to you and me? It basically means as long as the application is written using CryptoAPI in the intended way (and all do that I am aware of in this context) those CAs out there cannot be used to issue SSL certificates (or any other usages not listed) that would be “valid”, they can of course sign code as Microsoft which is a larger issue in my book.

Anyway over the years I have proposed in IETF that this same behavior be adopted, it was always rejected as an evil Microsoft conspiracy (I was at Microsoft at the time) it of course was nothing of the sort but in the end I gave up. Recently I have started trying to convince the browsers directly to implement this same behavior as I feel it is beneficial, for example here is a NSS bug tracking the same request, if implemented that would take care of Chrome and Firefox, that still leaves Safari and Opera but it’s a step in the right direction.

 

Additional Resources

http://lists.randombit.net/pipermail/cryptography/2012-June/002966.html

MSRC 2718704 and the Terminal Services Licensing Protocol

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There has been a ton of chatter on the internet the last few days about this MSRC incident; it is an example of so many things gone wrong it’s just not funny anymore.

In this post I wanted to document the aspects of this issue as it relates to the Terminal Services Licensing protocol.

Let’s start with the background story; Microsoft introduced a licensing mechanism for Terminal Services that they used to enforce their Client Access Licenses (CAL) for the product. The protocol used in this mechanism is defined in [MS-RDPELE].

A quick review of this document shows the model goes something like this:

  1. Microsoft operates a X.509 Certificate Authority (CA).
  2. Enterprise customers who use Terminal Services also operate a CA; the terminal services team however calls it a “License server”.
  3. Every Enterprise “License server” is made a subordinate of the Microsoft Issuing CA.
  4. The license server issues “Client Access Licenses (CALs)” or aka X.509 certificates.

By itself this isn’t actually a bad design, short(ish) lived certificates are used so the enterprise has to stay current on its licenses, it uses a bunch of existing technology so that it can be quickly developed and its performance characteristics are known.

The devil however is in the details; let’s list a few of those details:

  1. The Issuing CA is made a subordinate of one of the “Microsoft Root CAs”.
  2. The Policy  CA  (“Microsoft Enforced Licensing Intermediate PCA”) on the Microsoft side was issued “Dec 10 01:55:35 2009 GMT”
  3. All of the certificates in the chain were signed using RSAwithMD5.
  4. The CAs were likely a standard Microsoft Certificate Authority (The certificate was issued with a Template Name of “SubCA”)
  5. The CAs were not using random serial numbers (its serial number of one was 7038)
  6. The certificates the license server issued were good for :
    • License Server Verification (1.3.6.1.4.1.311.10.6.2)
    • Key Pack Licenses (1.3.6.1.4.1.311.10.6.1)
    • Code Signing (1.3.6.1.5.5.7.3.3)
  7. There were no name constraints in the “License server” certificate restricting the names that it could make assertions about.
  8. The URLs referred to in the certificates were invalid (URLs to CRLs and issuer certificates) and may have been for a long time (looks like they may not have been published since 2009).

OK, let’s explore #1 for a moment; they needed a Root CA they controlled to ensure they could prevent people from minting their own licenses. They probably decided to re-use the product Root Certificate Authority so that they could re-use the CertVerifiyCertificateChainPolicy and its CERT_CHAIN_POLICY_MICROSOFT_ROOT policy. No one likes maintaining an array of hashes that you have to keep up to date on all the machines doing the verification. This decision also meant that they could have those who operated the existing CAs manage this new one for them.

This is was also the largest mistake they made, the decision to operationally have this CA chained under the production product roots made this system an attack vector for the bad guys. It made it more valuable than any third-party CA.

They made this problem worse by having every license server be a subordinate of this CA; these license servers were storing their keys in software. The only thing protecting them is DPAPI and that’s not much protection.

They made the problem even worse by making each of those CAs capable of issuing code signing certificates.

That’s right, every single enterprise user of Microsoft Terminal Services on the planet had a CA key and certificate that could issue as many code signing certificates they wanted and for any name they wanted – yes even the Microsoft name!

It doesn’t end there though, the certificates were signed using RSAwithMD5 the problem is that MD5 has been known to be susceptible to collision attacks for a long time and they were found to be practical by December 2008 – before the CA was made.

Also these certificates were likely issued using the Microsoft Server 2003 Certificate Authority (remember this was 2009) as I believe that the Server 2008 CA used random serial numbers by default (I am not positive though – it’s been a while).

It also seems based on the file names in the certificates AIAs that the CRLs had not been updated since the CA went live and maybe were never even published, this signals that this was not being managed at all since then.

Then there is the fact that even though Name Constraints was both supported by the Windows platform at the time this solution was created they chose not to include them in these certificates – doing so would have further restricted the value to the attacker.

And finally there is the fact that MD5 has been prohibited from use in Root Programs for some time, the Microsoft Root Program required third-party CAs to not to have valid certificates that used MD5 after January 15, 2009. The CA/Browser Forum has been a bit more lenient in the Baseline Requirements by allowing their use until December 2010. Now it’s fair to say the Microsoft Roots are not “members” of the root program but one would think they would have met the same requirements.

So how did all of this happen, I don’t know honestly.

The Security Development Lifecycle (SDL) used at Microsoft was in full force in 2008 when this appears to have been put together; it should have been caught via that process, it should have been caught by a number of other checks and balances also.

It wasn’t though and here we are.

P.S. Thanks to Brad Hill, Marsh Ray and Nico Williams for chatting about this today 🙂

Additional Resources

Microsoft Update and The Nightmare Scenario

Microsoft Certificate Was Used to Sign “Flame” Malware

How old is Flame

Security Advisory 2718704: Update to Phased Mitigation Strategy

The Contrition of a Security Practitioner

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The Encarta World Dictionary says that Contrition is “the deep and genuine feelings of guilt and remorse”. Having been involved in information security for 20 years, now, I think I can sincerely say that many security practitioners would say this is how they feel about the early days of their careers.

Why, you ask? Well, in my case, I started my career doing work for large financial institutions and governments. Back then these sorts of customers often had a “security at any price” mantra. While one would need to assess the risk of a system to secure it, these sorts of customers would also plan to mitigate as many of the identified risks as possible.

For these customers this was not necessarily a bad approach, but that had more to do with what was at risk than it did with the approach being a sound one.

Today the world is a different place; security is something that even the smallest businesses need to consider. This change did not occur overnight. It was gradual and I guess this is where the contrition comes in.

You see many applied the same approaches that worked with those financial and government customers with Fortune 500 and later Fortune 1000 companies. While in some cases this was appropriate, in most cases it was not.

The modern security practitioner needs to take a more holistic look at the business and platform they are servicing to understand its schedule and technological needs along with what the immediate business risks are.

Beyond that, the breadth of the role has changed and expanded. Security practitioners are now commonly responsible for Compliance, Reliability and Privacy, as well.

This puts the security practitioner in an interesting position; with this more complete view they can now help improve:

  • time to market, by recommending solutions that are risk-appropriate for the business;
  • engineering efficiencies, by identifying areas where work is being done inefficiently;
  • systems and processes, by identifying gaps and potential failure points that can negatively impact the business;
  • how teams allocate their scarce resources, by identifying opportunities where they’ll do the most good, based on risk vs. return.

This represents a significant shift from a decade or two ago, and requires the security practitioner to no longer simply be an outside expert but become part of the development team they support.

This is one of the reasons the Security Champion model is used in many teams here at Microsoft. While it has its challenges, as a member of the feature team a champion has the opportunity to have and share these more holistic insights as I called out above.

A good example of this is the application of cryptography to solve business problems. Cryptography is a powerful tool, but it’s often misapplied, introducing fragility and operational overhead that can be avoided; I think this is best summed up by this quotation:

If you think cryptography will solve your problem, then you don’t understand cryptography… and you don’t understand your problem. — Bruce Schneier

So, my ask of you as an engineering manager is to have a formal Security Assurance program for your team and as a software engineer incorporate your security specialists early and often. They either have direct experience in the areas I discussed here, or are in the position to bring those resources to your aid … to not only help you secure your offerings, but to do so in record time, as well.

What is a wildcard certificate and why are they a bad idea?

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Wildcard certificates are SSL/TLS server certificates that unlike their traditional counterparts bind a entire domain (or sub-domain) to a single private key, for information on the kind of wildcards one can specify see: http://support.microsoft.com/kb/258858.

Why would someone want to have a certificate like this? Well in my experience the decision is most often made on cost factors (acquisition, management, and politics), in other words I would rather buy onecertificate for my entire server farm instead of one for each server.

What makes the use of these certificates a bad idea? Well there are several reasons:

First, SSL/TLS typically provides two key properties; authentication of the server and confidentiality of the session; the core value of that pair is the authentication of the server as you should not even consider submitting data if you don’t know who its going to and if you never submit data you don’t need to worry about if the session is encrypted (most of the time).

Lets explore why we care about “who” were talking to and why that need to specific, certification authorities only issue certificates to entities that agree to something commonly referred to as a subscriber agreement; this agreement obliges the subscriber to have certain practices, the most basic of which might be not to publish the associated private key on the web or to not host malicious content.

With a wildcard certificate you may or may not know if the site content your experiencing agreed to such an agreement, more over you have a pretty high probability that the private key associatedwith the certificate exists in multiple locations, this also increases the likely hood that the private key is in software and not hardware making it a practical possibility a remote exploit could expose the private key to an attacker.

Then there is the question of who your talking to, many sites use sub-domains (for example WordPress), this becoming more and more common with the exhaustion of easy to remember domains and new community sites that what remember-able URLs for their members, each of these “subs-sites” typically have control over their own content and knowing who their hosting provider is doesn’t tell you anything about them in particular.

Now I am not suggesting that having a certificate tells you that the content being served from that host is somehow more trustworthy but knowing who you are talking to went to the trouble to prove who they are to you is useful when making a trust decision.

Secondly there is the practical issue of key management, as I mentioned earlier you know that the key has (very likely) been shared amongst multiple hosts, and keys that have been spread out like the dogs breakfast are much less trustworthy than ones that have never been shared.

If these things are so bad why did they get developed? Well there are a few reasons, certainly cost was one of the but I actually believe it was done in the hope to enable intermediary SSL/TLS accelerators too to service multiple sites; the thing is that these devices have always been able to handle different certificates for each of the hosts they secure so this would have just been a excuse.

In my opinion all browsers should have a setting that allows users to disable the wildcard behavior, in-fact I would go so far to say that this should be the default.

There is another similar case to the wildcard certificate, it is possible to bind multiple identities to a single key pair, one can put in multiple Subject Alternate Names, in other words I can create a certificatethat binds 100 host names into a single certificate; this still has many of the problems of a wildcard certificate and I would argue that this case should also be covered by any setting that disabled ambiguously bound certificates.

Alun Jones has a good blog post from a while ago on this topic that is worth a read too.