Monthly Archives: June 2012

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