Thursday, June 2, 2016

Restricting the scope of CA certificates

The granting of an intermediate CA certificate to a surveillance firm generated quite some fuss. Setting theories aside, the main reason behind that outcry, is the fact that any intermediate CA certificate trusted by the browsers has unlimited powers to certify any web site on the Internet. Servers can protect themselves against an arbitrary CA generating a valid certificate for their web site, using certificate pinning, but there is very little end-users can do. In practice, end-users either trust the whole bundled CA list in their browser/system or not.

An option for end-users is to utilize trust on first use, but that is not a widespread practice, and few software, besides for SSH, support it. A way for me as a user to defend against a believed to be rogue CA, is by disabling or removing that CA from my trusted bundle. But what if I trust that CA for a particular web site or domain, but not for the whole Internet?

On this post I'll try to provide more information on some lesser documented aspects of p11-kit, which provide additional control over the CA certificate bundle in a system. That is, I'll explain how we can do better than disabling CAs, and how we can restrict CAs to particular domains. The following instructions are limited to Fedora 22+ which has deployed a shared trust database for certificates based on p11-kit. This database, is not only an archive of trusted certificates, but also provides the option to attach additional attributes to CA certificates in the form of PKIX extensions. These extensions are called stapled extensions in p11-kit jargon and they override any extensions available in the trust certificates. That, allows to enforce additional restrictions to the purpose and scope of a certificate.

I'll attempt to demonstrate this feature using an example. Let's consider the case where your employer's IT department provided you with a CA certificate to trust for communications within the company. Let's also assume that the company's internal domain is called "example.com". In that scenario as a user I'd like to restrict the provided CA certificate to example.com domain to prevent anyone with access to the corporate private key from being able to hijack any connection outside the company scope. This is not only out of paranoia against a potential corporate big-brother but also to keep a good security practice and avoid having master keys. A stolen corporate CA key which is trusted for everything under the sun provides a potential attacker not only with access to company's internal communication, but also with access to Internet communication of any corporate user.

How would we install such certificate in a way that it is restricted only to example.com? Assuming that the CA certificate is provided at the example.com-root.pem file, the following command will add the company's certificate to the trusted list.
$ sudo trust anchor example.com-root.pem

That will create a file in /etc/pki/ca-trust/source containing the CA certificate (for more information on adding and removing CA certificates in Fedora see the update-ca-trust manpage).

If we edit this file we will see something like the following.
[p11-kit-object-v1]
trusted: true
x-distrusted: false
private: false
certificate-category: authority
-----BEGIN CERTIFICATE-----
MIIDsDCCAxmgAwIBAgIBATANBgkqhkiG9w0BAQUFADCBnTELMAkGA1UEBhMCVVMx
...
-----END CERTIFICATE-----

This contains the certificate of the CA as well as various basic flags set to it.
How can we now attach a stapled extension to it?

We need to add another object in that database containing the extension. But let's see the process step by step. First we need to extract the certificate's public key because that's how p11-kit identifies existing objects. A command to achieve that is the following:
$ certool --pubkey-info --infile example.com-root.pem --outfile example.com-pubkey.pem

 The output file will contain a public key in PEM format (identifiable by the "-----BEGIN PUBLIC KEY-----" header). We now edit the p11-kit file in  /etc/pki/ca-trust/source containing our certificate and append the following.
[p11-kit-object-v1]
class: x-certificate-extension
label: "Example.com CA restriction"
object-id: 2.5.29.30
value: "%30%1a%06%03%55%1d%1e%04%13%30%11%a0%0f%30%0d%82%0b%65%78%61%6d%70%6c%65%2e%63%6f%6d"
-----BEGIN PUBLIC KEY-----
...
-----END PUBLIC KEY-----

Where the public key part is copied from the example.com-pubkey.pem file.

This added object, is a stapled extension containing a PKIX name constraints extension which allows this CA to be used for certificates under the "example.com" domain. If you attempt to connect to a host with a certificate of this CA you will get the following error:
$ gnutls-cli www.no-example.com
...
Status: The certificate is NOT trusted. The certificate chain violates the signer's constraints.
*** PKI verification of server certificate failed...

Note that, although NSS and openssl applications check some extensions (such as key purpose) from this trust database, they do not consider the name constraints extension. This may change in the future, but currently only GnuTLS applications under Fedora will honor this extension. The reason it works under Fedora distribution is because GnuTLS is compiled using the --with-default-trust-store-pkcs11="pkcs11:" configuration option which makes it use the p11-kit trust DB directly.

A question at this point, after seeing the p11-kit object format, is how can we generate the "value" listed above containing the desired constraints? The value contains a DER encoded certificate extension which corresponds to the object identifier "object-id" field. In this case the object-id field contains the object identifier for NameConstraints extension (2.5.29.30).

Unfortunately there are no available tools to generate this value, that I'm aware of. I created a sample application which will generate a valid name constraints value to be set above. The tool can be found at this github repository.

After you compile, run:
$ ./nconstraints mydomain.com myotherdomain.com
%30%30%06%03%55%1d%1e%04%29%30%27%a0%25%30%0e%82%0c%6d%79%64%6f%6d%61%69%6e%2e%63%6f%6d%30%13%82%11%6d%79%6f%74%68%65%72%64%6f%6d%61%69%6e%2e%63%6f%6d

and as you see, this command will provide the required string.

Happy hacking!

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