PSA: Ubuntu backports OpenSSL fix, breaks PKIX metadata signature validation in SP

[Etienne Dysli-Metref]

Hi list,

We received several reports this week of SPs failing to validate the
SWITCHaai federation metadata. Investigation of the issue revealed that
a change in the OpenSSL library provided by Ubuntu last week triggers a
bug in XMLTooling-C that prevents the validation of the certificate
chain of trust used to sign our federation's metadata (we use PKIX,
that's uncommon). The bug in question is already fixed since August
2015, but affects all XMLTooling-C versions except the latest release
i.e. 1.5.6. The end result is the SP rejects the metadata and thus will
refuse to operate with IdPs of the SWITCHaai federation.

You will find more details below about what we were able to find out. If
your SP doesn't run on Ubuntu, you can keep reading and snarkily laugh
because your favourite Linux distribution didn't do this to you... ;)

TL;DR: use XMLTooling-C 1.5.6


Affected software
-----------------

Operating systems:
- Ubuntu 12.04 LTS "Precise"
- Ubuntu 14.04 LTS "Trusty"
- probably other Ubuntu releases too

Libraries:
- XMLTooling-C 1.5.5 and all previous releases, when used with OpenSSL
1.0.1 or 1.0.2 provided by Ubuntu

Note: Debian is *not* affected because their OpenSSL maintainers did not
include the problematic backported patch.


Remediation
-----------

1. Upgrade XMLTooling-C to version 1.5.6
2. Restart the shibd daemon

There are a number of possible workarounds but they all make your SP
less secure, so upgrading is really recommended.


The gory details
----------------

On 7th July 2015, OpenSSL tightened their code around certificate
verification [1], changing the way the library should be used by
developers. This change made XMLTooling-C's use of OpenSSL wrong when
checking CRL with the PKIX trust engine. We reported this bug on 4th
August 2015 [2] and Scott Cantor promptly fixed it and released
XMLTooling-C version 1.5.6 [3]. Later, on 8th February 2016, Ubuntu
maintainers backported a series of commits from OpenSSL including the
change mentioned above (look for "alt-cert-chains-7.patch" in [4]). This
was released to Ubuntu 12.04 and 14.04 on 24th February 2016 [5]
unfortunately without mentioning the behaviour change.

[1]
https://git.openssl.org/gitweb/?p=openssl.git;a=commit;h=b3b1eb5735c5b3d566a9fc3bf745bf716a29afa0
[2] https://issues.shibboleth.net/jira/browse/CPPXT-105
[3] https://shibboleth.net/pipermail/announce/2015-August/000121.html
[4]
https://launchpadlibrarian.net/236983031/openssl_1.0.1-4ubuntu5.33_1.0.1-4ubuntu5.34.diff.gz
[5] http://www.ubuntu.com/usn/usn-2913-3/

Fun times! :)
  Etienne

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