[Python-Dev] ssl module and LibreSSL CVE-2018-8970

[Brett Cannon]

Nice work! Something to add to our "finding C compiler bugs" list of
accomplishments. 😁

On Wed, Apr 4, 2018, 13:39 Christian Heimes, <christian at python.org> wrote:

> Hi,
>
> I like to share the story of a critical security bug with you. Contrary
> to other issues in TLS/SSL, it's a story with happy ending. Nobody was
> harmed. The bug was fixed before it affected the general population.
>
>
> Introduction
> ------------
>
> Python's ssl.match_hostname() function was a source of several CVEs and
> other security bugs. After a long struggle, I decided to drop support
> for old OpenSSL releases and uses a new OpenSSL method to offload host
> name verification to OpenSSL. The improvement [1] eventually landed in
> Python 3.7. Nowadays OpenSSL verifies host name or IP address during the
> TLS/SSL handshake.
>
> Later I discovered that LibreSSL <= 2.6 did not have
> X509_VERIFY_PARAM_set1_host() [2]. We had to temporarily suspend support
> for LibreSSL. About two months later, LibreSSL caught up and released
> version 2.7.0 with support for the function.
>
>
> The bug
> -------
>
> One day after the release of LibreSSL 2.7.0, I started to port Python
> 3.7 to LibreSSL. In matter of minutes I got the ssl module to compile
> and work with LibreSSL. All tests were passing -- except for negative
> the host name verification tests. LibreSSL was accepting all invalid
> host names as correct! Python's vigorous test suite had discovered a
> critical security bug in LibreSSL.
>
> It turned out that LibreSSL copied the implementation of
> X509_VERIFY_PARAM_set1_host(param, name, namelen) from BoringSSL and the
> documentation from OpenSSL. BoringSSL's implementation didn't support
> the special case of 0 as namelen parammeter. OpenSSL supports namelen =
> 0, which is interpreted as namelen=strlen(name). It is documented in
> OpenSSL's man page and was even recommended on OpenSSL's wiki as
> preferred way.
>
>
> Happy Ending
> ------------
>
> So I got in contact with LibreSSL's security team and BoringSSL's
> security team [3]. Less than a day later, both libraries released fixes
> for the bug [4]. Mitre has assigned CVE-2018-8970 [5] to the bug.
> Disaster averted!
>
> BoringSSL's security issue [3] contains more information. Adam Langley
> lifted the restriction about an hour ago.
>
> I like to thank Bob Beck (LibreSSL), Adam Langley (Google) and David
> Benjamin (Google) for their assistance and cooperation.
>
> Regards,
> Christian
>
> [1] https://bugs.python.org/issue31399
> [2] https://github.com/libressl-portable/portable/issues/381
> [3] https://bugs.chromium.org/p/chromium/issues/detail?id=824799
> [4] https://www.libressl.org/releases.html
> [5] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-8970
>
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