mbed TLS: CVE-2017-14032: Bypass of authentication of peer possible when the authentication mode is configured as 'optional'

[Salvatore Bonaccorso <carnil () debian org>]

Hi

MITRE has assigned CVE-2017-14032 for the following issue in mbed TLS:

https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2017-02

>   Title     Bypass of authentication of peer possible when the authentication
>                             mode is configured as 'optional'
> Date      28th August 2017
> Affects   All versions of mbed TLS from version 1.3.10 and up, including all
>           2.1 and later releases
> Not       mbed TLS 1.3.9 and earlier
> affected
> Impact    Use of the 'optional' authentication mode can permit the peer to
>           bypass peer authentication
> Severity  High
>
> Vulnerability
> -------------
> If a malicious peer supplies an X.509 certificate chain that has more than
> MBEDTLS_X509_MAX_INTERMEDIATE_CA intermediates (which by default is 8), it
> could bypass authentication of the certificates, when the authentication mode
> was set to 'optional' eg. MBEDTLS_SSL_VERIFY_OPTIONAL. The issue could be
> triggered remotely by both the client and server sides.
>
> If the authentication mode, which can be set by the function
> mbedtls_ssl_conf_authmode(), was set to 'required' eg.
> MBEDTLS_SSL_VERIFY_REQUIRED which is the default, authentication would occur
> normally as intended.
>
> Impact
> ------
> Depending on the platform, an attack exploiting this vulnerability could allow
> successful impersonation of the intended peer and permit man-in-the-middle
> attacks.
>
> Resolution
> ----------
> Affected users should upgrade to mbed TLS 1.3.21, mbed TLS 2.1.9 or mbed TLS
> 2.6.0.
>
> Workaround
> ----------
> Users should wherever possible upgrade to the newer version of mbed TLS. Where
> this is not practical, users should consider if changing the authentication to
> the 'required' mode MBEDTLS_SSL_VERIFY_REQUIRED is practical for their
> application.

References:
 -  https://github.com/ARMmbed/mbedtls/commit/31458a18788b0cf0b722acda9bb2f2fe13a3fb32
 - https://github.com/ARMmbed/mbedtls/commit/d15795acd5074e0b44e71f7ede8bdfe1b48591fc
 - https://bugs.debian.org/873557

Regards,
Salvatore