Last month mbedTLS issued security advisory CVE-2017-14032, Bypass of
authentication of peer [1], which affects versions prior to "mbed TLS
1.3.21, mbed TLS 2.1.9 or mbed TLS 2.6.0" (but not PolarSSL). libcurl
uses mbedTLS in the way described in the advisory, and is thus affected.

This is a serious vulnerability and trivial to exploit. An example can
be seen at [2]. This is not a vulnerability in libcurl, it is in
mbedTLS. The certificate verification done by the client (libcurl) can
be bypassed by exploiting this vulnerability. This would most
practically happen during a Man-in-the-middle attack (MITM) [3].

To determine if the curl tool is using a vulnerable version of mbedTLS
run curl -V, and for libcurl you can call curl_version(). Either will
give you a version string like this:

curl 7.50.0-DEV (i686-w64-mingw32) libcurl/7.50.0-DEV mbedTLS/2.2.1
zlib/1.2.8 libidn/1.32 libssh2/1.7.0 nghttp2/1.11.0

In this example mbedTLS/2.2.1 is before 2.6.0 and therefore is
vulnerable. That may seem confusing since 2.2.1 comes after the fixed
2.1.9. That is because mbed TLS has separate 1.3.x and 2.1.x series.
What that means is 2.2.x - 2.5.x is vulnerable.

We rarely post about the impacts on curl due to flaws in the TLS
libraries, however this one in particular does not seem to have gotten
enough attention. Maybe it was not well understood? We suggest upgrading
to the latest version of mbedTLS as soon as you can.

[1]:
https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2017-02
[2]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=873557#17
[3]: https://en.wikipedia.org/wiki/Man-in-the-middle_attack

.

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Received on 2017-09-15