weechat does not properly use gnutls and allow an attacker to bypass certificate verification

About WeeChat:
"WeeChat is a fast, light and extensible chat client. It runs on many platforms
(including Linux, BSD and Mac OS).
Development is very active, and bug fixes are very fast!"

The vuln:
Weechat does not use the GnuTLS API properly to check certificates, potentially
exposing users to man-in-the-middle attacks.

Weechat registers a callback function to be called by GnuTLS during the TLS/SSL
handshake. The function perform checks on the server
certificate and optionally, send a client certificate.
The mentioned code is located in src/core/wee-network.c in the network_init
function:

gnutls_certificate_client_set_retrieve_function (gnutls_xcred,

&hook_connect_gnutls_set_certificates);

Excerpt from gnutls's doc:

gnutls_certificate_client_set_retrieve_function sets a callback to be called

in order to retrieve the certificate to be used in the handshake.
(…)
If the callback function is provided then gnutls will call it, in the
handshake, after the certificate request message has been received.

This callback function will only be called when the server ask for a client
certificate during the handshake, but weechat also use this callback
to check the server certificate.

As specified in the rfc2246 at 7.4.6., the certificate request is optionnal:

7.4.6. Client certificate

   When this message will be sent:
       This is the first message the client can send after receiving a
       server hello done message. This message is only sent if the
       server requests a certificate.

So when the server does not request a client certificate,
hook_connect_gnutls_set_certificates is never called and weechat does not
perform any check on the server certificate. It doesn't print any of the usual
information about the dh key size and the content
of the server certificate either.

POC:

$ openssl genrsa -out server.key 4096
$ openssl req -new -key server.key -out server.csr
$ openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt
$ openssl dhparam -outform PEM -out dhparam.pem 4096
$ openssl s_server -cert server.crt -key server.key -dhparam dhparam.pem -accept
6697 &>./log &
$ weechat-curses ircs://127.0.0.1:6697 # will not check the certificate
$ fg
^C
$ openssl s_server -cert server.crt -key server.key -dhparam dhparam.pem -accept
6697 -verify yes &>./log2 &
$ weechat-curses ircs://127.0.0.1:6697 # will print an error because the
certificate is self signed

This problem affects all versions. The maintainer has been contacted and a fix
should be published. someday…
A "beta" fix is availaible here: http://savannah.nongnu.org/patch/index.php?7459