Re: Certificate validation vulnerability in Git

[Zubin Mithra <zubin.mithra@gmail.com>]

On Mon, Feb 25, 2013 at 8:46 AM, Jeff King <peff@peff.net> wrote:
> On Sun, Feb 24, 2013 at 11:01:50PM +0530, Zubin Mithra wrote:
>
>> There seems to be a security issue in the way git uses openssl for
>> certificate validation. Similar occurrences have been found and
>> documented in other open source projects, the research can be found at
>> [1].
>>
>> -=========]
>> - imap-send.c
>>
>> Line 307
>>
>>  307   ret = SSL_connect(sock->ssl);
>>  308   if (ret <= 0) {
>>  309     socket_perror("SSL_connect", sock, ret);
>>  310     return -1;
>>  311   }
>>  312
>>
>> Certificate validation errors are signaled either through return
>> values of SSL_connect or by setting internal flags. The internal flags
>> need to be checked using the SSL_get_verify_result function. This is
>> not performed.
>
> I'm not sure what you mean. We use SSL_CTX_set_verify to turn on peer
> certificate verification, which will cause SSL_connect to return
> failure if the certificate signature cannot be traced back to a CA cert
> from our local store.
>
> Is there some case where this does not happen properly? If so, can you
> give an example? The paper you referenced says only that there are some
> special cases where SSL_connect does not notice the error, but then
> gives an example where the application does not turn on SSL_VERIFY_PEER.
> But git does. Are there are other cases that SSL_VERIFY_PEER does not
> handle?

Indeed -- it appears that I was mistaken. I had a quick look at the
openssl source code and it does seem that SSL_VERIFY_PEER is
equivalent to SSL_get_verify_result.

Thank you for your time!

- Zubin

>
> There is a _different_ problem not handled by the code you show above,
> which is that SSL_connect does not verify that the hostname we connected
> to matches the signed certificate. But that was fixed already by b62fb07
> (imap-send: the subject of SSL certificate must match the host,
> 2013-02-15), which is in git v1.8.1.4.
>
> -Peff