threads / discuss / 32995

Certificate validation vulnerability in Git

Subject: Certificate validation vulnerability in Git

## tl;dr

7 messages between Feb 24, 2013 and Feb 25, 2013.

replies: 6people: 4as markdown or json

Zubin Mithra· Feb 24, 2013, 17:31 UTC · lore
Hello,

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.

Kindly fix these issues, file a CVE and credit it to Dhanesh K. and Zubin Mithra. Thanks.

We are not subscribed to this list, so we'd appreciate it if you could CC us in the replies.

Hope this helps.

Thanks! Zubin

[1] http://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf
Andreas Ericsson· Feb 24, 2013, 18:46 UTC · re: Zubin Mithra · lore

Re: Certificate validation vulnerability in Git

On 02/24/2013 06:31 PM, Zubin Mithra wrote:
Show 27 quoted lines
> Hello,
> 
> 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.
> 
> Kindly fix these issues, file a CVE and credit it to Dhanesh K. and
> Zubin Mithra. Thanks.
>

The lack of certificate authority verification presents no attack vector for git imap-send. As such, it doesn't warrant a CVE. I'm sure you'll be credited with a "reported-by" line in the commit message if someone decides to fix it though. Personally, I'm not fussed.

> We are not subscribed to this list, so we'd appreciate it if you could
> CC us in the replies.
>

That's standard on this list. Please follow the same convention if/when you reply. Thanks.

-- 
Andreas Ericsson                   andreas.ericsson@op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

Considering the successes of the wars on alcohol, poverty, drugs and
terror, I think we should give some serious thought to declaring war
on peace.
Zubin Mithra· Feb 25, 2013, 02:28 UTC · re: Andreas Ericsson · lore

Re: Certificate validation vulnerability in Git

Hello,
On Mon, Feb 25, 2013 at 12:16 AM, Andreas Ericsson <ae@op5.se> wrote:
Show 33 quoted lines
> On 02/24/2013 06:31 PM, Zubin Mithra wrote:
>> Hello,
>>
>> 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.
>>
>> Kindly fix these issues, file a CVE and credit it to Dhanesh K. and
>> Zubin Mithra. Thanks.
>>
>
> The lack of certificate authority verification presents no attack vector
> for git imap-send. As such, it doesn't warrant a CVE. I'm sure you'll
> be credited with a "reported-by" line in the commit message if someone
> decides to fix it though. Personally, I'm not fussed.

I'd like to add in a few points -- generally SSL/TLS would be used in cases where the authenticity of the server and confidentiality of the messages transferred would be required. In this particular case, the threat scenarios would be :-

- Usage of an invalid attacker certificate could result in the
attacker gaining access to authentication information sent over the
wire.
- If the code repository were private, the patches thus generated are
also assumed to be kept private. An invalid certificate check at the
client side would enable an attacker to gain access to those patches.
Is there anything I'm missing? I believe this is a valid security issue.

Thanks, Zubin

Show 16 quoted lines
>
>> We are not subscribed to this list, so we'd appreciate it if you could
>> CC us in the replies.
>>
>
> That's standard on this list. Please follow the same convention if/when
> you reply. Thanks.
>
> --
> Andreas Ericsson                   andreas.ericsson@op5.se
> OP5 AB                             www.op5.se
> Tel: +46 8-230225                  Fax: +46 8-230231
>
> Considering the successes of the wars on alcohol, poverty, drugs and
> terror, I think we should give some serious thought to declaring war
> on peace.
Jeff King· Feb 25, 2013, 03:18 UTC · re: Andreas Ericsson · lore

Re: Certificate validation vulnerability in Git

On Sun, Feb 24, 2013 at 07:46:51PM +0100, Andreas Ericsson wrote:
> The lack of certificate authority verification presents no attack vector
> for git imap-send. As such, it doesn't warrant a CVE. I'm sure you'll
> be credited with a "reported-by" line in the commit message if someone
> decides to fix it though. Personally, I'm not fussed.

Sure it presents an attack vector. I can man-in-the-middle your imap-send client and read your otherwise secret patches. Or your otherwise secret imap password.

-Peff
Junio C Hamano· Feb 25, 2013, 05:35 UTC · re: Jeff King · lore

Re: Certificate validation vulnerability in Git

Jeff King <peff@peff.net> writes:
Show 10 quoted lines
> On Sun, Feb 24, 2013 at 07:46:51PM +0100, Andreas Ericsson wrote:
>
>> The lack of certificate authority verification presents no attack vector
>> for git imap-send. As such, it doesn't warrant a CVE. I'm sure you'll
>> be credited with a "reported-by" line in the commit message if someone
>> decides to fix it though. Personally, I'm not fussed.
>
> Sure it presents an attack vector. I can man-in-the-middle your
> imap-send client and read your otherwise secret patches. Or your
> otherwise secret imap password.

Yes, the lack of verification alone will not hurt the victim; you would need to also be able to insert yourself in the middle, perhaps by poisoning the victim's DNS. But one of the points of using SSL/TLS is to resist such an attack, and it certainly is an attack surfce, even though it may be of a lessor kind than other kinds of attacks.

Jeff King· Feb 25, 2013, 03:16 UTC · re: Zubin Mithra · lore

Re: Certificate validation vulnerability in Git

On Sun, Feb 24, 2013 at 11:01:50PM +0530, Zubin Mithra wrote:
Show 21 quoted lines
> 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?

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
Zubin Mithra· Feb 25, 2013, 15:42 UTC · re: Jeff King · lore

Re: Certificate validation vulnerability in Git

On Mon, Feb 25, 2013 at 8:46 AM, Jeff King <peff@peff.net> wrote:
Show 35 quoted lines
> 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
Show 8 quoted lines
>
> 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

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