** Description changed:
Recently, our group is trying to find SSL security problems by static
analysis. When using Openssl, people tend to miss the step or to
misunderstand the APIs when using SSL/TLS, which might cause severe man
in the middle attack and break the entire TLS mechanism. And static
analysis is a way of finding whether the APIs are called correctly.
- Now we just check whether a software verify the certitiface chain when
- using Openssl.
+ The source code we analysis was from ubuntu: apt-get source <package
name>.And we use this command in Ubuntu 12.04.
+ Now we just check whether a software verify the certitiface chain when using
Openssl.
一. How we ensure whether a software check the certificate chain or not?
We make a matching algorithm. If source code doesn't match this, the software
is not secure.
Typically, when Openssl clients want to verify a certificate, there are
the following choices:
1. Using built-in certificate verification(chain of trust verification,
expired validation, etc)
[Example 1]
/**
* set VERIFY_PEER flag before the establishment of a SSL connection
* OPENSSL will drop connection during handshake if verification fails
* No custom callback function used.
*/
SSL_CTX_set_verify(ctx,VERIFY_PEER,NULL);
[Example 2]
//check the built-in verification result after the SSL handshake
if(SSL_get_peer_certificate(ssl)!=NULL &&
SSL_get_verify_result(ssl)==X509_V_OK)
{
//PASS
}
else
{
//FAIL
}
2. Using custom verification.
[Example 3]
X509* usrcert = SSL_get_peer_certificate(ssl);
rootCertStore = X509_STORE_new();
.. ..
ctx = X509_STORE_CTX_new();
ret = X509_STORE_CTX_init(ctx,rootCertStore,usrCert,NULL);
ret = X509_verify_cert(ctx)
This example read the certificate out using SSL_get_peer_certificate
API. Then it use X509 API suite to do certificate verification. X509 API
is part of OPENSSL library. Theoretically, a developer can use any API
in any libraries to do this verification, but in practice, we only
identify the case above: using X509 API suite.
3. Add restrictions or relaxations to built-in certificate verification
The built-in certificate verification in OPENSSL library can be extended by
using custom callback functions. By default, this callback option is NULL,
indicating completely use built-in verification.
By adding this callback function, the developer can decide if they accept the
verify result by openssl, and they can modify the result whenever they what.
[Example 4]
SSL_CTX_set_verify(ctx,VERIFY_PEER,mycallback);
static mycallback(int preverify_ok, X509_STORE_CTX *ctx)
{
....
....
return preverify_ok;
}
二. The analysis result
Now, we find some SSL problems in crtmpserver, the following is details:
-----------------------------------------------------------------------------
file : crtmpserver-apps/crtmpserver-1.0~dfsg/thelib/src/protocols/ssl
/outboundsslprotocol.cpp
-----------------------------------------------------------------------------
function : OutboundSSLProtocol::DoHandshake()
-----------------------------------------------------------------------------
SSL method : \
-----------------------------------------------------------------------------
SSL_CTX_set_verify() argument : SSL_VERIFY_NONE
-----------------------------------------------------------------------------
Have SSL_CTX_set_verify ( SSL_set_verify) callback : NO
-----------------------------------------------------------------------------
call SSL_get_peer_certificate(): NO
-----------------------------------------------------------------------------
call SSL_get_verify_result(): NO
-----------------------------------------------------------------------------
According to the above result, we think crtmpserver doesn't check the
certificate chain when using OPENSSL.
PS: for more information, you can see the paper:
http://people.stfx.ca/x2011/x2011ucj/SSL/p38-georgiev.pdf
and more details you can contact with us, we will be very glad for your
responce.
Thanks.
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https://bugs.launchpad.net/bugs/1380298
Title:
some SSL security problems
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