** 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.
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 ftp-ssl, the following is details:
- ..（add a minute later）
+
+ -----------------------------------------------------------------------------
+ function : ssl_init
+ -----------------------------------------------------------------------------
+ SSL method : SSLv23
+ -----------------------------------------------------------------------------
+ call SSL_CTX_set_verify() : NOT FOUND
+ -----------------------------------------------------------------------------
+ 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.
To verify the result we make, we attack the software manually.
we take expired time check for example :
1. change the system time to 2200 to guarantee the certificate to be expired.
2. run command : ftp-ssl -z debug 192.168.68.130
(PS:192.168.68.130 is a norml ftp server)
3. result:succeed!!
The fetch succeeded again and no warning was given, indicating the
software didn't check whether the certificate expired or not.
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.
** 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.
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 ftp-ssl, the following is details:
-
-----------------------------------------------------------------------------
function : ssl_init
-----------------------------------------------------------------------------
SSL method : SSLv23
-----------------------------------------------------------------------------
call SSL_CTX_set_verify() : NOT FOUND
-----------------------------------------------------------------------------
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
+ According to the above result, we think ftp-ssl doesn't check the
certificate chain when using OPENSSL.
To verify the result we make, we attack the software manually.
we take expired time check for example :
1. change the system time to 2200 to guarantee the certificate to be expired.
2. run command : ftp-ssl -z debug 192.168.68.130
(PS:192.168.68.130 is a norml ftp server)
3. result:succeed!!
The fetch succeeded again and no warning was given, indicating the
software didn't check whether the certificate expired or not.
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/1380439
Title:
ftp-ssl's ssl connection is not secure
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