** Description changed:
- Recently, we are trying to find SSL security problems by static
- analysis. For example, as we all know, Hostname verification is an
- important step when verifying X509 certificates, however, 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.
+ 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 find some SSL problems in apf-client:
- 1. miss hostname check when verify x509 certificate
- 2. miss expired time check when verify x509 certificate
+ 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.
- More specifically , we can take function SSL_CTX_set_verify() for
- example, when using OPENSSL, if we call SSL_CTX_set_verify(ssl_ctx,
- SSL_VERIFY_NONE, null), we should verify the certificate by calling the
- function SSL_get_peer_certificate() to get the certificate at first.
- Then use X509 APIs or self-define function to verify the certificate we
- get. If the source code does not match this model, then we can deduce
- this code is vulnerable. And other APIs have similar problems.
+ 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 apf-client, the following is details:
+
+ -----------------------------------------------------------------------------
+ function : http_proxy_client
+ -----------------------------------------------------------------------------
+ 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(): YES
+ -----------------------------------------------------------------------------
+ call SSL_get_verify_result(): NO
+ -----------------------------------------------------------------------------
+
+ According to the above result, we think apf-client doesn't check the
+ certificate chain when using OPENSSL.
+
+ 三. How we prove the result we got?
To verify the result we make, we attack the software manually.
- 一. hostnmae check:
- 1. configure the file /etc/hosts in order to simulate DNS hijack:
- hackbyfun.com 127.0.0.1
- 2. server(127.0.0.1) : afserver -n 127.0.0.1 -p 9999 -m 2222 -v
- client (127.0.0.1) : afclient -n hackbyfun.com -p 21 -m 2222 -v
-
- 3. result : succeed!
- The fetch succeeded again and no warning was given, indicating the software
didn't check hostname.
-
- 二. expired time check :
+ expired time check :
1. change the system time to 2200 to guarantee the certificate to be expired.
2. server : afserver -n 127.0.0.1 -p 9999 -m 2222 -v
- client : afclient -n 127.0.0.1 -p 21 -m 2222 -v
+ client : afclient -n 127.0.0.1 -p 21 -m 2222 -v
3. result:succeed!!
The fetch succeeded again and no warning was given, indicating the
software didn't check the certificate expired.
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/1380452
Title:
apf-client doesn't check hostname or expired time when verifying x509
certificate
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