Fix alternate chains certificate forgery issue

During certificate verfification, OpenSSL will attempt to find an
alternative certificate chain if the first attempt to build such a chain
fails. An error in the implementation of this logic can mean that an
attacker could cause certain checks on untrusted certificates to be
bypassed, such as the CA flag, enabling them to use a valid leaf
certificate to act as a CA and "issue" an invalid certificate.

This occurs where at least one cert is added to the first chain from the
trust store, but that chain still ends up being untrusted. In that case
ctx->last_untrusted is decremented in error.

Patch provided by the BoringSSL project.

CVE-2015-1793

Reviewed-by: Stephen Henson <steve@openssl.org>

diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
index 16db4c025a14539427f02eede2018bd7264660e3..a0083b552d3a53402ab6c751b95f8d45eeb0183e 100644 (file)
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -328,8 +328,8 @@ int X509_verify_cert(X509_STORE_CTX *ctx)
                         xtmp = sk_X509_pop(ctx->chain);
                         X509_free(xtmp);
                         num--;
-                        ctx->last_untrusted--;
                     }
+                    ctx->last_untrusted = sk_X509_num(ctx->chain);
                     retry = 1;
                     break;
                 }