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>
openssl · Jul 7, 2015 · 2aacec8 · 2aacec8
1 parent 9cf315e
commit 2aacec8
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
@@ -392,8 +392,8 @@ 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;
                 }