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 8ce41f9c9a8ef4444d5f872298f448bcec86b131..33896fbc64ea4902f36027167c629fe680b89a44 100644 (file)
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -389,8 +389,8 @@ 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;
                 }