diff --git a/ChangeLog b/ChangeLog
index 2149ff2..62ef470 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -131,6 +131,9 @@
      digits. Found and fixed by Guido Vranken.
    * Fix unlisted DES configuration dependency in some pkparse test cases. Found
      by inestlerode. #555
+   * Fix a potential integer overflow in the version verification for DER
+     encoded X509 certificates. The overflow would enable maliciously
+     constructed certificates to bypass the certificate verification check.
 
 = mbed TLS 2.1.6 branch released 2016-10-17
 
diff --git a/library/x509_crt.c b/library/x509_crt.c
index d8819fd..d2aaa92 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -751,14 +751,14 @@
         return( ret );
     }
 
-    crt->version++;
-
-    if( crt->version > 3 )
+    if( crt->version < 0 || crt->version > 2 )
     {
         mbedtls_x509_crt_free( crt );
         return( MBEDTLS_ERR_X509_UNKNOWN_VERSION );
     }
 
+    crt->version++;
+
     if( ( ret = mbedtls_x509_get_sig_alg( &crt->sig_oid, &sig_params1,
                                   &crt->sig_md, &crt->sig_pk,
                                   &crt->sig_opts ) ) != 0 )