diff --git a/ChangeLog b/ChangeLog
index 9bf6a17..d35457b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,9 @@
    * Certificate verification functions now set flags to -1 in case the full
      chain was not verified due to an internal error (including in the verify
      callback) or chain length limitations.
+   * With authmode set to optional, handshake is now aborted if the
+     verification of the peer's certificate failed due to an overlong chain or
+     a fatal error in the vrfy callback.
 
 = mbed TLS 2.5.1 released 2017-06-21
 
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 5e549f6..31591e2 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -71,7 +71,7 @@
  * Name      ID  Nr of Errors
  * PEM       1   9
  * PKCS#12   1   4 (Started from top)
- * X509      2   19
+ * X509      2   20
  * PKCS5     2   4 (Started from top)
  * DHM       3   9
  * PK        3   14 (Started from top)
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 51c1c60..cc00070 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1052,7 +1052,7 @@
  *
  *                 If set, the verify callback is called for each
  *                 certificate in the chain. For implementation
- *                 information, please see \c x509parse_verify()
+ *                 information, please see \c mbedtls_x509_crt_verify()
  *
  * \param conf     SSL configuration
  * \param f_vrfy   verification function
diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index f219bf1..128eade 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -76,6 +76,7 @@
 #define MBEDTLS_ERR_X509_ALLOC_FAILED                     -0x2880  /**< Allocation of memory failed. */
 #define MBEDTLS_ERR_X509_FILE_IO_ERROR                    -0x2900  /**< Read/write of file failed. */
 #define MBEDTLS_ERR_X509_BUFFER_TOO_SMALL                 -0x2980  /**< Destination buffer is too small. */
+#define MBEDTLS_ERR_X509_FATAL_ERROR                      -0x3000  /**< A fatal error occured, eg the chain is too long or the vrfy callback failed. */
 /* \} name */
 
 /**
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 383e484..fd20336 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -267,7 +267,13 @@
  *
  *                 All flags left after returning from the callback
  *                 are also returned to the application. The function should
- *                 return 0 for anything but a fatal error.
+ *                 return 0 for anything (including invalid certificates)
+ *                 other than fatal error, as a non-zero return code
+ *                 immediately aborts the verification process. For fatal
+ *                 errors, a specific error code should be used (different
+ *                 from MBEDTLS_ERR_X509_CERT_VERIFY_FAILED which should not
+ *                 be returned at this point), or MBEDTLS_ERR_X509_FATAL_ERROR
+ *                 can be used if no better code is available.
  *
  * \note           In case verification failed, the results can be displayed
  *                 using \c mbedtls_x509_crt_verify_info()
diff --git a/library/error.c b/library/error.c
index dd2db0c..db42381 100644
--- a/library/error.c
+++ b/library/error.c
@@ -480,6 +480,8 @@
             mbedtls_snprintf( buf, buflen, "X509 - Read/write of file failed" );
         if( use_ret == -(MBEDTLS_ERR_X509_BUFFER_TOO_SMALL) )
             mbedtls_snprintf( buf, buflen, "X509 - Destination buffer is too small" );
+        if( use_ret == -(MBEDTLS_ERR_X509_FATAL_ERROR) )
+            mbedtls_snprintf( buf, buflen, "X509 - A fatal error occured, eg the chain is too long or the vrfy callback failed" );
 #endif /* MBEDTLS_X509_USE_C || MBEDTLS_X509_CREATE_C */
         // END generated code
 
diff --git a/library/x509_crt.c b/library/x509_crt.c
index ee5f27e..ec5f772 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -2057,8 +2057,8 @@
     /* path_cnt is 0 for the first intermediate CA */
     if( 1 + path_cnt > MBEDTLS_X509_MAX_INTERMEDIATE_CA )
     {
-        *flags |= MBEDTLS_X509_BADCERT_NOT_TRUSTED;
-        return( MBEDTLS_ERR_X509_CERT_VERIFY_FAILED );
+        /* return immediately as the goal is to avoid unbounded recursion */
+        return( MBEDTLS_ERR_X509_FATAL_ERROR );
     }
 
     if( mbedtls_x509_time_is_past( &child->valid_to ) )
@@ -2310,6 +2310,10 @@
     }
 
 exit:
+    /* prevent misuse of the vrfy callback */
+    if( ret == MBEDTLS_ERR_X509_CERT_VERIFY_FAILED )
+        ret = MBEDTLS_ERR_X509_FATAL_ERROR;
+
     if( ret != 0 )
     {
         *flags = (uint32_t) -1;
diff --git a/tests/suites/test_suite_x509parse.data b/tests/suites/test_suite_x509parse.data
index 6df5298..ea56f3f 100644
--- a/tests/suites/test_suite_x509parse.data
+++ b/tests/suites/test_suite_x509parse.data
@@ -1204,7 +1204,7 @@
 
 X509 CRT verify long chain (max intermediate CA + 1)
 depends_on:MBEDTLS_SHA256_C:MBEDTLS_ECDSA_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED
-mbedtls_x509_crt_verify_max:"data_files/dir-maxpath/00.crt":"data_files/dir-maxpath":MBEDTLS_X509_MAX_INTERMEDIATE_CA+1:MBEDTLS_ERR_X509_CERT_VERIFY_FAILED:-1
+mbedtls_x509_crt_verify_max:"data_files/dir-maxpath/00.crt":"data_files/dir-maxpath":MBEDTLS_X509_MAX_INTERMEDIATE_CA+1:MBEDTLS_ERR_X509_FATAL_ERROR:-1
 
 X509 CRT verify chain #1 (zero pathlen intermediate)
 depends_on:MBEDTLS_SHA256_C:MBEDTLS_RSA_C