Restrict --x509-alt-username extension types

The code never supported all extension types.  Make this explicit by only
allowing subjectAltName and issuerAltName (for which the current code does
work).

Using unsupported extension fields would most likely cause OpenVPN to crash
as soon as a client connects.  This does not have a real-world security
impact, as such a configuration would not be possible to use in practice.

This bug was discovered, analysed and reported to the OpenVPN team by
Guido Vranken.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Acked-by: Guido Vranken <guidovranken@gmail.com>
Message-Id: <1497864520-12219-5-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/search?l=mid&q=1497864520-12219-5-git-send-email-steffan.karger@fox-it.com
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/Changes.rst b/Changes.rst
index 89cfae87dfadbb6c079bf0ba14cf29c03a1885fc..6fa1c0c66fe6f6af8d5ac81e3c7cf7895340afc7 100644 (file)
--- a/Changes.rst
+++ b/Changes.rst
@@ -324,6 +324,9 @@ User-visible Changes
 - ``--verify-hash`` can now take an optional flag which changes the hashing
   algorithm. It can be either SHA1 or SHA256.  The default if not provided is
   SHA1 to preserve backwards compatibility with existing configurations.
+- Restrict the supported --x509-alt-username extension fields to subjectAltName
+  and issuerAltName.  Other extensions probably didn't work anyway, and would
+  cause OpenVPN to crash when a client connects.
 
 Bugfixes
 --------

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index 3f183e642d00c06a647c3125220a3429df4bf5a1..20bdd91b3e59a72cc2fd5a1c3e1521b12ac97a0e 100644 (file)
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -5307,6 +5307,8 @@ option will match against the chosen
 .B fieldname
 instead of the Common Name.
 
+Only the subjectAltName and issuerAltName X.509 extensions are supported.
+
 .B Please note:
 This option has a feature which will convert an all-lowercase
 .B fieldname

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 76a8550668c8f51994c9f9a1215f874d7e35535a..505c5b2e3bef78324d5b7db7f01399e109561980 100644 (file)
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -8083,6 +8083,10 @@ add_option(struct options *options,
                     "configuration", p[1]);
             }
         }
+        else if (!x509_username_field_ext_supported(s+4))
+        {
+            msg(msglevel, "Unsupported x509-username-field extension: %s", s);
+        }
         options->x509_username_field = p[1];
     }
 #endif /* ENABLE_X509ALTUSERNAME */

diff --git a/src/openvpn/ssl_verify_backend.h b/src/openvpn/ssl_verify_backend.h
index 978e54fdff6ff0a5a9d2f75c759d05b477134b25..e8eaabe9e584182d2849604591ec936d7720e9fb 100644 (file)
--- a/src/openvpn/ssl_verify_backend.h
+++ b/src/openvpn/ssl_verify_backend.h
@@ -124,6 +124,14 @@ struct buffer x509_get_sha256_fingerprint(openvpn_x509_cert_t *cert,
 result_t backend_x509_get_username(char *common_name, int cn_len,
                                    char *x509_username_field, openvpn_x509_cert_t *peer_cert);
 
+#ifdef ENABLE_X509ALTUSERNAME
+/**
+ * Return true iff the supplied extension field is supported by the
+ * --x509-username-field option.
+ */
+bool x509_username_field_ext_supported(const char *extname);
+#endif
+
 /*
  * Return the certificate's serial number in decimal string representation.
  *

diff --git a/src/openvpn/ssl_verify_openssl.c b/src/openvpn/ssl_verify_openssl.c
index 7c1a481c485c0defe693189ab864626195d6990f..08451f29ad69b03e7b0d016db1e34e494e8d9720 100644 (file)
--- a/src/openvpn/ssl_verify_openssl.c
+++ b/src/openvpn/ssl_verify_openssl.c
@@ -113,16 +113,29 @@ cleanup:
 }
 
 #ifdef ENABLE_X509ALTUSERNAME
+bool x509_username_field_ext_supported(const char *fieldname)
+{
+    int nid = OBJ_txt2nid(fieldname);
+    return nid == NID_subject_alt_name || nid == NID_issuer_alt_name;
+}
+
 static
 bool
 extract_x509_extension(X509 *cert, char *fieldname, char *out, int size)
 {
     bool retval = false;
     char *buf = 0;
-    GENERAL_NAMES *extensions;
-    int nid = OBJ_txt2nid(fieldname);
 
-    extensions = (GENERAL_NAMES *)X509_get_ext_d2i(cert, nid, NULL, NULL);
+    if (!x509_username_field_ext_supported(fieldname))
+    {
+        msg(D_TLS_ERRORS,
+            "ERROR: --x509-alt-username field 'ext:%s' not supported",
+            fieldname);
+        return false;
+    }
+
+    int nid = OBJ_txt2nid(fieldname);
+    GENERAL_NAMES *extensions = X509_get_ext_d2i(cert, nid, NULL, NULL);
     if (extensions)
     {
         int numalts;