session mbedtls, openssl UPDATE enforce Extended Key Usage (EKU) and Key Usage (KU) set by client, optional through compile-time flag

Signed-off-by: Niklas Moser <niklas.moser@srs.io>

Author: niklas-moser

CMakeLists.txt

@@ -96,6 +96,7 @@ option(ENABLE_DNSSEC "Enable support for SSHFP retrieval using DNSSEC for SSH (r
 option(ENABLE_PAM "Detect and use PAM" ON)
 option(ENABLE_COMMON_TARGETS "Define common custom target names such as 'doc' or 'uninstall', may cause conflicts when using add_subdirectory() to build this project" ON)
 option(ENABLE_IP_FREEBIND "Enable the IP_FREEBIND/IPV6_FREEBIND options on the listening TCP socket" OFF)
+option(ENABLE_COMPLY_WITH_ORAN_WG11 "Comply with the O-RAN WG11 security spec (e.g. enforce the clientAuth Extended Key Usage and the digitalSignature Key Usage on client certificates)" OFF)
 option(BUILD_SHARED_LIBS "By default, shared libs are enabled. Turn off for a static build." ON)
 set(READ_INACTIVE_TIMEOUT 20 CACHE STRING "Maximum number of seconds waiting for new data once some data have arrived")
 set(READ_ACTIVE_TIMEOUT 300 CACHE STRING "Maximum number of seconds for receiving a full message")
@@ -181,6 +182,11 @@ if(ENABLE_DNSSEC AND NOT ENABLE_SSH_TLS)
     set(ENABLE_DNSSEC OFF)
 endif()
 
+if(ENABLE_COMPLY_WITH_ORAN_WG11 AND NOT ENABLE_SSH_TLS)
+    message(WARNING "O-RAN WG11 compliance cannot be used without TLS support.")
+    set(ENABLE_COMPLY_WITH_ORAN_WG11 OFF)
+endif()
+
 if(ENABLE_VALGRIND_TESTS)
     find_program(VALGRIND_FOUND valgrind)
     if(NOT VALGRIND_FOUND)
@@ -309,6 +315,11 @@ if(ENABLE_SSH_TLS)
 
     # set compiler flag
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DNC_ENABLED_SSH_TLS")
+
+    # comply with the O-RAN WG11 security spec (e.g. client cert Extended Key Usage and Key Usage checks)
+    if(ENABLE_COMPLY_WITH_ORAN_WG11)
+        set(NC_COMPLY_WITH_ORAN_WG11 ON)
+    endif()
 endif()
 
 # dependencies - libval

README.md

@@ -137,6 +137,19 @@ interaction. Enable it with the following command.
 $ cmake -DENABLE_DNSSEC=ON ..
 ```
 
+### O-RAN WG11 Compliance
+
+By default, **libnetconf2** implements only the base YANG and NETCONF
+specifications. The O-RAN WG11 security specification (section 6.9.3) imposes
+additional requirements on the client certificate used for TLS authentication,
+namely that its end-entity certificate must carry the `clientAuth` Extended Key
+Usage and the `digitalSignature` Key Usage. This behavior is not part of the
+base specification, so it is disabled by default and can be enabled with the
+following command (requires TLS support).
+```
+$ cmake -DENABLE_COMPLY_WITH_ORAN_WG11=ON ..
+```
+
 ### Build Modes
 
 There are two build modes:

src/config.h.in

@@ -92,4 +92,7 @@
 /* Enable IP_FREEBIND/IPV6_FREEBIND on listening sockets. */
 #cmakedefine NC_ENABLE_IP_FREEBIND
 
+/* Comply with the O-RAN WG11 security spec (client cert Extended Key Usage and Key Usage checks). */
+#cmakedefine NC_COMPLY_WITH_ORAN_WG11
+
 #endif /* NC_CONFIG_H_ */

src/session_mbedtls.c

@@ -715,6 +715,51 @@ nc_server_tls_prepend_issuer_chain(mbedtls_x509_crt *cert, mbedtls_x509_crt **ch
     return -1;
 }
 
+#ifdef NC_COMPLY_WITH_ORAN_WG11
+
+/**
+ * @brief Check that a client certificate complies with the O-RAN WG11 6.9.3 key usage requirements.
+ *
+ * The end-entity (client) certificate MUST carry the clientAuth Extended Key Usage and a Key Usage
+ * extension with the digitalSignature bit set (the client signs the CertificateVerify handshake message).
+ *
+ * @param[in] session Session for logging.
+ * @param[in] cert Client (leaf) certificate to check.
+ * @param[in,out] flags Verification flags, set on failure.
+ * @return 0 if the certificate complies, non-zero otherwise.
+ */
+static int
+nc_server_tls_check_oran_wg11_usage(struct nc_session *session, mbedtls_x509_crt *cert, uint32_t *flags)
+{
+    const mbedtls_x509_sequence *usage;
+    int has_client_auth = 0;
+
+    /* the client certificate MUST carry the clientAuth Extended Key Usage */
+    for (usage = &cert->ext_key_usage; usage && usage->buf.p; usage = usage->next) {
+        if (!MBEDTLS_OID_CMP(MBEDTLS_OID_CLIENT_AUTH, &usage->buf)) {
+            has_client_auth = 1;
+            break;
+        }
+    }
+    if (!has_client_auth) {
+        ERR(session, "Cert verify: fail (client certificate lacks the required clientAuth Extended Key Usage).");
+        *flags |= MBEDTLS_X509_BADCERT_EXT_KEY_USAGE;
+        return 1;
+    }
+
+    /* the client certificate MUST also carry a Key Usage extension with the digitalSignature bit set */
+    if (!mbedtls_x509_crt_has_ext_type(cert, MBEDTLS_X509_EXT_KEY_USAGE) ||
+            mbedtls_x509_crt_check_key_usage(cert, MBEDTLS_X509_KU_DIGITAL_SIGNATURE)) {
+        ERR(session, "Cert verify: fail (client certificate lacks the required digitalSignature Key Usage).");
+        *flags |= MBEDTLS_X509_BADCERT_KEY_USAGE;
+        return 1;
+    }
+
+    return 0;
+}
+
+#endif /* NC_COMPLY_WITH_ORAN_WG11 */
+
 /**
  * @brief Verify a certificate.
  *
@@ -789,6 +834,13 @@ nc_server_tls_verify_cb(void *cb_data, mbedtls_x509_crt *cert, int depth, uint32
     } else if (!ret) {
         /* success */
         if ((depth == 0) && (!data->session->opts.server.client_cert)) {
+#ifdef NC_COMPLY_WITH_ORAN_WG11
+            /* O-RAN WG11 6.9.3: enforce the clientAuth Extended Key Usage and digitalSignature Key Usage */
+            if (nc_server_tls_check_oran_wg11_usage(data->session, cert, flags)) {
+                return 0;
+            }
+#endif /* NC_COMPLY_WITH_ORAN_WG11 */
+
             /* copy the client cert */
             data->session->opts.server.client_cert = nc_tls_cert_dup(cert);
             if (!data->session->opts.server.client_cert) {

src/session_openssl.c

@@ -379,6 +379,58 @@ nc_server_tls_set_tls_versions_wrap(void *tls_cfg, enum nc_tls_version min, enum
     return 0;
 }
 
+#ifdef NC_COMPLY_WITH_ORAN_WG11
+
+/**
+ * @brief Check that a client certificate complies with the O-RAN WG11 6.9.3 key usage requirements.
+ *
+ * The end-entity (client) certificate MUST carry the clientAuth Extended Key Usage and a Key Usage
+ * extension with the digitalSignature bit set (the client signs the CertificateVerify handshake message).
+ *
+ * @param[in] session Session for logging.
+ * @param[in] cert Client (leaf) certificate to check.
+ * @return 0 if the certificate complies, non-zero otherwise.
+ */
+static int
+nc_server_tls_check_oran_wg11_usage(struct nc_session *session, X509 *cert)
+{
+    EXTENDED_KEY_USAGE *eku;
+    ASN1_BIT_STRING *ku;
+    int has_client_auth = 0, has_digital_signature = 0, i;
+
+    /* the client certificate MUST carry the clientAuth Extended Key Usage */
+    eku = X509_get_ext_d2i(cert, NID_ext_key_usage, NULL, NULL);
+    if (eku) {
+        for (i = 0; i < sk_ASN1_OBJECT_num(eku); ++i) {
+            if (OBJ_obj2nid(sk_ASN1_OBJECT_value(eku, i)) == NID_client_auth) {
+                has_client_auth = 1;
+                break;
+            }
+        }
+        EXTENDED_KEY_USAGE_free(eku);
+    }
+    if (!has_client_auth) {
+        ERR(session, "Cert verify: fail (client certificate lacks the required clientAuth Extended Key Usage).");
+        return 1;
+    }
+
+    /* the client certificate MUST also carry a Key Usage extension with the digitalSignature bit set */
+    ku = X509_get_ext_d2i(cert, NID_key_usage, NULL, NULL);
+    if (ku) {
+        /* bit 0 of the Key Usage bit string is digitalSignature */
+        has_digital_signature = ASN1_BIT_STRING_get_bit(ku, 0);
+        ASN1_BIT_STRING_free(ku);
+    }
+    if (!has_digital_signature) {
+        ERR(session, "Cert verify: fail (client certificate lacks the required digitalSignature Key Usage).");
+        return 1;
+    }
+
+    return 0;
+}
+
+#endif /* NC_COMPLY_WITH_ORAN_WG11 */
+
 /**
  * @brief Verify a certificate.
  *
@@ -463,6 +515,13 @@ nc_server_tls_verify_cb(int preverify_ok, X509_STORE_CTX *x509_ctx)
     } else if (!ret) {
         /* success */
         if ((depth == 0) && (!data->session->opts.server.client_cert)) {
+#ifdef NC_COMPLY_WITH_ORAN_WG11
+            /* O-RAN WG11 6.9.3: enforce the clientAuth Extended Key Usage and digitalSignature Key Usage */
+            if (nc_server_tls_check_oran_wg11_usage(data->session, cert)) {
+                return 0;
+            }
+#endif /* NC_COMPLY_WITH_ORAN_WG11 */
+
             /* copy the client cert */
             data->session->opts.server.client_cert = X509_dup(cert);
             NC_CHECK_ERRMEM_RET(!data->session->opts.server.client_cert, 0);