obgm · mrdeep1 · Jun 27, 2026 · Jun 25, 2026
diff --git a/examples/coap-client.c b/examples/coap-client.c
@@ -1648,17 +1648,18 @@ setup_pki(coap_context_t *ctx, coap_dtls_role_t role) {
    * but this is used to define what checking actually takes place.
    */
   dtls_pki.verify_peer_cert        = verify_peer_cert;
-  dtls_pki.check_common_ca         = !root_ca_file;
-  dtls_pki.allow_self_signed       = 1;
+  dtls_pki.check_common_ca         = ca_file != NULL;
+  dtls_pki.allow_self_signed       = !(root_ca_file || ca_file);;
   dtls_pki.allow_expired_certs     = 1;
   dtls_pki.cert_chain_validation   = 1;
   dtls_pki.cert_chain_verify_depth = 2;
   dtls_pki.check_cert_revocation   = 1;
   dtls_pki.allow_no_crl            = 1;
   dtls_pki.allow_expired_crl       = 1;
-  dtls_pki.is_rpk_not_cert = is_rpk_not_cert;
-  dtls_pki.use_cid = setup_cid;
-  dtls_pki.validate_cn_call_back = verify_cn_callback;
+  dtls_pki.is_rpk_not_cert         = is_rpk_not_cert;
+  dtls_pki.use_cid                 = setup_cid;
+  dtls_pki.allow_sni_cn_mismatch   = 0; /* Not recommended to set */
+  dtls_pki.validate_cn_call_back   = verify_cn_callback;
   if (role == COAP_DTLS_ROLE_CLIENT) {
     if (proxy.host.length) {
       snprintf(client_sni, sizeof(client_sni), "%*.*s", (int)proxy.host.length, (int)proxy.host.length,

diff --git a/examples/tls_backend_testcases.sh b/examples/tls_backend_testcases.sh
@@ -661,7 +661,7 @@ run_pki_client () {
 
   if [ -n "$root_ca_file" ]; then
     client_args="$client_args -R $root_ca_file"
-  else
+  elif  [ -n "$ca_file" ]; then
     client_args="$client_args -C $ca_file"
   fi
 
@@ -786,7 +786,8 @@ run_psk_identity_hint_callback () {
   run_client "$case_name" "$PSK_KEY" "" "$CLIENT_TIMEOUT" "$TARGET_URI" \
     "$hint_file"
 
-  if assert_contains "$LOGDIR/$case_name.client" "does not support Identity Hint selection"; then
+  if assert_contains "$LOGDIR/$case_name.client" "does not support Identity Hint" ||
+     assert_contains "$LOGDIR/$case_name.server" "does not support Identity Hint" ; then
     skip_case "Identity Hint not supported"
   elif assert_contains "$LOGDIR/$case_name.client" "Identity Hint '$PSK_HINT' provided" &&
      assert_contains "$LOGDIR/$case_name.client" "Switching to using '$PSK_IDENTITY' identity \\+ '$PSK_KEY' key" &&
@@ -1167,7 +1168,7 @@ run_pki_chain_depth_too_long () {
      assert_not_contains "$LOGDIR/$case_name.client" "2\\.05" &&
      assert_not_contains "$LOGDIR/$case_name.server" "call handler for pseudo resource '.well-known/core'" &&
      assert_either_contains "$LOGDIR/$case_name.client" "$LOGDIR/$case_name.server" \
-       "certificate chain too long|constraint limits|unknown_ca|0x400000f|alert|No response received"; then
+       "The certificate's verify depth is too long|certificate chain too long|constraint limits|unknown_ca|0x400000f|alert|No response received"; then
     pass_case
   else
     fail_case "$case_name" "PKI chain beyond configured depth limit was accepted"
@@ -1189,7 +1190,7 @@ run_pki_self_signed_leaf () {
     return
   fi
 
-  run_pki_client "$case_name" "" "$CLIENT_TIMEOUT" no "$pki_dir/bad_ca.pem"
+  run_pki_client "$case_name" "" "$CLIENT_TIMEOUT"
 
   if assert_contains "$LOGDIR/$case_name.client" "COAP_EVENT_DTLS_CONNECTED" &&
      assert_contains "$LOGDIR/$case_name.client" "2\\.05" &&
@@ -1241,7 +1242,7 @@ run_pki_non_self_signed_override () {
     return
   fi
 
-  run_pki_client "$case_name" "" "$CLIENT_TIMEOUT" no "$pki_dir/bad_ca.pem"
+  run_pki_client "$case_name" "" "$CLIENT_TIMEOUT"
 
   if assert_not_contains "$LOGDIR/$case_name.client" "COAP_EVENT_DTLS_CONNECTED" &&
      assert_not_contains "$LOGDIR/$case_name.server" "call handler for pseudo resource '.well-known/core'" &&

diff --git a/include/coap3/coap_dtls.h b/include/coap3/coap_dtls.h
@@ -338,14 +338,17 @@ struct coap_dtls_pki_t {
   uint8_t use_cid;                 /**< 1 if DTLS Connection ID is to be
                                     *     used (Client only, server always enabled)
                                     *     if supported */
-  uint8_t reserved[2];             /**< Reserved - must be set to 0 for
+  uint8_t allow_sni_cn_mismatch:1; /**< 1 if SNI and returnd CN allowed to mismatch
+                                    *     (Client only). Not normally set */
+  uint8_t reserved[1];             /**< Reserved - must be set to 0 for
                                         future compatibility */
-  /* Size of 2 chosen to align to next
+  /* Size of 1 chosen to align to next
    * parameter, so if newly defined option
    * it can use one of the reserved slots so
    * no need to change
    * COAP_DTLS_PKI_SETUP_VERSION and just
    * decrement the reserved[] count.
+   * [Now just adding 1 bit at a time].
    */
 
   /** CN check callback function.

diff --git a/man/coap-client.txt.in b/man/coap-client.txt.in
@@ -298,12 +298,13 @@ definitions have to be in DER, not PEM, format.  Otherwise all of
   Disable remote peer certificate checking.
 
 *-C* cafile::
-PEM file or PKCS11 URI for the CA certificate and any intermediate CAs that was
+  PEM file or PKCS11 URI for the CA certificate and any intermediate CAs that was
   used to sign the server
   certfile. Ideally the client certificate should be signed by the same CA so that
   mutual authentication can take place. The contents of cafile are added
   to the trusted store of root CAs. Using the *-C* or *-R* options will trigger
-  the validation of the server certificate unless overridden by the *-n* option.
+  the validation of the server certificate (including not allowing self-signed
+  certificates) unless overridden by the *-n* option.
 
 *-J* pkcs11_pin::
   The user pin to unlock access to the PKCS11 token.
@@ -320,8 +321,8 @@ PEM file or PKCS11 URI for the CA certificate and any intermediate CAs that was
   to be in this list and is trusted for the validation. Using
   *-R trust_casfile* disables common CA mutual authentication which can only
   be done by using *-C cafile*. Using the *-C* or *-R* options will
-  trigger the validation of the server certificate unless overridden by the
-  *-n* option.
+  trigger the validation of the server certificate (including not allowing
+  self-signed certificates) unless overridden by the *-n* option.
 
 *-Y* ::
   Do not load the default system Trusted Root CA Store.

diff --git a/man/coap_encryption.txt.in b/man/coap_encryption.txt.in
@@ -515,6 +515,8 @@ typedef struct coap_dtls_pki_t {
   uint8_t use_cid;                  /* 1 if DTLS Connection ID is to be
                                      *   used (Client only, server always enabled)
                                      *   if supported */
+  uint8_t allow_sni_cn_mismatch:1;  /* 1 if SNI and returnd CN allowed to mismatch
+                                     * (Client only). Not normally set */
   uint8_t reserved[2];              /* Reserved - must be set to 0 for
                                        future compatibility */
 
@@ -574,8 +576,8 @@ for different versions of the coap_dtls_pki_t structure in the future.
 *verify_peer_cert* Set to 1 to check that the peer's certificate is valid if
 provided, else 0. If not set, check_common_ca, allow_self_signed,
 allow_expired_certs, cert_chain_validation, cert_chain_verify_depth,
-check_cert_revocation, allow_no_crl, allow_expired_crl, allow_bad_md_hash
-and allow_short_rsa_length settings are all ignored.
+check_cert_revocation, allow_no_crl, allow_expired_crl, allow_bad_md_hash,
+allow_short_rsa_length and allow_sni_cn_mismatch settings are all ignored.
 
 *check_common_ca* Set to 1 to check that the CA that signed the peer's
 certificate is the same CA that signed the local certificate
@@ -624,7 +626,11 @@ allow_no_crl, allow_expired_crl, allow_bad_md_hash and
 allow_short_rsa_length settings are all ignored.
 
 *use_cid* Set to 1 if the DTLS Client is to try to used Connection-ID.
-Server is alwys enabled if support available.
+Server is always enabled if support available.
+
+*allow_sni_cn_mismatch* Set to 1 if the DTLS Client is not to compare the
+Common Name (CN) of the received certificate with the sent SNI hostname.
+This is not recommended.
 
 *SECTION: PKI/RPK: coap_dtls_pki_t: Reserved*
 
@@ -1168,6 +1174,8 @@ setup_server_context_pki(const char *public_cert_file,
   dtls_pki.allow_bad_md_hash       = 0;
   dtls_pki.allow_short_rsa_length  = 0;
   dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
+  dtls_pki.use_cid                 = 0;
+  dtls_pki.allow_sni_cn_mismatch   = 0;
   dtls_pki.validate_cn_call_back   = verify_cn_callback;
   dtls_pki.cn_call_back_arg        = valid_cn_list;
   dtls_pki.validate_sni_call_back  = verify_pki_sni_callback;

diff --git a/man/coap_endpoint_client.txt.in b/man/coap_endpoint_client.txt.in
@@ -425,6 +425,8 @@ setup_client_session_pki(const char *host,
   dtls_pki.allow_bad_md_hash       = 0;
   dtls_pki.allow_short_rsa_length  = 0;
   dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
+  dtls_pki.use_cid                 = 0;
+  dtls_pki.allow_sni_cn_mismatch   = 0;
   dtls_pki.validate_cn_call_back   = verify_cn_callback;
   dtls_pki.cn_call_back_arg        = NULL;
   dtls_pki.validate_sni_call_back  = NULL;

diff --git a/man/coap_endpoint_server.txt.in b/man/coap_endpoint_server.txt.in
@@ -390,6 +390,8 @@ setup_server_context_pki(const char *public_cert_file,
   dtls_pki.allow_bad_md_hash       = 0;
   dtls_pki.allow_short_rsa_length  = 0;
   dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
+  dtls_pki.use_cid                 = 0;
+  dtls_pki.allow_sni_cn_mismatch   = 0;
   dtls_pki.validate_cn_call_back   = verify_cn_callback;
   dtls_pki.cn_call_back_arg        = valid_cn_list;
   dtls_pki.validate_sni_call_back  = verify_pki_sni_callback;

diff --git a/man/coap_tls_library.txt.in b/man/coap_tls_library.txt.in
@@ -211,6 +211,9 @@ setup_server_context_pki(const char *public_cert_file,
   dtls_pki.check_cert_revocation   = 1;
   dtls_pki.allow_no_crl            = 1;
   dtls_pki.allow_expired_crl       = 1;
+  dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
+  dtls_pki.use_cid                 = 0;
+  dtls_pki.allow_sni_cn_mismatch   = 0;
   dtls_pki.pki_key.key_type                      = COAP_PKI_KEY_DEFINE;
   dtls_pki.pki_key.key.define.ca.s_byte          = ca_file;
   dtls_pki.pki_key.key.define.public_cert.s_byte = public_cert_file;

diff --git a/src/coap_gnutls.c b/src/coap_gnutls.c
@@ -867,8 +867,18 @@ cert_verify_gnutls(gnutls_session_t g_session) {
       goto fail;
   }
 
-  G_CHECK(gnutls_certificate_verify_peers(g_session, NULL, 0, &status),
-          "gnutls_certificate_verify_peers");
+  if (c_session->type == COAP_SESSION_TYPE_CLIENT && g_context->setup_data.client_sni) {
+    gnutls_typed_vdata_st host;
+
+    host.type = GNUTLS_DT_DNS_HOSTNAME;
+    host.data = (uint8_t *)g_context->setup_data.client_sni;
+    host.size = strlen(g_context->setup_data.client_sni);
+    G_CHECK(gnutls_certificate_verify_peers(g_session, &host, 1, &status),
+            "gnutls_certificate_verify_peers");
+  } else {
+    G_CHECK(gnutls_certificate_verify_peers(g_session, NULL, 0, &status),
+            "gnutls_certificate_verify_peers");
+  }
 
   coap_dtls_log(COAP_LOG_DEBUG, "error %x cert '%s'\n",
                 status, cert_info.san_or_cn);
@@ -922,6 +932,23 @@ cert_verify_gnutls(gnutls_session_t g_session) {
                       OUTPUT_CERT_NAME);
       }
     }
+    if (status & (GNUTLS_CERT_UNEXPECTED_OWNER)) {
+      status &= ~(GNUTLS_CERT_UNEXPECTED_OWNER);
+      if (g_context->setup_data.allow_sni_cn_mismatch) {
+        coap_log_info("   %s: %s: overridden: '%s' v '%s'\n",
+                      coap_session_str(c_session),
+                      "The SNI does not match the returned CN",
+                      g_context->setup_data.client_sni,
+                      OUTPUT_CERT_NAME);
+      } else {
+        fail = 1;
+        coap_log_warn("   %s: %s: '%s' v '%s'\n",
+                      coap_session_str(c_session),
+                      "The SNI does not match the returned CN",
+                      g_context->setup_data.client_sni,
+                      OUTPUT_CERT_NAME);
+      }
+    }
     if (status & (GNUTLS_CERT_REVOCATION_DATA_SUPERSEDED|
                   GNUTLS_CERT_REVOCATION_DATA_ISSUED_IN_FUTURE)) {
       status &= ~(GNUTLS_CERT_REVOCATION_DATA_SUPERSEDED|

diff --git a/src/coap_mbedtls.c b/src/coap_mbedtls.c
@@ -679,12 +679,15 @@ cert_verify_callback_mbedtls(void *data, mbedtls_x509_crt *crt,
   char *cn = NULL;
 
   cn = get_san_or_cn_from_cert(crt);
+  coap_dtls_log(COAP_LOG_DEBUG, "depth %d flags %x cert '%s'\n",
+                depth, *flags, cn);
+  if (depth == 0 && c_session->type == COAP_SESSION_TYPE_CLIENT && setup_data->client_sni && cn &&
+      strcmp(cn, setup_data->client_sni)) {
+    *flags |= MBEDTLS_X509_BADCERT_CN_MISMATCH;
+  }
   if (setup_data->validate_cn_call_back) {
     int ret;
 
-    if (*flags & MBEDTLS_X509_BADCERT_CN_MISMATCH) {
-      *flags &= ~MBEDTLS_X509_BADCERT_CN_MISMATCH;
-    }
     coap_lock_callback_ret(ret,
                            setup_data->validate_cn_call_back(cn,
                                                              crt->raw.p,
@@ -697,6 +700,14 @@ cert_verify_callback_mbedtls(void *data, mbedtls_x509_crt *crt,
       *flags |= MBEDTLS_X509_BADCERT_CN_MISMATCH;
     }
   }
+  if (setup_data->cert_chain_validation &&
+      depth > (setup_data->cert_chain_verify_depth + 2)) {
+    *flags |= MBEDTLS_X509_BADCERT_OTHER;
+    coap_log_warn("   %s: %s: '%s' depth %d\n",
+                  coap_session_str(c_session),
+                  "The certificate's verify depth is too long",
+                  cn ? cn : "?", depth);
+  }
   if (*flags == 0) {
     if (cn)
       mbedtls_free(cn);
@@ -735,6 +746,16 @@ cert_verify_callback_mbedtls(void *data, mbedtls_x509_crt *crt,
                     "The certificate has a short RSA length", cn ? cn : "?", depth);
     }
   }
+  if (*flags & MBEDTLS_X509_BADCERT_CN_MISMATCH) {
+    if (setup_data->allow_sni_cn_mismatch) {
+      *flags &= ~MBEDTLS_X509_BADCERT_CN_MISMATCH;
+      coap_log_info("   %s: %s: overridden: '%s' v '%s' depth %d\n",
+                    coap_session_str(c_session),
+                    "The SNI does not match the returned CN",
+                    setup_data->client_sni ? setup_data->client_sni : "",
+                    cn ? cn : "?", depth);
+    }
+  }
   if (*flags & MBEDTLS_X509_BADCERT_NOT_TRUSTED) {
     uint32_t lflags;
     int self_signed = !mbedtls_x509_crt_verify(crt, crt, NULL, NULL, &lflags,
@@ -785,14 +806,6 @@ cert_verify_callback_mbedtls(void *data, mbedtls_x509_crt *crt,
       *flags &= ~MBEDTLS_X509_BADCRL_FUTURE;
     }
   }
-  if (setup_data->cert_chain_validation &&
-      depth > (setup_data->cert_chain_verify_depth + 1)) {
-    *flags |= MBEDTLS_X509_BADCERT_OTHER;
-    coap_log_warn("   %s: %s: '%s' depth %d\n",
-                  coap_session_str(c_session),
-                  "The certificate's verify depth is too long",
-                  cn ? cn : "?", depth);
-  }
 
   if (*flags != 0) {
     char buf[128];
@@ -2401,6 +2414,9 @@ coap_dtls_context_set_spsk(coap_context_t *c_context,
     coap_log_warn("Mbed TLS not compiled for EC-JPAKE support\n");
 #endif /* ! MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
   }
+  if (setup_data->psk_info.hint.length) {
+    coap_log_warn("CoAP Server with Mbed TLS does not support Identity Hint\n");
+  }
   m_context->psk_pki_enabled |= IS_PSK;
   return 1;
 }

diff --git a/src/coap_openhitls.c b/src/coap_openhitls.c
@@ -796,10 +796,8 @@ coap_hitls_validate_cn_cert(coap_session_t *session,
   uint8_t *der = NULL;
   uint32_t der_len = 0;
   BSL_Buffer cn;
-  int ret;
+  int ret = 1;
 
-  if (!setup_data->validate_cn_call_back)
-    return 1;
   memset(&cn, 0, sizeof(cn));
   (void)HITLS_X509_CertCtrl(cert, HITLS_X509_GET_ENCODELEN,
                             &der_len, sizeof(der_len));
@@ -809,11 +807,18 @@ coap_hitls_validate_cn_cert(coap_session_t *session,
   (void)HITLS_X509_CertCtrl(cert, HITLS_X509_GET_SUBJECT_CN_STR,
                             &cn, sizeof(cn));
 
-  coap_lock_callback_ret(ret,
-                         setup_data->validate_cn_call_back(
-                             cn.data ? (const char *)cn.data : "",
-                             der, der_len, session, depth, validated,
-                             setup_data->cn_call_back_arg));
+  if (setup_data->validate_cn_call_back) {
+    coap_lock_callback_ret(ret,
+                           setup_data->validate_cn_call_back(
+                               cn.data ? (const char *)cn.data : "",
+                               der, der_len, session, depth, validated,
+                               setup_data->cn_call_back_arg));
+  }
+  if (depth == 0 && session->type == COAP_SESSION_TYPE_CLIENT &&
+      setup_data->client_sni && !setup_data->allow_sni_cn_mismatch &&
+      cn.data && strcmp((const char *)cn.data, setup_data->client_sni)) {
+    ret = 0;
+  }
   if (cn.data)
     BSL_SAL_Free(cn.data);
   return ret;

diff --git a/src/coap_openssl.c b/src/coap_openssl.c
@@ -2469,7 +2469,6 @@ tls_verify_call_back(int preverify_ok, X509_STORE_CTX *ctx) {
   int err = X509_STORE_CTX_get_error(ctx);
   X509 *x509 = X509_STORE_CTX_get_current_cert(ctx);
   char *cn = x509 ? get_san_or_cn_from_cert(x509) : NULL;
-  int keep_preverify_ok = preverify_ok;
 
   coap_dtls_log(COAP_LOG_DEBUG, "depth %d error %x preverify %d cert '%s'\n",
                 depth, err, preverify_ok, cn);
@@ -2478,6 +2477,12 @@ tls_verify_call_back(int preverify_ok, X509_STORE_CTX *ctx) {
     OPENSSL_free(cn);
     return 0;
   }
+  if (depth == 0 && session->type == COAP_SESSION_TYPE_CLIENT && setup_data->client_sni &&
+      !setup_data->allow_sni_cn_mismatch && cn && strcmp(cn, setup_data->client_sni)) {
+    preverify_ok = 0;
+    X509_STORE_CTX_set_error(ctx, X509_V_ERR_SUBJECT_ISSUER_MISMATCH);
+    err = X509_V_ERR_SUBJECT_ISSUER_MISMATCH;
+  }
   if (!preverify_ok) {
     switch (err) {
     case X509_V_ERR_CERT_NOT_YET_VALID:
@@ -2508,6 +2513,10 @@ tls_verify_call_back(int preverify_ok, X509_STORE_CTX *ctx) {
       if (!setup_data->verify_peer_cert)
         preverify_ok = 1;
       break;
+    case X509_V_ERR_SUBJECT_ISSUER_MISMATCH:
+      if (setup_data->allow_sni_cn_mismatch)
+        preverify_ok = 1;
+      break;
     default:
       break;
     }
@@ -2534,7 +2543,7 @@ tls_verify_call_back(int preverify_ok, X509_STORE_CTX *ctx) {
     }
   }
   /* Certificate - depth == 0 is the Client Cert */
-  if (setup_data->validate_cn_call_back && keep_preverify_ok) {
+  if (setup_data->validate_cn_call_back) {
     int length = i2d_X509(x509, NULL);
     uint8_t *base_buf;
     uint8_t *base_buf2 = base_buf = length > 0 ? OPENSSL_malloc(length) : NULL;