commit 488b91929dc20d186913eb896202d634c769dbc4
author Gilles Peskine <Gilles.Peskine@arm.com> Thu Feb 13 14:39:02 2025 +0100
committer Gilles Peskine <Gilles.Peskine@arm.com> Mon Feb 24 18:48:49 2025 +0100
tree 339701f9dc29924035c39536726b1e0bc66e174c
parent 59a51170727c0a903c9a6dcbd4707b500d9cdaa3

Require calling mbedtls_ssl_set_hostname() for security

In a TLS client, when using certificate authentication, the client should
check that the certificate is valid for the server name that the client
expects. Otherwise, in most scenarios, a malicious server can impersonate
another server.

Normally, the application code should call mbedtls_ssl_set_hostname().
However, it's easy to forget. So raise an error if mandatory certificate
authentication is in effect and mbedtls_ssl_set_hostname() has not been
called. Raise the new error code
MBEDTLS_ERR_SSL_CERTIFICATE_VERIFICATION_WITHOUT_HOSTNAME, for easy
identification.

But don't raise the error if the backward compatibility option
MBEDTLS_SSL_CLI_ALLOW_WEAK_CERTIFICATE_VERIFICATION_WITHOUT_HOSTNAME is
enabled.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/ssl_tls.c

diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 998cac2..6c401b5 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -8872,6 +8872,10 @@
 {
     if (!mbedtls_ssl_has_set_hostname_been_called(ssl)) {
         MBEDTLS_SSL_DEBUG_MSG(1, ("Certificate verification without having set hostname"));
+        if (mbedtls_ssl_conf_get_endpoint(ssl->conf) == MBEDTLS_SSL_IS_CLIENT &&
+            ssl->conf->authmode == MBEDTLS_SSL_VERIFY_REQUIRED) {
+            return MBEDTLS_ERR_SSL_CERTIFICATE_VERIFICATION_WITHOUT_HOSTNAME;
+        }
     }
 
     *hostname = ssl->hostname;

tests/ssl-opt.sh

diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index ecff16e..8d417af 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -6049,12 +6049,13 @@
 run_test "Authentication: hostname unset, client required" \
          "$P_SRV" \
          "$P_CLI auth_mode=required set_hostname=no debug_level=2" \
-         0 \
+         1 \
          -C "does not match with the expected CN" \
          -c "Certificate verification without having set hostname" \
-         -c "Certificate verification without CN verification" \
+         -C "Certificate verification without CN verification" \
+         -c "get_hostname_for_verification() returned -" \
          -C "x509_verify_cert() returned -" \
-         -C "! mbedtls_ssl_handshake returned" \
+         -c "! mbedtls_ssl_handshake returned" \
          -C "X509 - Certificate verification failed"
 
 run_test "Authentication: hostname unset, client optional" \
@@ -6080,10 +6081,11 @@
 run_test "Authentication: hostname unset, client default, server picks cert, 1.2" \
          "$P_SRV force_version=tls12 force_ciphersuite=TLS-ECDHE-ECDSA-WITH-AES-128-CCM-8" \
          "$P_CLI psk=73776f726466697368 psk_identity=foo set_hostname=no debug_level=2" \
-         0 \
+         1 \
          -C "does not match with the expected CN" \
          -c "Certificate verification without having set hostname" \
-         -c "Certificate verification without CN verification" \
+         -C "Certificate verification without CN verification" \
+         -c "get_hostname_for_verification() returned -" \
          -C "x509_verify_cert() returned -" \
          -C "X509 - Certificate verification failed"
 
@@ -6091,10 +6093,11 @@
 run_test "Authentication: hostname unset, client default, server picks cert, 1.3" \
          "$P_SRV force_version=tls13 tls13_kex_modes=ephemeral" \
          "$P_CLI psk=73776f726466697368 psk_identity=foo set_hostname=no debug_level=2" \
-         0 \
+         1 \
          -C "does not match with the expected CN" \
          -c "Certificate verification without having set hostname" \
-         -c "Certificate verification without CN verification" \
+         -C "Certificate verification without CN verification" \
+         -c "get_hostname_for_verification() returned -" \
          -C "x509_verify_cert() returned -" \
          -C "X509 - Certificate verification failed"