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Require calling mbedtls_ssl_set_hostname() for security

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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>
This commit is contained in:
Gilles Peskine 2025-02-13 14:39:02 +01:00
parent 59a5117072
commit 488b91929d
2 changed files with 14 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
library/ssl_tls.c
View File

@ -8872,6 +8872,10 @@ static int get_hostname_for_verification(mbedtls_ssl_context *ssl,
{
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;

17
tests/ssl-opt.sh
View File

@ -6049,12 +6049,13 @@ run_test "Authentication: hostname null, client none" \
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 none" \
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 @@ requires_config_enabled MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_EPHEMERAL_ENABLED
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"