diff --git a/ChangeLog.d/tls13-fix-key-usage-checks.txt b/ChangeLog.d/tls13-fix-key-usage-checks.txt
new file mode 100644
index 0000000..f19bf52
--- /dev/null
+++ b/ChangeLog.d/tls13-fix-key-usage-checks.txt
@@ -0,0 +1,7 @@
+Security
+   * Fix check of certificate key usage in TLS 1.3. The usage of the public key
+     provided by a client or server certificate for authentication was not
+     checked properly when validating the certificate. This could cause a
+     client or server to be able to authenticate itself through a certificate
+     to an Mbed TLS TLS 1.3 server or client while it does not own a proper
+     certificate to do so.
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index c48922f..c7e00a9 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -546,6 +546,8 @@
     int authmode = MBEDTLS_SSL_VERIFY_REQUIRED;
     mbedtls_x509_crt *ca_chain;
     mbedtls_x509_crl *ca_crl;
+    const char *ext_oid;
+    size_t ext_len;
     uint32_t verify_result = 0;
 
     /* If SNI was used, overwrite authentication mode
@@ -627,12 +629,25 @@
     /*
      * Secondary checks: always done, but change 'ret' only if it was 0
      */
-    if( mbedtls_ssl_check_cert_usage( ssl->session_negotiate->peer_cert,
-                                      ssl->handshake->ciphersuite_info,
-                                      !ssl->conf->endpoint,
-                                      &verify_result ) != 0 )
+    if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate ( usage extensions )" ) );
+        ext_oid = MBEDTLS_OID_SERVER_AUTH;
+        ext_len = MBEDTLS_OID_SIZE( MBEDTLS_OID_SERVER_AUTH );
+    }
+    else
+    {
+        ext_oid = MBEDTLS_OID_CLIENT_AUTH;
+        ext_len = MBEDTLS_OID_SIZE( MBEDTLS_OID_CLIENT_AUTH );
+    }
+
+    if( ( mbedtls_x509_crt_check_key_usage(
+              ssl->session_negotiate->peer_cert,
+              MBEDTLS_X509_KU_DIGITAL_SIGNATURE ) != 0 ) ||
+        ( mbedtls_x509_crt_check_extended_key_usage(
+              ssl->session_negotiate->peer_cert,
+              ext_oid, ext_len ) != 0 ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate (usage extensions)" ) );
         if( ret == 0 )
             ret = MBEDTLS_ERR_SSL_BAD_CERTIFICATE;
     }
diff --git a/programs/ssl/ssl_test_common_source.c b/programs/ssl/ssl_test_common_source.c
index 0e66895..511ede9 100644
--- a/programs/ssl/ssl_test_common_source.c
+++ b/programs/ssl/ssl_test_common_source.c
@@ -285,6 +285,9 @@
 #if defined(MBEDTLS_SHA224_C)
     MBEDTLS_SSL_SIG_ALG( MBEDTLS_SSL_HASH_SHA224 )
 #endif
+#if defined(MBEDTLS_RSA_C) && defined(MBEDTLS_SHA256_C)
+    MBEDTLS_TLS1_3_SIG_RSA_PSS_RSAE_SHA256,
+#endif /* MBEDTLS_RSA_C && MBEDTLS_SHA256_C */
 #if defined(MBEDTLS_SHA1_C)
     /* Allow SHA-1 as we use it extensively in tests. */
     MBEDTLS_SSL_SIG_ALG( MBEDTLS_SSL_HASH_SHA1 )
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 9ec2ffa..41d69a3 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -6016,7 +6016,6 @@
             0 \
             -c "Ciphersuite is TLS-[EC]*DHE-RSA-WITH-"
 
-
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
 run_test    "keyUsage srv: RSA, keyEncipherment -> RSA" \
             "$P_SRV key_file=data_files/server2.key \
@@ -6151,6 +6150,78 @@
             -c "Ciphersuite is TLS-" \
             -c "! Usage does not match the keyUsage extension"
 
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: DigitalSignature+KeyEncipherment, RSA: OK" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server2.key \
+             -cert data_files/server2.ku-ds_ke.crt" \
+            "$P_CLI debug_level=3" \
+            0 \
+            -C "bad certificate (usage extensions)" \
+            -C "Processing of the Certificate handshake message failed" \
+            -c "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: KeyEncipherment, RSA: fail" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server2.key \
+             -cert data_files/server2.ku-ke.crt" \
+            "$P_CLI debug_level=1" \
+            1 \
+            -c "bad certificate (usage extensions)" \
+            -c "Processing of the Certificate handshake message failed" \
+            -C "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: KeyAgreement, RSA: fail" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server2.key \
+             -cert data_files/server2.ku-ka.crt" \
+            "$P_CLI debug_level=1" \
+            1 \
+            -c "bad certificate (usage extensions)" \
+            -c "Processing of the Certificate handshake message failed" \
+            -C "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: DigitalSignature, ECDSA: OK" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.ku-ds.crt" \
+            "$P_CLI debug_level=3" \
+            0 \
+            -C "bad certificate (usage extensions)" \
+            -C "Processing of the Certificate handshake message failed" \
+            -c "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: KeyEncipherment, ECDSA: fail" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.ku-ke.crt" \
+            "$P_CLI debug_level=1" \
+            1 \
+            -c "bad certificate (usage extensions)" \
+            -c "Processing of the Certificate handshake message failed" \
+            -C "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli 1.3: KeyAgreement, ECDSA: fail" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.ku-ka.crt" \
+            "$P_CLI debug_level=1" \
+            1 \
+            -c "bad certificate (usage extensions)" \
+            -c "Processing of the Certificate handshake message failed" \
+            -C "Ciphersuite is"
+
 # Tests for keyUsage in leaf certificates, part 3:
 # server-side checking of client cert
 
@@ -6160,6 +6231,7 @@
             "$O_CLI -key data_files/server2.key \
              -cert data_files/server2.ku-ds.crt" \
             0 \
+            -s "Verifying peer X.509 certificate... ok" \
             -S "bad certificate (usage extensions)" \
             -S "Processing of the Certificate handshake message failed"
 
@@ -6187,6 +6259,7 @@
             "$O_CLI -key data_files/server5.key \
              -cert data_files/server5.ku-ds.crt" \
             0 \
+            -s "Verifying peer X.509 certificate... ok" \
             -S "bad certificate (usage extensions)" \
             -S "Processing of the Certificate handshake message failed"
 
@@ -6199,6 +6272,52 @@
             -s "bad certificate (usage extensions)" \
             -S "Processing of the Certificate handshake message failed"
 
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli-auth 1.3: RSA, DigitalSignature: OK" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server2.key \
+             -cert data_files/server2.ku-ds.crt" \
+            0 \
+            -s "Verifying peer X.509 certificate... ok" \
+            -S "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli-auth 1.3: RSA, KeyEncipherment: fail (soft)" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server2.key \
+             -cert data_files/server2.ku-ke.crt" \
+            0 \
+            -s "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli-auth 1.3: ECDSA, DigitalSignature: OK" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.ku-ds.crt" \
+            0 \
+            -s "Verifying peer X.509 certificate... ok" \
+            -S "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "keyUsage cli-auth 1.3: ECDSA, KeyAgreement: fail (soft)" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.ku-ka.crt" \
+            0 \
+            -s "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
 # Tests for extendedKeyUsage, part 1: server-side certificate/suite selection
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
@@ -6271,6 +6390,54 @@
             -c "Processing of the Certificate handshake message failed" \
             -C "Ciphersuite is TLS-"
 
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli 1.3: serverAuth -> OK" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.eku-srv.crt" \
+            "$P_CLI debug_level=1" \
+            0 \
+            -C "bad certificate (usage extensions)" \
+            -C "Processing of the Certificate handshake message failed" \
+            -c "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli 1.3: serverAuth,clientAuth -> OK" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.eku-srv_cli.crt" \
+            "$P_CLI debug_level=1" \
+            0 \
+            -C "bad certificate (usage extensions)" \
+            -C "Processing of the Certificate handshake message failed" \
+            -c "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli 1.3: codeSign,anyEKU -> OK" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.eku-cs_any.crt" \
+            "$P_CLI debug_level=1" \
+            0 \
+            -C "bad certificate (usage extensions)" \
+            -C "Processing of the Certificate handshake message failed" \
+            -c "Ciphersuite is"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli 1.3: codeSign -> fail" \
+            "$O_NEXT_SRV_NO_CERT -tls1_3 -num_tickets=0 -key data_files/server5.key \
+             -cert data_files/server5.eku-cs.crt" \
+            "$P_CLI debug_level=1" \
+            1 \
+            -c "bad certificate (usage extensions)" \
+            -c "Processing of the Certificate handshake message failed" \
+            -C "Ciphersuite is"
+
 # Tests for extendedKeyUsage, part 3: server-side checking of client cert
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
@@ -6318,6 +6485,50 @@
             -s "bad certificate (usage extensions)" \
             -s "Processing of the Certificate handshake message failed"
 
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli-auth 1.3: clientAuth -> OK" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.eku-cli.crt" \
+            0 \
+            -S "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli-auth 1.3: serverAuth,clientAuth -> OK" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.eku-srv_cli.crt" \
+            0 \
+            -S "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli-auth 1.3: codeSign,anyEKU -> OK" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.eku-cs_any.crt" \
+            0 \
+            -S "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_openssl_tls1_3
+requires_config_disabled MBEDTLS_SSL_PROTO_TLS1_2
+run_test    "extKeyUsage cli-auth 1.3: codeSign -> fail (soft)" \
+            "$P_SRV debug_level=1 auth_mode=optional" \
+            "$O_NEXT_CLI_NO_CERT -key data_files/server5.key \
+             -cert data_files/server5.eku-cs.crt" \
+            0 \
+            -s "bad certificate (usage extensions)" \
+            -S "Processing of the Certificate handshake message failed"
+
 # Tests for DHM parameters loading
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2