diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index ae5eb8d..c9f9325 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -5780,7 +5780,8 @@
  *
  * \param ssl         SSL context from which to export keys. Must have finished the handshake.
  * \param out         Output buffer of length at least key_len bytes.
- * \param key_len     Length of the key to generate in bytes. Must be < 2^16 in TLS 1.3.
+ * \param key_len     Length of the key to generate in bytes. In TLS 1.3, this can be at most
+ *                    8160 if SHA256 is used as hash function or 12240 if SHA384 is used.
  * \param label       Label for which to generate the key of length label_len.
  * \param label_len   Length of label in bytes. Must be < 251 in TLS 1.3.
  * \param context     Context of the key. Can be NULL if context_len or use_context is 0.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index d631420..82cb36c 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -10124,11 +10124,16 @@
     const size_t hash_len = PSA_HASH_LENGTH(hash_alg);
     const unsigned char *secret = ssl->session->app_secrets.exporter_master_secret;
 
-    /* Check that the label and key_len fit into the HkdfLabel struct as defined
-     * in RFC 8446, Section 7.1. key_len must fit into an uint16 and the label
-     * must be at most 250 bytes long. (The struct allows up to 256 bytes for
-     * the label, but it is prefixed with "tls13 ".) */
-    if (key_len > UINT16_MAX || label_len > 250) {
+    /* Validate the length of the label and the desired key length. The key
+     * length can be at most 255 * hash_len by definition of HKDF-Expand in
+     * RFC 5869.
+     *
+     * The length of the label must be at most 250 bytes long to fit into the
+     * HkdfLabel struct as defined in RFC 8446, Section 7.1. This struct also
+     * requires that key_len fits into a uint16, but until we have to deal with
+     * a hash function with more than 2048 bits of output, the 255 * hash_len
+     * limit will guarantee that. */
+    if (key_len > 255 * hash_len || label_len > 250) {
         return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
     }
 
diff --git a/tests/suites/test_suite_ssl.data b/tests/suites/test_suite_ssl.data
index b566d7c..79e56b4 100644
--- a/tests/suites/test_suite_ssl.data
+++ b/tests/suites/test_suite_ssl.data
@@ -3489,7 +3489,7 @@
 
 TLS 1.3 Keying Material Exporter: Consistent results, large keys
 depends_on:MBEDTLS_SSL_PROTO_TLS1_3
-ssl_tls_exporter_consistent_result:MBEDTLS_SSL_VERSION_TLS1_3:1024:0
+ssl_tls_exporter_consistent_result:MBEDTLS_SSL_VERSION_TLS1_3:255 * 32:0
 
 TLS 1.3 Keying Material Exporter: Uses label
 depends_on:MBEDTLS_SSL_PROTO_TLS1_3
@@ -3505,7 +3505,7 @@
 
 TLS 1.3 Keying Material Exporter: Exported key too long
 depends_on:MBEDTLS_SSL_PROTO_TLS1_3
-ssl_tls_exporter_rejects_bad_parameters:MBEDTLS_SSL_VERSION_TLS1_3:UINT16_MAX + 1:20:20
+ssl_tls_exporter_rejects_bad_parameters:MBEDTLS_SSL_VERSION_TLS1_3:255 * 48 + 1:20:20
 
 TLS 1.3 Keying Material Exporter: Label too long
 depends_on:MBEDTLS_SSL_PROTO_TLS1_3