diff --git a/src/ssl.c b/src/ssl.c
index 10fd73ca60..869af20011 100644
--- a/src/ssl.c
+++ b/src/ssl.c
@@ -5726,6 +5726,13 @@ int wolfSSL_export_keying_material(WOLFSSL *ssl,
         return WOLFSSL_FAILURE;
     }
 
+    /* Sanity check contextLen to prevent integer overflow when cast to word32
+     * and to ensure it fits in the 2-byte length encoding (max 65535). */
+    if (use_context && contextLen > WOLFSSL_MAX_16BIT) {
+        WOLFSSL_MSG("contextLen too large");
+        return WOLFSSL_FAILURE;
+    }
+
     /* clientRandom + serverRandom
      * OR
      * clientRandom + serverRandom + ctx len encoding + ctx */
diff --git a/src/tls13.c b/src/tls13.c
index 101b31541a..025819bd9f 100644
--- a/src/tls13.c
+++ b/src/tls13.c
@@ -1023,6 +1023,11 @@ int Tls13_Exporter(WOLFSSL* ssl, unsigned char *out, size_t outLen,
     if (ret != 0)
         return ret;
 
+    /* Sanity check contextLen to prevent truncation when cast to word32. */
+    if (contextLen > WOLFSSL_MAX_32BIT) {
+        return BAD_FUNC_ARG;
+    }
+
     /* Hash(context_value) */
     ret = wc_Hash(hashType, context, (word32)contextLen, hashOut, WC_MAX_DIGEST_SIZE);
     if (ret != 0)
diff --git a/tests/api.c b/tests/api.c
index 2cdd81c79b..d8b0677007 100644
--- a/tests/api.c
+++ b/tests/api.c
@@ -24070,6 +24070,11 @@ static int test_export_keying_material_cb(WOLFSSL_CTX *ctx, WOLFSSL *ssl)
             NULL, 0, 0), 0);
     ExpectIntEQ(wolfSSL_export_keying_material(ssl, ekm, sizeof(ekm),
             "key expansion", XSTR_SIZEOF("key expansion"), NULL, 0, 0), 0);
+    /* contextLen overflow: values exceeding UINT16_MAX must be rejected to
+     * prevent integer overflow in seedLen calculation (ZD #21242). */
+    ExpectIntEQ(wolfSSL_export_keying_material(ssl, ekm, sizeof(ekm),
+            "Test label", XSTR_SIZEOF("Test label"), ekm,
+            (size_t)0xFFFF + 1, 1), 0);
 
     return EXPECT_RESULT();
 }