From 23c131dcac92952c21c04b6831cd1c45cbcd7289 Mon Sep 17 00:00:00 2001
From: Eric Blankenhorn <eric@wolfssl.com>
Date: Thu, 9 Apr 2026 15:12:06 -0500
Subject: [PATCH] Fix TLSX_EchChangeSNI to check hostname length

---
 src/tls.c   |  3 +++
 tests/api.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/src/tls.c b/src/tls.c
index 18aad71d39..8e2c7a3915 100644
--- a/src/tls.c
+++ b/src/tls.c
@@ -16110,6 +16110,9 @@ static int TLSX_EchChangeSNI(WOLFSSL* ssl, TLSX** pEchX,
                 hostNameSz = MAX_PUBLIC_NAME_SZ;
 
             XMEMCPY(serverName, hostName, hostNameSz);
+            /* Guarantee NUL termination after truncation so that
+             * TLSX_EchRestoreSNI's XSTRLEN cannot read past the buffer. */
+            serverName[hostNameSz - 1] = '\0';
         }
 
         /* only swap the SNI if one was found; extensions is non-NULL if an
diff --git a/tests/api.c b/tests/api.c
index 5eca4071a8..2fb098ba60 100644
--- a/tests/api.c
+++ b/tests/api.c
@@ -15042,6 +15042,49 @@ static int test_wolfSSL_Tls13_ECH_disable_conn(void)
 
     return EXPECT_RESULT();
 }
+/* Regression test: an inner SNI hostname >= MAX_PUBLIC_NAME_SZ (256) bytes
+ * must not cause a stack-buffer-overflow in TLSX_EchRestoreSNI.  Before the
+ * fix, the truncated copy omitted the NUL terminator and XSTRLEN read past
+ * the buffer. */
+static int test_wolfSSL_Tls13_ECH_long_SNI(void)
+{
+    EXPECT_DECLS;
+#if !defined(NO_WOLFSSL_CLIENT)
+    test_ssl_memio_ctx test_ctx;
+    /* 300 chars > MAX_PUBLIC_NAME_SZ (256) to exercise truncation */
+    char longName[300];
+
+    XMEMSET(longName, 'a', sizeof(longName) - 1);
+    longName[sizeof(longName) - 1] = '\0';
+
+    XMEMSET(&test_ctx, 0, sizeof(test_ctx));
+
+    test_ctx.s_cb.method = wolfTLSv1_3_server_method;
+    test_ctx.c_cb.method = wolfTLSv1_3_client_method;
+
+    test_ctx.s_cb.ctx_ready = test_ech_server_ctx_ready;
+    test_ctx.s_cb.ssl_ready = test_ech_server_ssl_ready;
+
+    ExpectIntEQ(test_ssl_memio_setup(&test_ctx), TEST_SUCCESS);
+
+    /* Set ECH configs on the client */
+    ExpectIntEQ(wolfSSL_SetEchConfigs(test_ctx.c_ssl, echCbTestConfigs,
+        echCbTestConfigsLen), WOLFSSL_SUCCESS);
+
+    /* Set the over-long SNI as the inner hostname */
+    ExpectIntEQ(wolfSSL_UseSNI(test_ctx.c_ssl, WOLFSSL_SNI_HOST_NAME,
+        longName, (word16)XSTRLEN(longName)), WOLFSSL_SUCCESS);
+
+    /* The handshake triggers TLSX_EchChangeSNI / TLSX_EchRestoreSNI.
+     * Before the fix this would stack-buffer-overflow in XSTRLEN.
+     * The connection may fail (SNI mismatch) but must not crash. */
+    (void)test_ssl_memio_do_handshake(&test_ctx, 10, NULL);
+
+    test_ssl_memio_cleanup(&test_ctx);
+#endif /* !NO_WOLFSSL_CLIENT */
+
+    return EXPECT_RESULT();
+}
 #endif /* HAVE_SSL_MEMIO_TESTS_DEPENDENCIES */
 
 /* verify that ECH can be enabled/disabled without issue */
@@ -35783,6 +35826,7 @@ TEST_CASE testCases[] = {
     TEST_DECL(test_wolfSSL_Tls13_ECH_new_config),
     TEST_DECL(test_wolfSSL_Tls13_ECH_GREASE),
     TEST_DECL(test_wolfSSL_Tls13_ECH_disable_conn),
+    TEST_DECL(test_wolfSSL_Tls13_ECH_long_SNI),
 #endif
     TEST_DECL(test_wolfSSL_Tls13_ECH_enable_disable),
 #endif /* WOLFSSL_TLS13 && HAVE_ECH */