Fix use-after-free in SSLClient destructor with mbedTLS (Fix #2492)

SSLClient::~SSLClient() freed the TLS context before shutting down
the SSL session. mbedTLS sessions hold a raw pointer into the
context's mbedtls_ssl_config, so a live keep-alive session's
close_notify would read freed memory. Shut down the session first,
then free the context.

Add a regression test that destructs an SSLClient while a keep-alive
mbedTLS session is still open.

httplib.h

@@ -16107,11 +16107,18 @@ inline bool SSLServer::update_certs_pem(const char *cert_pem,
 
 // SSL HTTP client implementation
 inline SSLClient::~SSLClient() {
-  if (ctx_) { tls::free_context(ctx_); }
   // Make sure to shut down SSL since shutdown_ssl will resolve to the
   // base function rather than the derived function once we get to the
   // base class destructor, and won't free the SSL (causing a leak).
+  // This must happen before the context is freed below: some backends
+  // (e.g. mbedTLS) have the SSL session borrow a raw pointer into the
+  // context, so freeing the context first leaves close_notify reading
+  // freed memory.
   shutdown_ssl_impl(socket_, true);
+  if (ctx_) {
+    tls::free_context(ctx_);
+    ctx_ = nullptr;
+  }
 }
 
 inline bool SSLClient::is_valid() const { return ctx_ != nullptr; }

test/test.cc

@@ -18293,6 +18293,39 @@ TEST(SSLClientServerTest, CustomizeServerSSLCtxMbedTLS) {
   ASSERT_TRUE(res);
   ASSERT_EQ(StatusCode::OK_200, res->status);
 }
+
+// Regression test for a use-after-free where ~SSLClient freed the mbedTLS
+// context (owning mbedtls_ssl_config) before shutting down a still-open
+// keep-alive SSL session, which reads through that config in
+// mbedtls_ssl_close_notify.
+TEST(SSLClientServerTest, DestructWithLiveKeepAliveSessionMbedTLS) {
+  SSLServer svr(SERVER_CERT_FILE, SERVER_PRIVATE_KEY_FILE);
+  ASSERT_TRUE(svr.is_valid());
+
+  svr.Get("/test", [&](const Request & /*req*/, Response &res) {
+    res.set_content("test", "text/plain");
+  });
+
+  thread t = thread([&]() { ASSERT_TRUE(svr.listen(HOST, PORT)); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  {
+    SSLClient cli(HOST, PORT);
+    cli.enable_server_certificate_verification(false);
+    cli.set_keep_alive(true);
+
+    auto res = cli.Get("/test");
+    ASSERT_TRUE(res);
+    ASSERT_EQ(StatusCode::OK_200, res->status);
+    // cli is destructed here with the keep-alive SSL session still open.
+  }
+}
 #endif
 
 // WebSocket Tests