diff --git a/httplib.h b/httplib.h
index 0114e27f..b261b80a 100644
--- a/httplib.h
+++ b/httplib.h
@@ -3968,6 +3968,7 @@ bool is_field_content(const std::string &s);
 bool is_field_value(const std::string &s);
 bool is_field_valid(const std::string &name, const std::string &value);
 bool is_request_target(const std::string &s);
+bool is_host(const std::string &s);
 
 } // namespace fields
 } // namespace detail
@@ -10322,6 +10323,17 @@ inline bool is_request_target(const std::string &s) {
   return std::all_of(s.begin(), s.end(), is_field_vchar);
 }
 
+// RFC 9110 §7.2: the bytes of uri-host [ ":" port ], i.e. unreserved,
+// sub-delims, "%", ":" and the IP-literal brackets. An empty value is valid.
+inline bool is_host(const std::string &s) {
+  return std::all_of(s.begin(), s.end(), [](char c) {
+    return is_ascii_alnum(c) || c == '-' || c == '.' || c == '_' || c == '~' ||
+           c == '!' || c == '$' || c == '&' || c == '\'' || c == '(' ||
+           c == ')' || c == '*' || c == '+' || c == ',' || c == ';' ||
+           c == '=' || c == '%' || c == ':' || c == '[' || c == ']';
+  });
+}
+
 } // namespace fields
 
 inline bool perform_websocket_handshake(Stream &strm, Request &req,
@@ -14580,6 +14592,16 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
     return write_response(strm, close_connection, req, res);
   }
 
+  // RFC 9112 §3.2: Reject more than one Host field line or an invalid Host
+  // value, so that every recipient of the request sees the same authority.
+  if (req.get_header_value_count("Host") > 1 ||
+      !detail::fields::is_host(req.get_header_value("Host"))) {
+    connection_closed = true;
+    res.status = StatusCode::BadRequest_400;
+    output_error_log(Error::InvalidHeaders, &req);
+    return write_response(strm, close_connection, req, res);
+  }
+
   // Check if the request URI doesn't exceed the limit
   if (req.target.size() > CPPHTTPLIB_REQUEST_URI_MAX_LENGTH) {
     connection_closed = true;
diff --git a/test/test.cc b/test/test.cc
index 33a91092..0541ff72 100644
--- a/test/test.cc
+++ b/test/test.cc
@@ -19686,6 +19686,58 @@ TEST(RequestSmugglingTest, DuplicateContentLengthSameValues) {
   ASSERT_EQ("HTTP/1.1 200 OK", response.substr(0, response.find("\r\n")));
 }
 
+TEST(HostHeaderValidationTest, AmbiguousOrInvalidHostRejected) {
+  Server svr;
+  svr.Get("/test", [](const Request &, Response &res) {
+    res.set_content("ok", "text/plain");
+  });
+
+  auto port = svr.bind_to_any_port(HOST);
+  thread t = thread([&] { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  auto status_line = [&](const std::string &host_lines) {
+    std::string res;
+    send_request(
+        5, "GET /test HTTP/1.1\r\n" + host_lines + "Connection: close\r\n\r\n",
+        &res, port);
+    return res.substr(0, res.find("\r\n"));
+  };
+
+  const char *accepted[] = {
+      "Host: localhost\r\n",
+      "Host: a.example:8080\r\n",
+      "Host: my_service\r\n",
+      "Host: [::1]:8080\r\n",
+      "Host: [fe80::1%25eth0]\r\n",
+      "Host: \r\n", // RFC 9110 §7.2: the target URI has no authority
+  };
+  for (auto host_lines : accepted) {
+    EXPECT_EQ("HTTP/1.1 200 OK", status_line(host_lines)) << host_lines;
+  }
+
+  const char *rejected[] = {
+      "Host: a.example\r\nHost: b.example\r\n",
+      "Host: a.example\r\nHost: a.example\r\n",
+      "Host: a@b.example\r\n",
+      "Host: a.example/b\r\n",
+      "Host: a.example\\b\r\n",
+      "Host: a.example?b\r\n",
+      "Host: a.example#b\r\n",
+      "Host: a.example b.example\r\n",
+  };
+  for (auto host_lines : rejected) {
+    EXPECT_EQ("HTTP/1.1 400 Bad Request", status_line(host_lines))
+        << host_lines;
+  }
+}
+
 TEST(HeaderSmugglingTest, ChunkedTrailerHeadersMerged) {
   Server svr;