Escape quote, CR and LF in multipart field names and filenames

detail::serialize_multipart_formdata_item_begin() concatenated item.name
and item.filename into the Content-Disposition quoted-string without any
escaping. A '"' in either value silently terminated the quoted-string
early, and embedded CR/LF allowed injecting arbitrary part headers or
forging part boundaries when a filename comes from external input.

Escape both values following the WHATWG HTML standard ('"' -> %22,
CR -> %0D, LF -> %0A), which matches what browsers send in
multipart/form-data bodies. Behavior only changes for inputs that
previously produced malformed HTTP.

httplib.h

@@ -8318,14 +8318,32 @@ inline bool is_multipart_boundary_chars_valid(const std::string &boundary) {
   return valid;
 }
 
+// Escape a multipart field name/filename following the WHATWG HTML standard
+// ("escape a multipart form-data name"), which is what browsers send:
+// '"' -> %22, CR -> %0D, LF -> %0A
+inline std::string escape_multipart_field(const std::string &s) {
+  std::string result;
+  result.reserve(s.size());
+  for (auto c : s) {
+    switch (c) {
+    case '"': result += "%22"; break;
+    case '\r': result += "%0D"; break;
+    case '\n': result += "%0A"; break;
+    default: result += c; break;
+    }
+  }
+  return result;
+}
+
 template <typename T>
 inline std::string
 serialize_multipart_formdata_item_begin(const T &item,
                                         const std::string &boundary) {
   std::string body = "--" + boundary + "\r\n";
-  body += "Content-Disposition: form-data; name=\"" + item.name + "\"";
+  body += "Content-Disposition: form-data; name=\"" +
+          escape_multipart_field(item.name) + "\"";
   if (!item.filename.empty()) {
-    body += "; filename=\"" + item.filename + "\"";
+    body += "; filename=\"" + escape_multipart_field(item.filename) + "\"";
   }
   body += "\r\n";
   if (!item.content_type.empty()) {

test/test.cc

@@ -6531,6 +6531,66 @@ TEST_F(ServerTest, PostMultipartPlusBoundary) {
   EXPECT_EQ(StatusCode::OK_200, res->status);
 }
 
+TEST(MultipartFormDataTest, FieldEscaping) {
+  Server svr;
+
+  svr.Post("/post", [&](const Request &req, Response & /*res*/,
+                        const ContentReader &content_reader) {
+    ASSERT_TRUE(req.is_multipart_form_data());
+    std::vector<FormData> received;
+    content_reader(
+        [&](const FormData &file) {
+          received.push_back(file);
+          return true;
+        },
+        [&](const char *data, size_t data_length) {
+          received.back().content.append(data, data_length);
+          return true;
+        });
+
+    // '"', CR and LF in names and filenames are escaped following the
+    // WHATWG HTML standard, so each part still parses as a single part
+    // with no injected headers.
+    ASSERT_EQ(3U, received.size());
+
+    EXPECT_EQ("na%22me", received[0].name);
+    EXPECT_EQ("quoted name", received[0].content);
+
+    EXPECT_EQ("file", received[1].name);
+    EXPECT_EQ("evil%0D%0AContent-Type: text/evil%0D%0A%0D%0A.pdf",
+              received[1].filename);
+    EXPECT_EQ("application/octet-stream", received[1].content_type);
+    EXPECT_EQ("injected", received[1].content);
+
+    EXPECT_EQ("my%0Dna%0Ame", received[2].name);
+    EXPECT_EQ("my%22file.txt", received[2].filename);
+    EXPECT_EQ("cr lf name", received[2].content);
+  });
+
+  auto port = svr.bind_to_any_port("localhost");
+  auto t = std::thread([&]() { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+
+  svr.wait_until_ready();
+
+  Client cli("localhost", port);
+
+  UploadFormDataItems items = {
+      {"na\"me", "quoted name", "", ""},
+      {"file", "injected", "evil\r\nContent-Type: text/evil\r\n\r\n.pdf",
+       "application/octet-stream"},
+      {"my\rna\nme", "cr lf name", "my\"file.txt", "text/plain"},
+  };
+
+  auto res = cli.Post("/post", items);
+  ASSERT_TRUE(res);
+  EXPECT_EQ(StatusCode::OK_200, res->status);
+}
+
 TEST_F(ServerTest, PostContentReceiverGzip) {
   cli_.set_compress(true);
   auto res = cli_.Post("/content_receiver", "content", "text/plain");