diff --git a/frankenphp.go b/frankenphp.go
index e560031e62..3f5df594d6 100644
--- a/frankenphp.go
+++ b/frankenphp.go
@@ -783,16 +783,53 @@ func go_read_cookies(threadIndex C.uintptr_t) *C.char {
 		return nil
 	}
 
-	cookie := strings.Join(request.Header.Values("Cookie"), "; ")
-	if cookie == "" {
+	return cookieHeader(request.Header.Values("Cookie"))
+}
+
+// cookieHeader joins Cookie fields and removes NUL bytes directly in the final
+// C buffer, which is freed in frankenphp_free_request_context().
+func cookieHeader(values []string) *C.char {
+	const maxInt = int(^uint(0) >> 1)
+	rawLen, size := 0, 0
+	if len(values) > 1 {
+		if 2 >= maxInt/(len(values)-1) {
+			panic("strings: Join output length overflow")
+		}
+		rawLen = 2 * (len(values) - 1)
+		size = rawLen
+	}
+	for _, value := range values {
+		if len(value) > maxInt-rawLen {
+			panic("strings: Join output length overflow")
+		}
+		rawLen += len(value)
+		size += len(value) - strings.Count(value, "\x00")
+	}
+	// Preserve the distinction between no cookie and a nonempty field that
+	// becomes empty after removing NUL bytes.
+	if rawLen == 0 {
 		return nil
 	}
 
-	// remove potential null bytes
-	cookie = strings.ReplaceAll(cookie, "\x00", "")
-
-	// freed in frankenphp_free_request_context()
-	return C.CString(cookie)
+	ptr := C.malloc(C.size_t(size + 1))
+	buf := unsafe.Slice((*byte)(ptr), size+1)
+	pos := 0
+	for i, value := range values {
+		if i > 0 {
+			pos += copy(buf[pos:], "; ")
+		}
+		for {
+			j := strings.IndexByte(value, 0)
+			if j < 0 {
+				pos += copy(buf[pos:], value)
+				break
+			}
+			pos += copy(buf[pos:], value[:j])
+			value = value[j+1:]
+		}
+	}
+	buf[pos] = 0
+	return (*C.char)(ptr)
 }
 
 // getLogger returns the logger and context safely even if phpThreads have not been created yet
diff --git a/frankenphp_test.go b/frankenphp_test.go
index 84199ba81d..5d37adab27 100644
--- a/frankenphp_test.go
+++ b/frankenphp_test.go
@@ -428,6 +428,45 @@ func TestMalformedCookie(t *testing.T) {
 	}, &testOptions{nbParallelRequests: 1})
 }
 
+func TestCookieHeaderFields(t *testing.T) {
+	for _, workerScript := range []string{"", "cookies.php"} {
+		mode := "module"
+		if workerScript != "" {
+			mode = "worker"
+		}
+		t.Run(mode, func(t *testing.T) {
+			for _, tc := range []struct {
+				name   string
+				fields []string
+				want   []string
+			}{
+				{"absent", nil, []string{"array (\n)"}},
+				{"empty", []string{""}, []string{"array (\n)"}},
+				{"empty fields", []string{"", ""}, []string{"array (\n)"}},
+				{"NUL only", []string{"\x00\x00"}, []string{"array (\n)"}},
+				{"multiple fields", []string{"foo=bar", "second=test"}, []string{"'foo' => 'bar'", "'second' => 'test'"}},
+				{"empty separators", []string{"", "foo=bar", ""}, []string{"'foo' => 'bar'"}},
+				{"NUL within values", []string{"foo=ba\x00r", "second=te\x00st"}, []string{"'foo' => 'bar'", "'second' => 'test'"}},
+				{"first duplicate wins", []string{"foo=first", "foo=second"}, []string{"'foo' => 'first'"}},
+				{"long values", []string{"foo=" + strings.Repeat("a", 256), "second=" + strings.Repeat("b", 256)}, []string{"'foo' => '" + strings.Repeat("a", 256) + "'", "'second' => '" + strings.Repeat("b", 256) + "'"}},
+			} {
+				t.Run(tc.name, func(t *testing.T) {
+					runTest(t, func(handler func(http.ResponseWriter, *http.Request), _ *httptest.Server, _ int) {
+						req := httptest.NewRequest("GET", "http://example.com/cookies.php", nil)
+						for _, field := range tc.fields {
+							req.Header.Add("Cookie", field)
+						}
+						body, _ := testRequest(req, handler, t)
+						for _, want := range tc.want {
+							assert.Contains(t, body, want)
+						}
+					}, &testOptions{workerScript: workerScript, nbParallelRequests: 1})
+				})
+			}
+		})
+	}
+}
+
 func TestSession_module(t *testing.T) { testSession(t, nil) }
 func TestSession_worker(t *testing.T) {
 	testSession(t, &testOptions{workerScript: "session.php"})