diff --git a/go.mod b/go.mod
index d0d77ae..39b4332 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module git.jpi.io/JPI/ranpass
 
-go 1.14
+go 1.25
diff --git a/main.go b/main.go
index f1721b1..49d0793 100644
--- a/main.go
+++ b/main.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"log"
@@ -8,6 +9,29 @@ import (
 	"strconv"
 )
 
+const maxPasswordLength = 512
+
+// parseParam parses a positive integer form field. If the field is absent or
+// empty, defaultVal is returned. If present but invalid or out of [1, max],
+// an error is returned.
+func parseParam(r *http.Request, name string, defaultVal, max int) (int, error) {
+	v := r.FormValue(name)
+	if v == "" {
+		return defaultVal, nil
+	}
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return 0, fmt.Errorf("%s: must be an integer", name)
+	}
+	if n <= 0 {
+		return 0, fmt.Errorf("%s: must be greater than zero", name)
+	}
+	if n > max {
+		return 0, fmt.Errorf("%s: must be at most %d", name, max)
+	}
+	return n, nil
+}
+
 type passwordData struct {
 	Length     int
 	Digits     int
@@ -17,11 +41,9 @@ type passwordData struct {
 	Password   string
 }
 
-var d, defaults passwordData
+var defaults passwordData
 
 func generatePassword(w http.ResponseWriter, r *http.Request) {
-	var err error
-
 	if r.URL.Path != "/" {
 		http.Error(w, "404 not found.", http.StatusNotFound)
 		return
@@ -29,56 +51,53 @@ func generatePassword(w http.ResponseWriter, r *http.Request) {
 
 	switch r.Method {
 	case "GET":
-		d.Password, err = generate(defaults.Length, defaults.Digits, defaults.Symbols, false, false)
+		password, err := generate(defaults.Length, defaults.Digits, defaults.Symbols, false, false)
 		if err != nil {
-			d.Password = "Error: " + err.Error()
 			w.WriteHeader(http.StatusInternalServerError)
-			w.Write([]byte(d.Password))
+			w.Write([]byte("Error: " + err.Error()))
 			return
 		}
 		w.WriteHeader(http.StatusOK)
-		w.Write([]byte(d.Password))
+		w.Write([]byte(password))
 	case "POST":
-		if err = r.ParseForm(); err != nil {
+		var d passwordData
+
+		if err := r.ParseForm(); err != nil {
 			w.WriteHeader(http.StatusUnprocessableEntity)
 			w.Write([]byte(fmt.Sprintf("ParseForm() err: %v", err)))
 			return
 		}
-		d.Length, err = strconv.Atoi(r.FormValue("length"))
-		if err != nil {
-			d.Length = defaults.Length
-		}
 
-		d.Digits, err = strconv.Atoi(r.FormValue("digits"))
-		if err != nil {
-			d.Digits = defaults.Digits
-		}
+		var err error
+		var validationErr error
 
-		d.Symbols, err = strconv.Atoi(r.FormValue("symbols"))
-		if err != nil {
-			d.Symbols = defaults.Symbols
-		}
+		d.Length, err = parseParam(r, "length", defaults.Length, maxPasswordLength)
+		validationErr = errors.Join(validationErr, err)
 
-		if r.FormValue("noupper") == "on" {
-			d.NoUpper = true
-		}
+		d.Digits, err = parseParam(r, "digits", defaults.Digits, maxPasswordLength)
+		validationErr = errors.Join(validationErr, err)
 
-		if r.FormValue("denyrepeat") == "on" {
-			d.DenyRepeat = true
-		}
+		d.Symbols, err = parseParam(r, "symbols", defaults.Symbols, maxPasswordLength)
+		validationErr = errors.Join(validationErr, err)
 
-		d.Password, err = generate(d.Length, d.Digits, d.Symbols, d.NoUpper, !d.DenyRepeat)
-		if err != nil {
-			d.Password = "Error: " + err.Error()
-			w.WriteHeader(http.StatusInternalServerError)
-			w.Write([]byte(d.Password))
+		if validationErr != nil {
+			w.WriteHeader(http.StatusBadRequest)
+			w.Write([]byte(validationErr.Error()))
 			return
 		}
-		if d.Length == 0 {
-			d.Password = "Error: password can not have zero length"
+
+		d.NoUpper = r.FormValue("noupper") == "on"
+		d.DenyRepeat = r.FormValue("denyrepeat") == "on"
+
+		var password string
+		password, err = generate(d.Length, d.Digits, d.Symbols, d.NoUpper, !d.DenyRepeat)
+		if err != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte("Error: " + err.Error()))
+			return
 		}
 		w.WriteHeader(http.StatusOK)
-		w.Write([]byte(d.Password))
+		w.Write([]byte(password))
 	default:
 		w.WriteHeader(http.StatusBadRequest)
 		w.Write([]byte("Sorry, only GET and POST methods are supported."))