Logging in returns to the page that was asked for (closes #384)

A logged-out GET of an admin page now redirects to /pages/login with its path and query in a next parameter, when they fit in 2048 bytes. The login form carries next as a hidden field; a successful login redirects there, and a failed one shows the page again with the same next. A POST still redirects to plain /pages/login.

The value is client-chosen, so every read of it goes through one check: after percent-decoding it must start with exactly one / and contain no backslash or control character; anything else becomes /. gosec's open-redirect rule is suppressed on the two redirects that follow it. The login page's navigation bar no longer links to itself.

Model: opus-5-5

internal/handlers/auth.go

@@ -2,19 +2,56 @@ package handlers
 
 import (
 	"net/http"
+	"net/url"
 	"strconv"
+	"strings"
+	"unicode"
 
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/logfield"
+	"sneak.berlin/go/webhooker/internal/middleware"
 )
 
+// loginDestination returns where a successful login sends the
+// browser: next when it is a path on this site, otherwise "/", which
+// leads to the webhook list.
+//
+// A browser reads "//host" as another site, reads "\" as "/", and
+// drops tabs and newlines before reading at all. So the value must
+// start with exactly one "/" and hold no "\" or control character
+// anywhere: http.Redirect cleans "/a/../\host" down to "/\host". It
+// is checked after percent-decoding, so an encoded form of any of
+// these is refused too.
+func loginDestination(next string) string {
+	if len(next) > middleware.MaxNextBytes {
+		return "/"
+	}
+
+	decoded, err := url.PathUnescape(next)
+	if err != nil ||
+		!strings.HasPrefix(decoded, "/") ||
+		strings.HasPrefix(decoded, "//") ||
+		strings.Contains(decoded, `\`) ||
+		strings.ContainsFunc(decoded, unicode.IsControl) {
+		return "/"
+	}
+
+	return next
+}
+
 // HandleLoginPage returns a handler for the login page (GET)
 func (h *Handlers) HandleLoginPage() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+		next := loginDestination(
+			r.URL.Query().Get(middleware.NextParam),
+		)
+
 		// Check if already logged in
 		sess, err := h.session.Get(r)
 		if err == nil && h.session.IsAuthenticated(sess) {
-			http.Redirect(w, r, "/", http.StatusSeeOther)
+			http.Redirect( //nolint:gosec // checked by loginDestination
+				w, r, next, http.StatusSeeOther,
+			)
 
 			return
 		}
@@ -22,6 +59,7 @@ func (h *Handlers) HandleLoginPage() http.HandlerFunc {
 		// Render login page
 		data := map[string]any{
 			tmplKeyError: "",
+			tmplKeyNext:  next,
 		}
 
 		h.renderTemplate(w, r, "login.html", data)
@@ -77,8 +115,13 @@ func (h *Handlers) HandleLoginSubmit() http.HandlerFunc {
 			"user_id", user.ID,
 		)
 
-		// Redirect to home page
-		http.Redirect(w, r, "/", http.StatusSeeOther)
+		// The form value is the client's to set, so it is checked
+		// again here rather than trusted from the rendered page.
+		http.Redirect( //nolint:gosec // checked by loginDestination
+			w, r,
+			loginDestination(r.PostFormValue(middleware.NextParam)),
+			http.StatusSeeOther,
+		)
 	}
 }
 
@@ -91,6 +134,9 @@ func (h *Handlers) renderLoginError(
 ) {
 	data := map[string]any{
 		tmplKeyError: msg,
+		tmplKeyNext: loginDestination(
+			r.PostFormValue(middleware.NextParam),
+		),
 	}
 
 	w.WriteHeader(status)