Secure Back Redirects on login form

app/auth/forms.py

@@ -4,9 +4,10 @@
 
 TODO: à revoir complètement pour reprendre ZScoUsers et les pages d'authentification
 """
-
+from urllib.parse import urlparse, urljoin
+from flask import request, url_for, redirect
 from flask_wtf import FlaskForm
-from wtforms import StringField, PasswordField, BooleanField, SubmitField
+from wtforms import BooleanField, HiddenField, PasswordField, StringField, SubmitField
 from wtforms.validators import ValidationError, DataRequired, Email, EqualTo
 from app.auth.models import User, is_valid_password
 
@@ -14,13 +15,45 @@ from app.auth.models import User, is_valid_password
 _ = lambda x: x  # sans babel
 _l = _
 
+# See http://flask.pocoo.org/snippets/63/
+def is_safe_url(target):
+    ref_url = urlparse(request.host_url)
+    test_url = urlparse(urljoin(request.host_url, target))
+    return test_url.scheme in ("http", "https") and ref_url.netloc == test_url.netloc
 
-class LoginForm(FlaskForm):
+
+def get_redirect_target():
+    for target in request.args.get("next"), request.referrer:
+        if not target:
+            continue
+        if is_safe_url(target):
+            return target
+
+
+class RedirectForm(FlaskForm):
+    next = HiddenField()
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if not self.next.data:
+            self.next.data = get_redirect_target() or ""
+
+    def redirect(self, endpoint="index", **values):
+        if is_safe_url(self.next.data):
+            return redirect(self.next.data)
+        target = get_redirect_target()
+        return redirect(target or url_for(endpoint, **values))
+
+
+class LoginForm(RedirectForm):
     user_name = StringField(_l("Nom d'utilisateur"), validators=[DataRequired()])
     password = PasswordField(_l("Mot de passe"), validators=[DataRequired()])
     remember_me = BooleanField(_l("mémoriser la connexion"))
     submit = SubmitField(_l("Suivant"))
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
 
 class UserCreationForm(FlaskForm):
     user_name = StringField(_l("Nom d'utilisateur"), validators=[DataRequired()])

app/auth/routes.py

@@ -42,10 +42,10 @@ def login():
             return redirect(url_for("auth.login"))
         login_user(user, remember=form.remember_me.data)
         current_app.logger.info("login: success (%s)", form.user_name.data)
-        next_page = request.args.get("next")
+        # next_page = request.args.get("next")
-        if not next_page or url_parse(next_page).netloc != "":
+        # if not next_page or url_parse(next_page).netloc != "":
-            next_page = url_for("scodoc.index")
+        #    next_page = url_for("scodoc.index")
-        return redirect(next_page)
+        return form.redirect("scodoc.index")
     message = request.args.get("message", "")
     return render_template(
         "auth/login.html", title=_("Sign In"), form=form, message=message