2 files changed, 27 insertions, 1 deletions
diff --git a/webmentions_ssg/forms/__init__.py b/webmentions_ssg/forms/__init__.py
index 98962ad..0c54999 100644
--- a/webmentions_ssg/forms/__init__.py
+++ b/webmentions_ssg/forms/__init__.py
@@ -4,7 +4,7 @@ from flask_wtf import FlaskForm
 from wtforms import PasswordField, StringField, SubmitField
 from wtforms.validators import URL, InputRequired, Regexp
 
-from .validators import AllowedHostname, NotEqualTo
+from .validators import AllowedHostname, NotEqualTo, PublicURL
 
 
 class LoginForm(FlaskForm):
@@ -25,6 +25,7 @@ class EndpointForm(FlaskForm):
                 message="source must begin with http or https",
             ),
             NotEqualTo("target"),
+            PublicURL(),
         ],
     )
     target = StringField(
diff --git a/webmentions_ssg/forms/validators.py b/webmentions_ssg/forms/validators.py
index 20d9ebf..e979644 100644
--- a/webmentions_ssg/forms/validators.py
+++ b/webmentions_ssg/forms/validators.py
@@ -3,6 +3,12 @@ from urllib.parse import urlsplit
 from flask import current_app
 from wtforms import ValidationError
 
+from ..url_security import (
+    AddressResolutionError,
+    NonPublicAddressError,
+    ensure_public_url,
+)
+
 
 class NotEqualTo:
     """
@@ -60,3 +66,22 @@ class AllowedHostname:
             message = field.gettext("Invalid input.")
 
         raise ValidationError(message)
+
+
+class PublicURL:
+    def __init__(self, message=None):
+        self.message = message
+
+    def __call__(self, form, field):
+        try:
+            ensure_public_url(field.data)
+        except (
+            AddressResolutionError,
+            NonPublicAddressError,
+        ) as exc:
+            message = self.message
+
+            if message is None:
+                message = field.gettext("URL must resolve to a public address.")
+
+            raise ValidationError(message) from exc