Reserve privileged alias addresses for admins

Any user could create aliases like postmaster@, webmaster@ or abuse@ on
their domain. CAs use these addresses to validate domains, so a family
account (or a compromised one) could obtain TLS certificates or catch
abuse and system mails.

- Only admins may create aliases with a reserved local part (admin,
  administrator, webmaster, hostmaster, postmaster, abuse, security,
  noc, root, mailer-daemon). The acting user counts, so admins can
  still assign such aliases to other users.
- Escape the domains in the email regex, so "finn.st" no longer
  matches "finnXst".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/private_static/i18n/de.json

@@ -65,5 +65,6 @@
     "Used as an alias by {alias}.": "Word für den Alias {alias} verwendet",
     "Invalid domain": "Invalide Domain",
     "Can use different alias domains": "Kann unterschiedliche Aliasdomains benutzen",
-    "Can use different alias domains is not set": "Kann unterschiedliche Aliasdomains benutzen ist nicht gesetzt"
+    "Can use different alias domains is not set": "Kann unterschiedliche Aliasdomains benutzen ist nicht gesetzt",
+    "This address is reserved and can only be created by an admin.": "Diese Adresse ist reserviert und kann nur von einem Admin angelegt werden."
 }

max/routes/alias/create.py

@@ -42,6 +42,12 @@ class AliasCreate(
         full_email = f"{self.email}@{self.domain}"
         if not self.is_email_valid(full_email):
             error = True
+        elif not self.auth_user.is_admin and self.is_reserved_email(full_email):
+            flash(
+                t("This address is reserved and can only be created by an admin."),
+                category="error",
+            )
+            error = True
 
         enabled = self.request.form.get("enabled")
         if enabled == "on":

max/routes/check_email_mixin.py

@@ -5,14 +5,34 @@ from flask import flash
 from max.db import get_domains
 from max.translations import t
 
+RESERVED_LOCAL_PARTS = frozenset(
+    (
+        "admin",
+        "administrator",
+        "webmaster",
+        "hostmaster",
+        "postmaster",
+        "abuse",
+        "security",
+        "noc",
+        "root",
+        "mailer-daemon",
+    )
+)
+
 
 class CheckEmailMixin:
     def is_email_valid(self, email: str):
         """Checks for the email being valid and that it has a valid domain."""
-        regex = re.compile(f"^[a-zA-Z0-9+_-]+@({'|'.join(get_domains())})$")
+        domains = "|".join(map(re.escape, get_domains()))
+        regex = re.compile(f"^[a-zA-Z0-9+_-]+@({domains})$")
         match = regex.match(email)
         if not match:
             flash(t("The email is not valid"), category="error")
             return False
 
         return True
+
+    def is_reserved_email(self, email: str):
+        local_part = email.split("@")[0]
+        return local_part.lower() in RESERVED_LOCAL_PARTS