diff --git a/tests/users/models/test_domain.py b/tests/users/models/test_domain.py
index e8e6860..408fdf8 100644
--- a/tests/users/models/test_domain.py
+++ b/tests/users/models/test_domain.py
@@ -3,6 +3,36 @@ import pytest
 from users.models import Domain
 
 
+def test_valid_domain():
+    """
+    Tests that a valid domain is valid
+    """
+
+    assert Domain.is_valid_domain("example.com")
+    assert Domain.is_valid_domain("xn----gtbspbbmkef.xn--p1ai")
+    assert Domain.is_valid_domain("underscore_subdomain.example.com")
+    assert Domain.is_valid_domain("something.versicherung")
+    assert Domain.is_valid_domain("11.com")
+    assert Domain.is_valid_domain("a.cn")
+    assert Domain.is_valid_domain("sub1.sub2.sample.co.uk")
+    assert Domain.is_valid_domain("somerandomexample.xn--fiqs8s")
+    assert not Domain.is_valid_domain("über.com")
+    assert not Domain.is_valid_domain("example.com:4444")
+    assert not Domain.is_valid_domain("example.-com")
+    assert not Domain.is_valid_domain("foo@bar.com")
+    assert not Domain.is_valid_domain("example.")
+    assert not Domain.is_valid_domain("example.com.")
+    assert not Domain.is_valid_domain("-example.com")
+    assert not Domain.is_valid_domain("_example.com")
+    assert not Domain.is_valid_domain("_example._com")
+    assert not Domain.is_valid_domain("example_.com")
+    assert not Domain.is_valid_domain("example")
+    assert not Domain.is_valid_domain("a......b.com")
+    assert not Domain.is_valid_domain("a.123")
+    assert not Domain.is_valid_domain("123.123")
+    assert not Domain.is_valid_domain("123.123.123.123")
+
+
 @pytest.mark.django_db
 def test_recursive_block():
     """
diff --git a/users/models/domain.py b/users/models/domain.py
index d9fe38f..eb82436 100644
--- a/users/models/domain.py
+++ b/users/models/domain.py
@@ -1,5 +1,6 @@
 import json
 import logging
+import re
 import ssl
 from functools import cached_property
 from typing import Optional
@@ -8,6 +9,7 @@ import httpx
 import pydantic
 import urlman
 from django.conf import settings
+from django.core.exceptions import ValidationError
 from django.db import models
 
 from core.models import Config
@@ -53,6 +55,14 @@ class DomainStates(StateGraph):
         return cls.outdated
 
 
+def _domain_validator(value: str):
+    if not Domain.is_valid_domain(value):
+        raise ValidationError(
+            "%(value)s is not a valid domain",
+            params={"value": value},
+        )
+
+
 class Domain(StatorModel):
     """
     Represents a domain that a user can have an account on.
@@ -71,7 +81,9 @@ class Domain(StatorModel):
     display domains for now, until we start doing better probing.
     """
 
-    domain = models.CharField(max_length=250, primary_key=True)
+    domain = models.CharField(
+        max_length=250, primary_key=True, validators=[_domain_validator]
+    )
     service_domain = models.CharField(
         max_length=250,
         null=True,
@@ -119,6 +131,19 @@ class Domain(StatorModel):
     class Meta:
         indexes: list = []
 
+    @classmethod
+    def is_valid_domain(cls, domain: str) -> bool:
+        """
+        Check if a domain is valid, domain must be lowercase
+        """
+        return (
+            re.match(
+                r"^(?:[a-z0-9](?:[a-z0-9-_]{0,61}[a-z0-9])?\.)+[a-z0-9][a-z0-9-_]{0,61}[a-z]$",
+                domain,
+            )
+            is not None
+        )
+
     @classmethod
     def get_remote_domain(cls, domain: str) -> "Domain":
         return cls.objects.get_or_create(domain=domain.lower(), local=False)[0]
diff --git a/users/shortcuts.py b/users/shortcuts.py
index 7357ee9..7621e90 100644
--- a/users/shortcuts.py
+++ b/users/shortcuts.py
@@ -18,6 +18,8 @@ def by_handle_or_404(request, handle, local=True, fetch=False) -> Identity:
         domain = domain_instance.domain
     else:
         username, domain = handle.split("@", 1)
+        if not Domain.is_valid_domain(domain):
+            raise Http404("Invalid domain")
         # Resolve the domain to the display domain
         domain_instance = Domain.get_domain(domain)
         if domain_instance is None: