Unresolvable hostnames should still be added, they are checked for security at fetch time

changedetectionio/processors/base.py

@@ -1,12 +1,15 @@
+import asyncio
 import re
 import hashlib
 
 from changedetectionio.browser_steps.browser_steps import browser_steps_get_valid_steps
 from changedetectionio.content_fetchers.base import Fetcher
 from changedetectionio.strtobool import strtobool
+from changedetectionio.validate_url import is_private_hostname
 from copy import deepcopy
 from abc import abstractmethod
 import os
+from urllib.parse import urlparse
 from loguru import logger
 
 SCREENSHOT_FORMAT_JPEG = 'JPEG'
@@ -95,6 +98,23 @@ class difference_detection_processor():
             self.last_raw_content_checksum = None
 
 
+    async def validate_iana_url(self):
+        """Pre-flight SSRF check — runs DNS lookup in executor to avoid blocking the event loop.
+        Covers all fetchers (requests, playwright, puppeteer, plugins) since every fetch goes
+        through call_browser().
+        """
+        if strtobool(os.getenv('ALLOW_IANA_RESTRICTED_ADDRESSES', 'false')):
+            return
+        parsed = urlparse(self.watch.link)
+        if not parsed.hostname:
+            return
+        loop = asyncio.get_running_loop()
+        if await loop.run_in_executor(None, is_private_hostname, parsed.hostname):
+            raise Exception(
+                f"Fetch blocked: '{self.watch.link}' resolves to a private/reserved IP address. "
+                f"Set ALLOW_IANA_RESTRICTED_ADDRESSES=true to allow."
+            )
+
     async def call_browser(self, preferred_proxy_id=None):
 
         from requests.structures import CaseInsensitiveDict
@@ -108,6 +128,8 @@ class difference_detection_processor():
                     "file:// type access is denied for security reasons."
                 )
 
+        await self.validate_iana_url()
+
         # Requests, playwright, other browser via wss:// etc, fetch_extra_something
         prefer_fetch_backend = self.watch.get('fetch_backend', 'system')
 

changedetectionio/tests/test_security.py

@@ -34,6 +34,7 @@ def test_favicon(client, live_server, measure_memory_usage, datastore_path):
                                                                             favicon_base_64=SVG_BASE64
                                                                             )
 
+
     res = client.get(url_for('static_content', group='favicon', filename=uuid))
     assert res.status_code == 200
     assert len(res.data) > 10
@@ -601,8 +602,6 @@ def test_ssrf_private_ip_blocked(client, live_server, monkeypatch, measure_memor
     from changedetectionio.validate_url import is_safe_valid_url, is_private_hostname
 
     monkeypatch.setenv('ALLOW_IANA_RESTRICTED_ADDRESSES', 'false')
-    # Clear any URL results cached while the env var was 'true'
-    is_safe_valid_url.cache_clear()
 
     # ------------------------------------------------------------------
     # 1. is_private_hostname() — unit tests across all reserved ranges
@@ -644,13 +643,11 @@ def test_ssrf_private_ip_blocked(client, live_server, monkeypatch, measure_memor
     # 3. ALLOW_IANA_RESTRICTED_ADDRESSES=true bypasses the block
     # ------------------------------------------------------------------
     monkeypatch.setenv('ALLOW_IANA_RESTRICTED_ADDRESSES', 'true')
-    is_safe_valid_url.cache_clear()
     assert is_safe_valid_url('http://127.0.0.1/'), \
         "Private IP should be allowed when ALLOW_IANA_RESTRICTED_ADDRESSES=true"
 
     # Restore the block for the remaining assertions
     monkeypatch.setenv('ALLOW_IANA_RESTRICTED_ADDRESSES', 'false')
-    is_safe_valid_url.cache_clear()
 
     # ------------------------------------------------------------------
     # 4. UI form rejects private-IP URLs
@@ -708,3 +705,35 @@ def test_ssrf_private_ip_blocked(client, live_server, monkeypatch, measure_memor
                     request_body=None,
                     request_method='GET',
                 )
+
+
+def test_unresolvable_hostname_is_allowed(client, live_server, monkeypatch):
+    """
+    Unresolvable hostnames must NOT be blocked at add-time when ALLOW_IANA_RESTRICTED_ADDRESSES=false.
+
+    DNS failure (gaierror) at add-time does not mean the URL resolves to a private IP —
+    the domain may simply be offline or not yet live. Blocking it would be a false positive.
+    The real DNS-rebinding protection happens at fetch-time in call_browser().
+    """
+    from changedetectionio.validate_url import is_safe_valid_url
+
+    monkeypatch.setenv('ALLOW_IANA_RESTRICTED_ADDRESSES', 'false')
+
+    url = 'http://this-host-does-not-exist-xyz987.invalid/some/path'
+
+    # Should pass URL validation despite being unresolvable
+    assert is_safe_valid_url(url), \
+        "Unresolvable hostname should pass is_safe_valid_url — DNS failure is not a private-IP signal"
+
+    # Should be accepted via the UI form and appear in the watch list
+    res = client.post(
+        url_for('ui.ui_views.form_quick_watch_add'),
+        data={'url': url, 'tags': ''},
+        follow_redirects=True
+    )
+    assert b'Watch protocol is not permitted or invalid URL format' not in res.data, \
+        "UI should not reject a URL just because its hostname is unresolvable"
+
+    res = client.get(url_for('watchlist.index'))
+    assert b'this-host-does-not-exist-xyz987.invalid' in res.data, \
+        "Unresolvable hostname watch should appear in the watch overview list"

changedetectionio/validate_url.py

@@ -61,7 +61,9 @@ def normalize_url_encoding(url):
 def is_private_hostname(hostname):
     """Return True if hostname resolves to an IANA-restricted (private/reserved) IP address.
 
-    Fails closed: unresolvable hostnames return True (block them).
+    Unresolvable hostnames return False (allow them) — DNS may be temporarily unavailable
+    or the domain not yet live. The actual DNS rebinding attack is mitigated by fetch-time
+    re-validation in requests.py, not by blocking unresolvable domains at add-time.
     Never cached — callers that need fresh DNS resolution (e.g. at fetch time) can call
     this directly without going through the lru_cached is_safe_valid_url().
     """
@@ -70,12 +72,11 @@ def is_private_hostname(hostname):
             ip = ipaddress.ip_address(info[4][0])
             if ip.is_private or ip.is_loopback or ip.is_link_local or ip.is_reserved:
                 return True
-    except socket.gaierror:
+    except socket.gaierror as e:
-        return True
+        logger.warning(f"{hostname} error checking {str(e)}")
+        return False
     return False
 
-
-@lru_cache(maxsize=10000)
 def is_safe_valid_url(test_url):
     from changedetectionio import strtobool
     from changedetectionio.jinja2_custom import render as jinja_render