fix(receiver): prevent requests to non-public addresses

Resolve source hostnames before fetching and reject addresses that are
not globally routable to prevent SSRF against local or private services.

Repeat the check for every HTTP request so redirects cannot bypass the
initial source validation. Treat DNS resolution failures during
verification as temporary fetch errors.

1 files changed, 42 insertions, 0 deletions

diff --git a/tests/test_url_security.py b/tests/test_url_security.py
new file mode 100644
index 0000000..fba6d7d
--- /dev/null
+++ b/tests/test_url_security.py
@@ -0,0 +1,42 @@
+import pytest
+
+from webmentions_ssg.url_security import (
+    NonPublicAddressError,
+    ensure_public_url,
+)
+
+
+class FakeAnswers:
+    def __init__(self, *addresses: str):
+        self._addresses = addresses
+
+    def addresses(self):
+        return iter(self._addresses)
+
+
+@pytest.mark.parametrize(
+    ("url", "resolved_addresses"),
+    [
+        ("http://127.0.0.1/", ["127.0.0.1"]),
+        ("http://127.0.0.1:8080/test", ["127.0.0.1"]),
+        ("http://[::1]/", ["::1"]),
+        ("http://10.0.0.1/", ["10.0.0.1"]),
+        ("http://172.16.0.1/", ["172.16.0.1"]),
+        ("http://192.168.1.1/", ["192.168.1.1"]),
+        ("http://169.254.169.254/", ["169.254.169.254"]),
+        ("http://localhost/", ["127.0.0.1", "::1"]),
+        ("http://internal.example/", ["192.168.1.10"]),
+    ],
+)
+def test_ensure_public_url_rejects_non_public_addresses(
+    monkeypatch: pytest.MonkeyPatch,
+    url: str,
+    resolved_addresses: list[str],
+) -> None:
+    monkeypatch.setattr(
+        "webmentions_ssg.url_security.dns.resolver.resolve_name",
+        lambda hostname: FakeAnswers(*resolved_addresses),
+    )
+
+    with pytest.raises(NonPublicAddressError):
+        ensure_public_url(url)