AP: verify incoming signatures!

for #315

activitypub.py

@@ -1,6 +1,8 @@
 """Handles requests for ActivityPub endpoints: actors, inbox, etc.
 """
+from base64 import b64encode
 import datetime
+from hashlib import sha256
 import logging
 import re
 import threading
@@ -10,6 +12,7 @@
 from google.cloud import ndb
 from google.cloud.ndb import OR
 from granary import as1, as2
+from httpsig import HeaderVerifier
 from oauth_dropins.webutil import flask_util, util
 from oauth_dropins.webutil.flask_util import error
 from oauth_dropins.webutil.util import json_dumps, json_loads
@@ -133,14 +136,41 @@ def inbox(domain=None):
     if type not in SUPPORTED_TYPES:
         error(f'Sorry, {type} activities are not supported yet.', status=501)
 
-    # TODO: verify signature if there is one
-
+    # load user
     user = None
     if domain:
         user = User.get_by_id(domain)
         if not user:
             return f'User {domain} not found', 404
 
+    # load actor
+    if actor and isinstance(actor, str):
+        actor = activity['actor'] = \
+            json_loads(common.get_object(actor, user=user).as2)
+
+    # optionally verify signature
+    # TODO: switch this from erroring to logging lots of detail. need to see
+    # which headers, key shapes, etc we get in the wild.
+    if request.headers.get('Signature'):
+        digest = request.headers.get('Digest')
+        if not digest:
+            error(f'Missing Digest header, required for HTTP Signature', status=401)
+
+        expected = b64encode(sha256(request.data).digest()).decode()
+        if digest.removeprefix('SHA-256=') != expected:
+            error(f'Invalid Digest header, required for HTTP Signature', status=401)
+
+        # TODO: check keyId
+        key = actor.get('publicKey', {}).get('publicKeyPem', {})
+        try:
+            if not HeaderVerifier(request.headers, key, method='GET', path=request.path,
+                                  required_headers=common.HTTP_SIG_HEADERS,
+                                  sign_header='signature').verify():
+                error(f'HTTP Signature verification failed', status=401)
+        except BaseException as e:
+            error(f'HTTP Signature verification failed: {e}', status=401)
+
+    # handle activity!
     if type == 'Undo' and obj_as2.get('type') == 'Follow':
         # skip actor fetch below; we don't need it to undo a follow
         undo_follow(redirect_unwrap(activity))
@@ -182,11 +212,6 @@ def inbox(domain=None):
         ndb.put_multi(followers)
         return 'OK'
 
-    # fetch actor if necessary so we have name, profile photo, etc
-    if actor and isinstance(actor, str):
-        actor = activity['actor'] = \
-            json_loads(common.get_object(actor, user=user).as2)
-
     # fetch object if necessary so we can render it in feeds
     if type in FETCH_OBJECT_TYPES and isinstance(activity.get('object'), str):
         obj_as2 = activity['object'] = \

common.py

@@ -78,6 +78,8 @@
 CACHE_TIME = timedelta(seconds=60)
 PAGE_SIZE = 20
 
+HTTP_SIG_HEADERS = ('Date', 'Host', 'Digest', '(request-target)')
+
 
 def host_url(path_query=None):
   base = request.host_url
@@ -202,10 +204,9 @@ def signed_request(fn, url, user, data=None, log_data=True, headers=None, **kwar
     # implementations require, eg Peertube.
     # https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-12#section-2.3
     # https://github.com/snarfed/bridgy-fed/issues/40
-    auth = HTTPSignatureAuth(
-      secret=user.private_pem(), key_id=key_id, algorithm='rsa-sha256',
-      sign_header='signature',
-      headers=('Date', 'Host', 'Digest', '(request-target)'))
+    auth = HTTPSignatureAuth(secret=user.private_pem(), key_id=key_id,
+                             algorithm='rsa-sha256', sign_header='signature',
+                             headers=HTTP_SIG_HEADERS)
 
     # make HTTP request
     kwargs.setdefault('gateway', True)

tests/test_activitypub.py

@@ -1,10 +1,14 @@
 # coding=utf-8
 """Unit tests for activitypub.py."""
+from base64 import b64encode
 import copy
 from datetime import datetime, timedelta
+from hashlib import sha256
 from unittest.mock import ANY, call, patch
 
+from google.cloud import ndb
 from granary import as2
+from httpsig import HeaderSigner, HeaderVerifier
 from oauth_dropins.webutil import util
 from oauth_dropins.webutil.testutil import requests_response
 from oauth_dropins.webutil.util import json_dumps, json_loads
@@ -13,7 +17,7 @@
 
 import activitypub
 import common
-from models import Follower, Object, Target, User
+from models import Follower, Object, User
 from . import testutil
 
 ACTOR = {
@@ -194,7 +198,6 @@ def setUp(self):
         super().setUp()
         self.user = User.get_or_create('foo.com', has_hcard=True,
                                        actor_as2=json_dumps(ACTOR))
-        activitypub.seen_ids.clear()
 
     def test_actor(self, *_):
         got = self.client.get('/foo.com')
@@ -223,7 +226,7 @@ def test_actor(self, *_):
             'publicKey': {
                 'id': 'http://localhost/foo.com',
                 'owner': 'http://localhost/foo.com',
-                'publicKeyPem': User.get_by_id('foo.com').public_pem().decode(),
+                'publicKeyPem': self.user.public_pem().decode(),
             },
         }, got.json)
 
@@ -598,6 +601,9 @@ def test_inbox_follow_use_instead_strip_www(self, mock_head, mock_get, mock_post
 
     def test_inbox_undo_follow(self, mock_head, mock_get, mock_post):
         mock_head.return_value = requests_response(url='https://foo.com/')
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
 
         Follower.get_or_create('foo.com', ACTOR['id'])
 
@@ -629,19 +635,30 @@ def test_inbox_follow_inactive(self, mock_head, mock_get, mock_post):
 
     def test_inbox_undo_follow_doesnt_exist(self, mock_head, mock_get, mock_post):
         mock_head.return_value = requests_response(url='https://foo.com/')
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
 
         got = self.client.post('/foo.com/inbox', json=UNDO_FOLLOW_WRAPPED)
         self.assertEqual(200, got.status_code)
 
     def test_inbox_undo_follow_inactive(self, mock_head, mock_get, mock_post):
         mock_head.return_value = requests_response(url='https://foo.com/')
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
+
         Follower.get_or_create('foo.com', ACTOR['id'], status='inactive')
 
         got = self.client.post('/foo.com/inbox', json=UNDO_FOLLOW_WRAPPED)
         self.assertEqual(200, got.status_code)
 
     def test_inbox_undo_follow_composite_object(self, mock_head, mock_get, mock_post):
         mock_head.return_value = requests_response(url='https://foo.com/')
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
+
         Follower.get_or_create('foo.com', ACTOR['id'], status='inactive')
 
         undo_follow = copy.deepcopy(UNDO_FOLLOW_WRAPPED)
@@ -683,23 +700,74 @@ def test_inbox_bad_object_url(self, mock_head, mock_get, mock_post):
 
         self.assertIsNone(Object.get_by_id(bad_url))
 
-    def test_delete_actor(self, _, __, ___):
+    def test_inbox_verify_http_signature(self, _, mock_get, ___):
+        # actor with a public key
+        mock_get.return_value = self.as2_resp({
+            **ACTOR,
+            'publicKey': {
+                'id': 'my-key-id',
+                'owner': 'http://sen/der',
+                'publicKeyPem': self.user.public_pem().decode(),
+            },
+        })
+
+        # manually construct signature
+        body = json_dumps(NOTE)
+        digest = b64encode(sha256(body.encode()).digest()).decode()
+        headers = {
+            'Date': 'Sun, 02 Jan 2022 03:04:05 GMT',
+            'Host': 'localhost',
+            'Content-Type': as2.CONTENT_TYPE,
+            'Digest': f'SHA-256={digest}',
+        }
+        hs = HeaderSigner('my-key-id', self.user.private_pem().decode(),
+                          algorithm='rsa-sha256', sign_header='signature',
+                          headers=('Date', 'Host', 'Digest', '(request-target)'))
+        headers = hs.sign(headers, method='GET', path='/inbox')
+
+        # valid signature
+        resp = self.client.post('/inbox', data=body, headers=headers)
+        self.assertEqual(200, resp.status_code)
+
+        # invalid signature, content changed
+        activitypub.seen_ids.clear()
+        obj_key = ndb.Key(Object, NOTE['id'])
+        obj_key.delete()
+        resp = self.client.post('/inbox', json={**NOTE, 'content': 'z'}, headers=headers)
+        self.assertEqual(401, resp.status_code)
+
+        # invalid signature, header changed
+        activitypub.seen_ids.clear()
+        obj_key.delete()
+        orig_date = headers['Date']
+        resp = self.client.post('/inbox', data=body, headers={**headers, 'Date': 'X'})
+        self.assertEqual(401, resp.status_code)
+
+    def test_delete_actor(self, _, mock_get, ___):
         follower = Follower.get_or_create('foo.com', DELETE['actor'])
         followee = Follower.get_or_create(DELETE['actor'], 'snarfed.org')
         # other unrelated follower
         other = Follower.get_or_create('foo.com', 'https://mas.to/users/other')
         self.assertEqual(3, Follower.query().count())
 
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
+
         got = self.client.post('/inbox', json=DELETE)
         self.assertEqual(200, got.status_code)
         self.assertEqual('inactive', follower.key.get().status)
         self.assertEqual('inactive', followee.key.get().status)
         self.assertEqual('active', other.key.get().status)
 
-    def test_delete_note(self, _, __, ___):
+    def test_delete_note(self, _, mock_get, ___):
         obj = Object(id='http://an/obj', as1='{}')
         obj.put()
 
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
+
         delete = {
             **DELETE,
             'object': 'http://an/obj',
@@ -714,14 +782,18 @@ def test_delete_note(self, _, __, ___):
         obj.deleted = True
         self.assert_entities_equal(obj, common.get_object.cache['http://an/obj'])
 
-    def test_update_note(self, *_):
+    def test_update_note(self, *mocks):
         Object(id='https://a/note', as1='{}').put()
-        self._test_update()
+        self._test_update(*mocks)
+
+    def test_update_unknown(self, *mocks):
+        self._test_update(*mocks)
 
-    def test_update_unknown(self, *_):
-        self._test_update()
+    def _test_update(self, _, mock_get, ___):
+        mock_get.side_effect = [
+            self.as2_resp(ACTOR),
+        ]
 
-    def _test_update(self):
         resp = self.client.post('/inbox', json=UPDATE_NOTE)
         self.assertEqual(200, resp.status_code)
 

tests/testutil.py

@@ -18,7 +18,7 @@
 import requests
 
 from app import app, cache
-import common
+import activitypub, common
 from models import Object, Target
 
 
@@ -29,6 +29,7 @@ def setUp(self):
         super().setUp()
         app.testing = True
         cache.clear()
+        activitypub.seen_ids.clear()
         common.get_object.cache.clear()
 
         self.client = app.test_client()