Merge pull request #6331 from matrix-org/rav/url_preview_limit_title

* commit '02f99906f': Apply suggestions from code review Strip overlong OpenGraph data from url preview
matrix-org · Mar 16, 2020 · 6e290f3 · 6e290f3
2 parents 4d02402 + 02f9990
commit 6e290f3
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diff --git a/changelog.d/6331.feature b/changelog.d/6331.feature
@@ -0,0 +1 @@
+Limit the length of data returned by url previews, to prevent DoS attacks.
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
@@ -56,6 +56,9 @@
 _charset_match = re.compile(br"<\s*meta[^>]*charset\s*=\s*([a-z0-9-]+)", flags=re.I)
 _content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
 
+OG_TAG_NAME_MAXLEN = 50
+OG_TAG_VALUE_MAXLEN = 1000
+
 
 class PreviewUrlResource(DirectServeResource):
     isLeaf = True
@@ -171,7 +174,7 @@ def _do_preview(self, url, user, ts):
             ts (int):
 
         Returns:
-            Deferred[str]: json-encoded og data
+            Deferred[bytes]: json-encoded og data
         """
         # check the URL cache in the DB (which will also provide us with
         # historical previews, if we have any)
@@ -272,6 +275,17 @@ def _do_preview(self, url, user, ts):
             logger.warning("Failed to find any OG data in %s", url)
             og = {}
 
+        # filter out any stupidly long values
+        keys_to_remove = []
+        for k, v in og.items():
+            if len(k) > OG_TAG_NAME_MAXLEN or len(v) > OG_TAG_VALUE_MAXLEN:
+                logger.warning(
+                    "Pruning overlong tag %s from OG data", k[:OG_TAG_NAME_MAXLEN]
+                )
+                keys_to_remove.append(k)
+        for k in keys_to_remove:
+            del og[k]
+
         logger.debug("Calculated OG for %s as %s", url, og)
 
         jsonog = json.dumps(og)
@@ -506,6 +520,10 @@ def _calc_og(tree, media_uri):
     og = {}
     for tag in tree.xpath("//*/meta[starts-with(@property, 'og:')]"):
         if "content" in tag.attrib:
+            # if we've got more than 50 tags, someone is taking the piss
+            if len(og) >= 50:
+                logger.warning("Skipping OG for page with too many 'og:' tags")
+                return {}
             og[tag.attrib["property"]] = tag.attrib["content"]
 
     # TODO: grab article: meta tags too, e.g.:

diff --git a/tests/rest/media/v1/test_url_preview.py b/tests/rest/media/v1/test_url_preview.py
@@ -247,6 +247,41 @@ def test_non_ascii_preview_content_type(self):
         self.assertEqual(channel.code, 200)
         self.assertEqual(channel.json_body["og:title"], "\u0434\u043a\u0430")
 
+    def test_overlong_title(self):
+        self.lookups["matrix.org"] = [(IPv4Address, "8.8.8.8")]
+
+        end_content = (
+            b"<html><head>"
+            b"<title>" + b"x" * 2000 + b"</title>"
+            b'<meta property="og:description" content="hi" />'
+            b"</head></html>"
+        )
+
+        request, channel = self.make_request(
+            "GET", "url_preview?url=http://matrix.org", shorthand=False
+        )
+        request.render(self.preview_url)
+        self.pump()
+
+        client = self.reactor.tcpClients[0][2].buildProtocol(None)
+        server = AccumulatingProtocol()
+        server.makeConnection(FakeTransport(client, self.reactor))
+        client.makeConnection(FakeTransport(server, self.reactor))
+        client.dataReceived(
+            (
+                b"HTTP/1.0 200 OK\r\nContent-Length: %d\r\n"
+                b'Content-Type: text/html; charset="windows-1251"\r\n\r\n'
+            )
+            % (len(end_content),)
+            + end_content
+        )
+
+        self.pump()
+        self.assertEqual(channel.code, 200)
+        res = channel.json_body
+        # We should only see the `og:description` field, as `title` is too long and should be stripped out
+        self.assertCountEqual(["og:description"], res.keys())
+
     def test_ipaddr(self):
         """
         IP addresses can be previewed directly.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Limit the length of data returned by url previews, to prevent DoS attacks.