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Strip overlong OpenGraph data from url preview
... to stop people causing DoSes with malicious web pages
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changelog.d/6331.feature
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1
changelog.d/6331.feature
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@ -0,0 +1 @@
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Limit the length of data returned by url previews, to prevent DoS attacks.
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@ -56,6 +56,9 @@ logger = logging.getLogger(__name__)
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_charset_match = re.compile(br"<\s*meta[^>]*charset\s*=\s*([a-z0-9-]+)", flags=re.I)
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_content_type_match = re.compile(r'.*; *charset="?(.*?)"?(;|$)', flags=re.I)
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OG_TAG_NAME_MAXLEN = 50
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OG_TAG_VALUE_MAXLEN = 1000
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class PreviewUrlResource(DirectServeResource):
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isLeaf = True
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@ -167,7 +170,7 @@ class PreviewUrlResource(DirectServeResource):
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ts (int):
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Returns:
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Deferred[str]: json-encoded og data
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Deferred[bytes]: json-encoded og data
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"""
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# check the URL cache in the DB (which will also provide us with
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# historical previews, if we have any)
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@ -268,6 +271,17 @@ class PreviewUrlResource(DirectServeResource):
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logger.warn("Failed to find any OG data in %s", url)
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og = {}
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# filter out any stupidly long values
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keys_to_remove = []
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for k, v in og.items():
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if len(k) > OG_TAG_NAME_MAXLEN or len(v) > OG_TAG_VALUE_MAXLEN:
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logger.warning(
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"Pruning overlong tag %s from OG data", k[:OG_TAG_NAME_MAXLEN]
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)
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keys_to_remove.append(k)
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for k in keys_to_remove:
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del og[k]
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logger.debug("Calculated OG for %s as %s" % (url, og))
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jsonog = json.dumps(og)
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@ -502,6 +516,10 @@ def _calc_og(tree, media_uri):
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og = {}
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for tag in tree.xpath("//*/meta[starts-with(@property, 'og:')]"):
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if "content" in tag.attrib:
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# if we've got more than 50 tags, someone is taking the piss
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if len(og) >= 50:
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logger.warning("skipping OG for page with too many og: tags")
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return {}
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og[tag.attrib["property"]] = tag.attrib["content"]
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# TODO: grab article: meta tags too, e.g.:
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@ -247,6 +247,40 @@ class URLPreviewTests(unittest.HomeserverTestCase):
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self.assertEqual(channel.code, 200)
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self.assertEqual(channel.json_body["og:title"], "\u0434\u043a\u0430")
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def test_overlong_title(self):
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self.lookups["matrix.org"] = [(IPv4Address, "8.8.8.8")]
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end_content = (
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b"<html><head>"
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b"<title>" + b"x" * 2000 + b"</title>"
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b'<meta property="og:description" content="hi" />'
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b"</head></html>"
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)
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request, channel = self.make_request(
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"GET", "url_preview?url=http://matrix.org", shorthand=False
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)
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request.render(self.preview_url)
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self.pump()
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client = self.reactor.tcpClients[0][2].buildProtocol(None)
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server = AccumulatingProtocol()
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server.makeConnection(FakeTransport(client, self.reactor))
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client.makeConnection(FakeTransport(server, self.reactor))
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client.dataReceived(
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(
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b"HTTP/1.0 200 OK\r\nContent-Length: %d\r\n"
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b'Content-Type: text/html; charset="windows-1251"\r\n\r\n'
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)
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% (len(end_content),)
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+ end_content
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)
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self.pump()
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self.assertEqual(channel.code, 200)
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res = channel.json_body
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self.assertCountEqual(["og:description"], res.keys())
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def test_ipaddr(self):
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"""
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IP addresses can be previewed directly.
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