diff --git a/CHANGES.md b/CHANGES.md
index 6faa4b8dce..9312dc2941 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,12 @@
+Synapse 1.5.1 (2019-11-06)
+==========================
+
+Features
+--------
+
+- Limit the length of data returned by url previews, to prevent DoS attacks. ([\#6331](https://github.com/matrix-org/synapse/issues/6331), [\#6334](https://github.com/matrix-org/synapse/issues/6334))
+
+
 Synapse 1.5.0 (2019-10-29)
 ==========================
 
diff --git a/debian/changelog b/debian/changelog
index acda7e5c63..c4415f460a 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,9 @@
+matrix-synapse-py3 (1.5.1) stable; urgency=medium
+
+  * New synapse release 1.5.1.
+
+ -- Synapse Packaging team <packages@matrix.org>  Wed, 06 Nov 2019 10:02:14 +0000
+
 matrix-synapse-py3 (1.5.0) stable; urgency=medium
 
   * New synapse release 1.5.0.
diff --git a/scripts-dev/build_debian_packages b/scripts-dev/build_debian_packages
index 93305ee9b1..84eaec6a95 100755
--- a/scripts-dev/build_debian_packages
+++ b/scripts-dev/build_debian_packages
@@ -20,11 +20,13 @@ from concurrent.futures import ThreadPoolExecutor
 DISTS = (
     "debian:stretch",
     "debian:buster",
+    "debian:bullseye",
     "debian:sid",
     "ubuntu:xenial",
     "ubuntu:bionic",
     "ubuntu:cosmic",
     "ubuntu:disco",
+    "ubuntu:eoan",
 )
 
 DESC = '''\
diff --git a/synapse/__init__.py b/synapse/__init__.py
index 8587ffa76f..ec16f54a49 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -36,7 +36,7 @@ try:
 except ImportError:
     pass
 
-__version__ = "1.5.0"
+__version__ = "1.5.1"
 
 if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     # We import here so that we don't have to install a bunch of deps when
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index 0c68c3aad5..ec9c4619c9 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -56,6 +56,9 @@ logger = logging.getLogger(__name__)
 _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
@@ -167,7 +170,7 @@ class PreviewUrlResource(DirectServeResource):
             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)
@@ -268,6 +271,18 @@ class PreviewUrlResource(DirectServeResource):
             logger.warn("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():
+            # values can be numeric as well as strings, hence the cast to str
+            if len(k) > OG_TAG_NAME_MAXLEN or len(str(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)
@@ -502,6 +517,10 @@ 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
index 976652aee8..852b8ab11c 100644
--- a/tests/rest/media/v1/test_url_preview.py
+++ b/tests/rest/media/v1/test_url_preview.py
@@ -247,6 +247,41 @@ class URLPreviewTests(unittest.HomeserverTestCase):
         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.