diff --git a/CHANGES.md b/CHANGES.md
index 9ba930e72..d94a802fa 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,7 +1,59 @@
 Synapse 1.12.0 (2020-03-23)
 ===========================
 
-No significant changes.
+No significant changes since 1.12.0rc1.
+
+Debian packages and Docker images are rebuilt using the letest versions of
+dependency libraries, including Twisted 20.3.0. **Please see security advisory
+below**.
+
+Security advisory
+-----------------
+
+Synapse may be vulnerable to request-smuggling attacks when it is used with a
+reverse-proxy. The vulnerabilties are fixed in Twisted 20.3.0, and are
+described in
+[CVE-2020-10108](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10108)
+and
+[CVE-2020-10109](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10109).
+For a good introduction to this class of request-smuggling attacks, see
+https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn.
+
+We are not aware of these vulnerabilities being exploited in the world, and
+do not believe that they are exploitable with current versions of any reverse
+proxies. Nevertheless, we recommend that all Synapse administrators ensure that
+they have the latest versions of the Twisted library to ensure that their
+installation remains secore.
+
+* Administrators using the [`matrix.org` Docker
+  image](https://hub.docker.com/r/matrixdotorg/synapse/) or the [Debian/Ubuntu
+  packages from
+  `matrix.org`](https://github.com/matrix-org/synapse/blob/master/INSTALL.md#matrixorg-packages)
+  should ensure that they have version 1.12.0 installed: these images include
+  Twisted 20.3.0.
+* Administrators who have [installed Synapse from
+  source](https://github.com/matrix-org/synapse/blob/master/INSTALL.md#installing-from-source)
+  should upgrade Twisted within their virtualenv by running:
+  ```sh
+  <path_to_virtualenv>/bin/pip install 'Twisted>=20.3.0'
+  ```
+* Administrators who have installed Synapse from distribution packages should
+  consult the information from their distributions.
+
+Advance notice of change to the default `git` branch for Synapse
+----------------------------------------------------------------
+
+Currently, the default `git` branch for Synapse is `master`, which tracks the
+latest release.
+
+After the release of Synapse 1.13.0, we intend to change this default to
+`develop`, which is the development tip. This is more consistent with common
+practice and modern `git` usage.
+
+Although we try to keep `develop` in a stable state, there may be occasions
+where regressions keep in. Developers and distributors who have scripts which
+run builds using the default branch of `Synapse` should therefore consider
+pinning their scripts to `master`.
 
 
 Synapse 1.12.0rc1 (2020-03-19)