diff --git a/CHANGELOG b/CHANGELOG
index af6140d..5eff0ee 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -405,3 +405,8 @@
 * Limit the size of HTTP responses read over federation. (#9833)
 * Fix a bug which could cause Synapse to get stuck in a loop of resyncing device lists. (#9867)
 * Fix a long-standing bug where errors from federation did not propagate to the client. (#9868)
+
+[1.23.1]
+* Update Synapse to 1.33.1
+* Fix bug where /sync would break if using the latest version of attrs dependency, by pinning to a previous version. (#9937)
+
diff --git a/CloudronManifest.json b/CloudronManifest.json
index 602eec0..9961964 100644
--- a/CloudronManifest.json
+++ b/CloudronManifest.json
@@ -5,7 +5,7 @@
   "description": "file://DESCRIPTION.md",
   "changelog": "file://CHANGELOG",
   "tagline": "Secure & decentralized communication",
-  "version": "1.23.0",
+  "version": "1.23.1",
   "healthCheckPath": "/",
   "httpPort": 8008,
   "memoryLimit": 536870912,
diff --git a/DESCRIPTION.md b/DESCRIPTION.md
index 0df9c2d..10b48b1 100644
--- a/DESCRIPTION.md
+++ b/DESCRIPTION.md
@@ -1,4 +1,4 @@
-This app packages Synapse <upstream>1.33.0</upstream>.
+This app packages Synapse <upstream>1.33.1</upstream>.
 
 **This package only provides the Matrix backend. A variety of clients are available
 [here](https://matrix.org/clients/). Riot is a popular web frontend for Matrix and
diff --git a/Dockerfile b/Dockerfile
index f27fc02..551a744 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -9,7 +9,7 @@ RUN virtualenv -p python3 /app/code/env
 ENV VIRTUAL_ENV=/app/code/env
 ENV PATH="$VIRTUAL_ENV/bin:$PATH"
 
-ARG VERSION=v1.33.0
+ARG VERSION=v1.33.1
 
 # Synapse (https://github.com/matrix-org/synapse/blob/master/INSTALL.md)
 # lxml - required for previews