diff --git a/.changeset/secure-phoenix-client.md b/.changeset/secure-phoenix-client.md
new file mode 100644
index 000000000..04aeb5d41
--- /dev/null
+++ b/.changeset/secure-phoenix-client.md
@@ -0,0 +1,5 @@
+---
+"@knocklabs/client": patch
+---
+
+Upgrade Phoenix to 1.8.9 to address CVE-2026-56812.
diff --git a/packages/client/package.json b/packages/client/package.json
index b05ea8aa0..14968f928 100644
--- a/packages/client/package.json
+++ b/packages/client/package.json
@@ -75,7 +75,7 @@
     "@types/phoenix": "^1.6.7",
     "eventemitter2": "^6.4.5",
     "nanoid": "^3.3.12",
-    "phoenix": "1.8.5",
+    "phoenix": "1.8.9",
     "urlpattern-polyfill": "^10.0.0"
   }
 }
diff --git a/yarn.lock b/yarn.lock
index 961df04b8..b9786bf07 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4632,7 +4632,7 @@ __metadata:
     eventemitter2: "npm:^6.4.5"
     jsonwebtoken: "npm:^9.0.3"
     nanoid: "npm:^3.3.12"
-    phoenix: "npm:1.8.5"
+    phoenix: "npm:1.8.9"
     prettier: "npm:^3.5.3"
     rimraf: "npm:^6.1.3"
     rollup: "npm:^4.46.2"
@@ -17150,10 +17150,10 @@ __metadata:
   languageName: node
   linkType: hard
 
-"phoenix@npm:1.8.5":
-  version: 1.8.5
-  resolution: "phoenix@npm:1.8.5"
-  checksum: 10c0/11e7a4c75a41524e25aa32001be709ad50d2a8d02650f0c67f30e4e69bd5d85211054137a51b7d4dcf9cfe65d1e65cf7bbefa22c4207506caf1afd36bf7cd551
+"phoenix@npm:1.8.9":
+  version: 1.8.9
+  resolution: "phoenix@npm:1.8.9"
+  checksum: 10c0/9f661db2c0e3986ec3cce4969ec4906a02e15675bd9123fbb8ae7a8caf61213ce20420be505f52dc0a9de9117021a21e9b1344964400530e26474e45d7819c86
   languageName: node
   linkType: hard