From 18fedf4119a1d6ba7fda53a2209cb59b2b864e30 Mon Sep 17 00:00:00 2001
From: Lukasz Samson <lukaszsamson@gmail.com>
Date: Sat, 25 Apr 2026 17:33:45 +0200
Subject: [PATCH] Correctly serialize non ASCII metadata

Fixes #6663
---
 assets/js/phoenix/serializer.js | 38 +++++++++++++++++++++--------
 assets/test/serializer_test.js  | 42 +++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 10 deletions(-)

diff --git a/assets/js/phoenix/serializer.js b/assets/js/phoenix/serializer.js
index 3df8eb3ef7..b2f7867d22 100644
--- a/assets/js/phoenix/serializer.js
+++ b/assets/js/phoenix/serializer.js
@@ -30,28 +30,46 @@ export default {
 
   binaryEncode(message){
     let {join_ref, ref, event, topic, payload} = message
-    let metaLength = this.META_LENGTH + join_ref.length + ref.length + topic.length + event.length
+    let encoder = new TextEncoder()
+    let joinRefBytes = encoder.encode(join_ref)
+    let refBytes = encoder.encode(ref)
+    let topicBytes = encoder.encode(topic)
+    let eventBytes = encoder.encode(event)
+
+    this.assertFieldSize(joinRefBytes.byteLength, "join_ref")
+    this.assertFieldSize(refBytes.byteLength, "ref")
+    this.assertFieldSize(topicBytes.byteLength, "topic")
+    this.assertFieldSize(eventBytes.byteLength, "event")
+
+    let metaLength = this.META_LENGTH + joinRefBytes.byteLength + refBytes.byteLength + topicBytes.byteLength + eventBytes.byteLength
     let header = new ArrayBuffer(this.HEADER_LENGTH + metaLength)
+    let headerBytes = new Uint8Array(header)
     let view = new DataView(header)
     let offset = 0
 
     view.setUint8(offset++, this.KINDS.push) // kind
-    view.setUint8(offset++, join_ref.length)
-    view.setUint8(offset++, ref.length)
-    view.setUint8(offset++, topic.length)
-    view.setUint8(offset++, event.length)
-    Array.from(join_ref, char => view.setUint8(offset++, char.charCodeAt(0)))
-    Array.from(ref, char => view.setUint8(offset++, char.charCodeAt(0)))
-    Array.from(topic, char => view.setUint8(offset++, char.charCodeAt(0)))
-    Array.from(event, char => view.setUint8(offset++, char.charCodeAt(0)))
+    view.setUint8(offset++, joinRefBytes.byteLength)
+    view.setUint8(offset++, refBytes.byteLength)
+    view.setUint8(offset++, topicBytes.byteLength)
+    view.setUint8(offset++, eventBytes.byteLength)
+    headerBytes.set(joinRefBytes, offset); offset += joinRefBytes.byteLength
+    headerBytes.set(refBytes, offset); offset += refBytes.byteLength
+    headerBytes.set(topicBytes, offset); offset += topicBytes.byteLength
+    headerBytes.set(eventBytes, offset); offset += eventBytes.byteLength
 
     var combined = new Uint8Array(header.byteLength + payload.byteLength)
-    combined.set(new Uint8Array(header), 0)
+    combined.set(headerBytes, 0)
     combined.set(new Uint8Array(payload), header.byteLength)
 
     return combined.buffer
   },
 
+  assertFieldSize(size, name){
+    if(size > 255){
+      throw new Error(`unable to convert ${name} to binary: must be less than or equal to 255 bytes, but is ${size} bytes`)
+    }
+  },
+
   binaryDecode(buffer){
     let view = new DataView(buffer)
     let kind = view.getUint8(0)
diff --git a/assets/test/serializer_test.js b/assets/test/serializer_test.js
index 78a56e8816..4965340d6e 100644
--- a/assets/test/serializer_test.js
+++ b/assets/test/serializer_test.js
@@ -81,6 +81,48 @@ describe("binary", () => {
     })
   })
 
+  it("encodes non-ASCII metadata as UTF-8", (done) => {
+    let buffer = binPayload()
+    let topic = "room:café"
+    let event = "π"
+    let joinRef = "🚀"
+    let ref = "1"
+    let encoder = new TextEncoder()
+    let topicBytes = encoder.encode(topic)
+    let eventBytes = encoder.encode(event)
+    let joinRefBytes = encoder.encode(joinRef)
+    let refBytes = encoder.encode(ref)
+
+    Serializer.encode({join_ref: joinRef, ref: ref, topic: topic, event: event, payload: buffer}, (result) => {
+      let bytes = new Uint8Array(result)
+      expect(bytes[0]).toBe(0) // push kind
+      expect(bytes[1]).toBe(joinRefBytes.byteLength) // 4 bytes for "🚀"
+      expect(bytes[2]).toBe(refBytes.byteLength)
+      expect(bytes[3]).toBe(topicBytes.byteLength) // 10 bytes for "room:café"
+      expect(bytes[4]).toBe(eventBytes.byteLength) // 2 bytes for "π"
+
+      let offset = 5
+      expect(Array.from(bytes.slice(offset, offset + joinRefBytes.byteLength))).toEqual(Array.from(joinRefBytes))
+      offset += joinRefBytes.byteLength
+      expect(Array.from(bytes.slice(offset, offset + refBytes.byteLength))).toEqual(Array.from(refBytes))
+      offset += refBytes.byteLength
+      expect(Array.from(bytes.slice(offset, offset + topicBytes.byteLength))).toEqual(Array.from(topicBytes))
+      offset += topicBytes.byteLength
+      expect(Array.from(bytes.slice(offset, offset + eventBytes.byteLength))).toEqual(Array.from(eventBytes))
+      offset += eventBytes.byteLength
+      expect(bytes[offset]).toBe(1) // payload byte
+      done()
+    })
+  })
+
+  it("throws when a metadata field exceeds 255 UTF-8 bytes", () => {
+    let buffer = binPayload()
+    let big = "a".repeat(256)
+    expect(() => {
+      Serializer.encode({join_ref: "0", ref: "1", topic: big, event: "e", payload: buffer}, () => {})
+    }).toThrow(/topic/)
+  })
+
   it("decodes broadcast", (done) => {
     let bin = "\x02\x03\ntopsome-event\x01\x01"
     let buffer = new TextEncoder().encode(bin).buffer