CVE-2026-56812: Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff

Summary

The Phoenix JavaScript presence client (assets/js/phoenix/presence.js) tests whether a presence already exists using a bare truthiness check (state[key]) rather than an own-property check. Because applications commonly track presences under a client-supplied username or id, the presence key can be attacker-controlled. A user who joins a channel and picks a key that names an Object.prototype member (proto, constructor, toString, hasOwnProperty, and similar) makes the lookup return the inherited Object.prototype object instead of undefined, which is truthy. The code then reads .metas.map(...) off it and throws an uncaught TypeError, breaking presence sync for every viewer of that channel topic. Any authenticated channel participant can trigger it.

Details

The victim is any browser subscribed to a presence channel. When it receives the server's presencestate message, it invokes Presence.syncState, which iterates the incoming presences and checks whether each one already exists locally via let currentPresence = state[key]. state is a plain object inheriting from Object.prototype. For an ordinary key like alice, state["alice"] is undefined (falsy) and the safe path runs. For the key proto (or constructor, toString, etc.), state["proto"] does not resolve to a tracked presence but to JavaScript's built-in Object.prototype, which is truthy. The if(currentPresence) guard passes, and the code evaluates currentPresence.metas.map(m => m.phxref). Since Object.prototype.metas is undefined, calling .map on it throws a TypeError.

Phoenix wraps no try/catch around channel binding callbacks, so the TypeError propagates out of the message handler: this.state is never updated and onSync() never fires. The malicious key is tracked server-side, so it is re-pushed on every presence update and keeps re-throwing, leaving presence permanently broken until the attacker leaves. Presence.syncDiff uses the same unsafe state[key] existence-check pattern, so presence diffs fail identically.

Two scoping points matter. The impact is per channel topic, not global: presence state is per-topic on the server and per-Presence-instance in the browser, so only viewers of the topic carrying the malicious key are affected. The bug is a read-time confusion of the prototype object, not prototype pollution: the crash occurs on the state["proto"] read in syncState, before any state[key] = ... write is reached, so Object.prototype is never mutated and nothing leaks across channels. The fix builds the state and accumulator objects with Object.create(null) (or a Map) and gates existence checks with Object.prototype.hasOwnProperty.call(obj, key).

If an application does not pass a client-controlled key to Presence.track, it is not affected.

PoC

1. Connect to an application that uses Phoenix.Presence and tracks presences under a client-chosen key (e.g. a username). 2. Join a presence channel choosing the key proto (or constructor, toString, hasOwnProperty). 3. The server tracks the presence and pushes presencestate / presencediff to every subscriber of that topic. 4. Each viewer's Presence.syncState (or syncDiff) reads state["proto"], gets the truthy Object.prototype, and throws an uncaught TypeError. 5. Presence sync stays broken for all viewers of the topic until the attacker leaves the channel.

Impact

An attacker with ordinary channel access can cause a persistent, stored client-side denial of service against every browser viewing a presence channel topic, freezing presence updates for all of them until the attacker disconnects. Any application driving the Phoenix JavaScript presence client with user-influenced presence keys is affected.