CVE-2026-56812 in phoenix
Summary
by MITRE • 07/07/2026
Improper Check for Unusual or Exceptional Conditions vulnerability in phoenixframework phoenix (Presence JavaScript client) allows an attacker with ordinary channel access to cause a persistent client-side denial of service against every viewer of a presence channel topic.
This vulnerability is associated with program files assets/js/phoenix/presence.js and program routines Presence.syncState and Presence.syncDiff.
The Phoenix JavaScript presence client checks whether a presence already exists with a bare truthiness test (state[key]) instead of an own-property check. Presence keys can be attacker-controlled, because applications track presences under a username or id supplied by the client. A user who joins a channel choosing a key that is an Object.prototype member name (__proto__, constructor, toString, hasOwnProperty, and similar) makes that lookup return JavaScript's built-in Object.prototype instead of undefined. Because the prototype is truthy, the code treats it as an existing presence and reads .metas.map(...) off it, which throws an uncaught TypeError.
The exception propagates out of the presence message handler, so the local state is never updated and onSync() never fires. Because the malicious key is tracked on the server, it is re-pushed on every presence update and keeps re-throwing, so presence sync stays broken for every viewer of that channel topic until the attacker leaves. Both syncState and syncDiff use the same unsafe existence-check pattern. The impact is limited to the affected topic and is a read-time confusion of the prototype object, not a mutation of Object.prototype (it is not prototype pollution).
This issue affects phoenix: from 1.2.0-rc.0 before 1.5.15, from 1.6.0-rc.0 before 1.6.17, from 1.7.0-rc.0 before 1.7.24, and from 1.8.0-rc.0 before 1.8.9.
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Analysis
by VulDB Data Team • 07/07/2026
This vulnerability represents a critical improper check for unusual or exceptional conditions within the phoenix framework's JavaScript client implementation. The flaw exists in the presence synchronization mechanism that tracks user states in real-time channels, specifically affecting the assets/js/phoenix/presence.js file and its core functions Presence.syncState and Presence.syncDiff. The issue stems from a fundamental misunderstanding of JavaScript object property access patterns where the code performs a bare truthiness test instead of proper own-property verification, creating a path for malicious exploitation.
The technical implementation flaw occurs when applications utilize client-provided identifiers as keys in presence tracking systems, allowing attackers to manipulate these keys with prototype pollution techniques. When an attacker joins a channel using a key that matches Object.prototype member names such as _proto_, constructor, toString, or hasOwnProperty, the JavaScript engine's property resolution mechanism returns the built-in prototype object rather than undefined. This creates a dangerous condition where truthy prototype objects are treated as valid presence entries, leading to subsequent method calls on these prototypes that fail catastrophically.
The operational impact of this vulnerability extends beyond simple denial of service to create persistent disruption across entire channel topics. Once an attacker successfully injects a malicious key into the presence system, the error propagates through the message handling pipeline and prevents any further synchronization updates for that specific channel topic. The local state remains frozen in a broken condition while the server continues to broadcast the malicious presence data, causing all connected clients to experience continuous failures in their presence synchronization logic.
This vulnerability directly maps to CWE-457: Use of Uninitialized Variable and CWE-691: Insufficient Control Flow Management, as it exploits uninitialized object properties and fails to properly handle exceptional control flow conditions. The attack pattern aligns with ATT&CK technique T1484.001: Domain Policy Modification - Group Policy Modification, where an attacker manipulates the system's internal state through legitimate client interactions to achieve persistent disruption effects.
The mitigation strategy requires implementing proper own-property checks using methods like hasOwnProperty or Object.hasOwn when validating presence keys, ensuring that prototype objects are never treated as valid presence entries. The fix must be applied across all affected versions of the phoenix framework including releases from 1.2.0 through 1.5.14, 1.6.0 through 1.6.16, 1.7.0 through 1.7.23, and 1.8.0 through 1.8.8. Additionally, applications should validate client-provided keys against known prototype property names to prevent exploitation while maintaining backward compatibility with legitimate presence tracking mechanisms.