CVE-2026-56812

What it is

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; 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.

At a glance

Exploitability

Can be exploited over the network without logging in. Limited direct impact.

Exploit odds

0.8% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-07-07 16:16 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-10-25 20:10 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.8% in the next 30 days (EPSS).
• Severity: 6.3 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-07-07 16:16 UTC

Last modified

2026-10-25 20:10 UTC

Exploit probability

0.8% · EPSS, next 30 days

Weakness

CWE-754

Record status

Modified

Affected

• Phoenixframework Phoenix < 1.5.15
• Phoenixframework Phoenix < 1.6.17
• Phoenixframework Phoenix < 1.7.24
• Phoenixframework Phoenix < 1.8.9

Attack profile CVSS 4.0 · 6.3

Attack vector

Network

Complexity

Low

Attack requirements

Present

Privileges needed

None

User interaction

None

Confidentiality

None

Integrity

None

Availability

Low

Is it in your code?

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