phoenixframework phoenix up to 1.5.14/1.6.16/1.7.23/1.8.8 Presence presence.js Presence.syncState/Presence.syncDiff denial of service

Summaryinfo

A vulnerability, which was classified as problematic, was found in phoenixframework phoenix up to 1.5.14/1.6.16/1.7.23/1.8.8. The impacted element is the function Presence.syncState/Presence.syncDiff of the file assets/js/phoenix/presence.js of the component Presence. Such manipulation leads to denial of service. This vulnerability is listed as CVE-2026-56812. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in phoenixframework phoenix up to 1.5.14/1.6.16/1.7.23/1.8.8. It has been rated as problematic. Affected by this issue is the function Presence.syncState/Presence.syncDiff of the file assets/js/phoenix/presence.js of the component Presence. The manipulation with an unknown input leads to a denial of service vulnerability. Using CWE to declare the problem leads to CWE-404. The product does not release or incorrectly releases a resource before it is made available for re-use. Impacted is availability. CVE summarizes:

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.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-56812 since 06/23/2026. The exploitation is known to be easy. The attack may be launched remotely. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1499 by the MITRE ATT&CK project.

Upgrading to version 1.5.15, 1.6.17, 1.7.24 or 1.8.9 eliminates this vulnerability.

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Productinfo

Vendor

• phoenixframework

Name

• phoenix

Version

• 1.5.0
• 1.5.1
• 1.5.2
• 1.5.3
• 1.5.4
• 1.5.5
• 1.5.6
• 1.5.7
• 1.5.8
• 1.5.9
• 1.5.10
• 1.5.11
• 1.5.12
• 1.5.13
• 1.5.14
• 1.6.0
• 1.6.1
• 1.6.2
• 1.6.3
• 1.6.4
• 1.6.5
• 1.6.6
• 1.6.7
• 1.6.8
• 1.6.9
• 1.6.10
• 1.6.11
• 1.6.12
• 1.6.13
• 1.6.14
• 1.6.15
• 1.6.16
• 1.7.0
• 1.7.1
• 1.7.2
• 1.7.3
• 1.7.4
• 1.7.5
• 1.7.6
• 1.7.7
• 1.7.8
• 1.7.9
• 1.7.10
• 1.7.11
• 1.7.12
• 1.7.13
• 1.7.14
• 1.7.15
• 1.7.16
• 1.7.17
• 1.7.18
• 1.7.19
• 1.7.20
• 1.7.21
• 1.7.22
• 1.7.23
• 1.8.0
• 1.8.1
• 1.8.2
• 1.8.3
• 1.8.4
• 1.8.5
• 1.8.6
• 1.8.7
• 1.8.8

Website

• Product: https://github.com/phoenixframework/phoenix/

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