---
id: CVE-2026-56812
title: >-
  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…
summary: >-
  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…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-754
vendor: phoenixframework
product: phoenix
affected:
  - 'phoenix >= 1.2.0, < 1.5.15'
  - 'phoenix >= 1.6.0, < 1.6.17'
  - 'phoenix >= 1.7.0, < 1.7.24'
  - 'phoenix >= 1.8.0, < 1.8.9'
patched:
  - phoenix 1.8.9
published: '2026-07-07'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T22:17:00.683'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-56812'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-56812.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/commit/2270aaf21bd02c6a6a1022820564efb605a97655
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/commit/7f7b971c1ea0994e3fbd1c11ddb05e780bd38ad8
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/commit/89a1c4be161e436241e12b2378a719904b9bd96f
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/commit/b90b22521465ece00eb5a19d5aa2b9465b209c85
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/commit/beffc4da1e787e572121f68902c63daf4fe7d9c2
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/security/advisories/GHSA-63mc-hw7g-86rr
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-56812'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/phoenixframework/phoenix/security/advisories/GHSA-63mc-hw7g-86rr
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-56812'
  - url: 'https://github.com/advisories/GHSA-63mc-hw7g-86rr'
tags:
  - nvd
  - cve.org
  - exploit-available
  - ghsa
  - erlang
epss: 0.00796
epssPercentile: 0.54606
aliases:
  - GHSA-63mc-hw7g-86rr
ecosystem: erlang
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-07-07T16:11:03.353715Z'
scores:
  nvd: 7.5
  cna: 6.3
ingestedAt: '2026-09-03T21:08:46.112Z'
---

## Overview

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.

## Affected

- `phoenix >= 1.2.0, < 1.5.15`
- `phoenix >= 1.6.0, < 1.6.17`
- `phoenix >= 1.7.0, < 1.7.24`
- `phoenix >= 1.8.0, < 1.8.9`

## Remediation

Upgrade past the affected range:

- `phoenix 1.8.9`

## Package advisory (CVE-2026-56812)

Affected packages:

- `phoenix >= 1.2.0-rc.0, < 1.5.15`
- `phoenix >= 1.6.0-rc.0, < 1.6.17`
- `phoenix >= 1.7.0-rc.0, < 1.7.24`
- `phoenix >= 1.8.0-rc.0, < 1.8.9`
- `phoenix >= 1.2.0-rc.0, < 1.5.15`
- `phoenix >= 1.6.0-rc.0, < 1.6.17`
- `phoenix >= 1.7.0-rc.0, < 1.7.24`
- `phoenix >= 1.8.0-rc.0, < 1.8.9`

Patched in:

- `phoenix 1.5.15`
- `phoenix 1.6.17`
- `phoenix 1.7.24`
- `phoenix 1.8.9`
- `phoenix 1.5.15`
- `phoenix 1.6.17`
- `phoenix 1.7.24`
- `phoenix 1.8.9`

Source: https://github.com/advisories/GHSA-63mc-hw7g-86rr