Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff
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
CVE-2026-56812
Primary source · Revision 1
Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.
Security context
Source reference: cve:CVE-2026-56812
phoenix
Source reference: /containers/cna/affected/0/product
- Affected versions
- Not stated in this record
- Fixed versions
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- Prerequisites
- Not stated in this record
phoenix
Source reference: /containers/cna/affected/1/product
- Affected versions
- Not stated in this record
- Fixed versions
- Not stated in this record
- Prerequisites
- Not stated in this record
phoenix
Source reference: /containers/cna/affected/2/product
- Affected versions
- Not stated in this record
- Fixed versions
- Not stated in this record
- Prerequisites
- Not stated in this record
Structured affected-version data
Original package identities and version rules from this source.
[
{
"cpes": [
"cpe:2.3:a:phoenixframework:phoenix:*:*:*:*:*:*:*:*"
],
"repo": "https://github.com/phoenixframework/phoenix",
"vendor": "phoenixframework",
"modules": [
"Presence"
],
"product": "phoenix",
"versions": [
{
"status": "affected",
"version": "1.2.0-rc.0",
"lessThan": "1.5.15",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.6.0-rc.0",
"lessThan": "1.6.17",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.7.0-rc.0",
"lessThan": "1.7.24",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.8.0-rc.0",
"lessThan": "1.8.9",
"versionType": "semver"
}
],
"packageURL": "pkg:hex/phoenix",
"packageName": "phoenix",
"programFiles": [
"assets/js/phoenix/presence.js"
],
"collectionURL": "https://repo.hex.pm",
"defaultStatus": "unaffected",
"programRoutines": [
{
"name": "Presence.syncState"
},
{
"name": "Presence.syncDiff"
}
]
},
{
"cpes": [
"cpe:2.3:a:phoenixframework:phoenix:*:*:*:*:*:*:*:*"
],
"repo": "https://github.com/phoenixframework/phoenix",
"vendor": "phoenixframework",
"modules": [
"Presence"
],
"product": "phoenix",
"versions": [
{
"status": "affected",
"version": "1.2.0-rc.0",
"lessThan": "1.5.15",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.6.0-rc.0",
"lessThan": "1.6.17",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.7.0-rc.0",
"lessThan": "1.7.24",
"versionType": "semver"
},
{
"status": "affected",
"version": "1.8.0-rc.0",
"lessThan": "1.8.9",
"versionType": "semver"
}
],
"packageURL": "pkg:npm/phoenix",
"packageName": "phoenix",
"programFiles": [
"assets/js/phoenix/presence.js"
],
"collectionURL": "https://registry.npmjs.org",
"defaultStatus": "unaffected",
"programRoutines": [
{
"name": "Presence.syncState"
},
{
"name": "Presence.syncDiff"
}
]
},
{
"cpes": [
"cpe:2.3:a:phoenixframework:phoenix:*:*:*:*:*:*:*:*"
],
"repo": "https://github.com/phoenixframework/phoenix",
"vendor": "phoenixframework",
"modules": [
"Presence"
],
"product": "phoenix",
"versions": [
{
"status": "affected",
"changes": [
{
"at": "7f7b971c1ea0994e3fbd1c11ddb05e780bd38ad8",
"status": "unaffected"
},
{
"at": "89a1c4be161e436241e12b2378a719904b9bd96f",
"status": "unaffected"
},
{
"at": "b90b22521465ece00eb5a19d5aa2b9465b209c85",
"status": "unaffected"
},
{
"at": "beffc4da1e787e572121f68902c63daf4fe7d9c2",
"status": "unaffected"
}
],
"version": "2270aaf21bd02c6a6a1022820564efb605a97655",
"lessThan": "*",
"versionType": "git"
}
],
"packageURL": "pkg:github/phoenixframework/phoenix",
"packageName": "phoenixframework/phoenix",
"programFiles": [
"assets/js/phoenix/presence.js"
],
"collectionURL": "https://github.com",
"defaultStatus": "unaffected",
"programRoutines": [
{
"name": "Presence.syncState"
},
{
"name": "Presence.syncDiff"
}
]
}
]- Related CVEs
- CVE-2026-56812
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- Event
- #1936 · Revision 1
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