Reference page — cumulative record through Wednesday, August 19, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.
phoenixframework phoenix — Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff
AV AC AT PR UI VC VI VA CVSS EPSS %ile KEV
N L P N N N N L 6.3 .0051 41.2 —
AFFECTED
Product Versions Fixed
phoenix 1.2.0-rc.0 – —
phoenix 1.2.0-rc.0 – —
phoenix 2270aaf21bd02c6a6a1022820564efb605a97655 – 7f7b971c1ea0994e3fbd1c11ddb05e780bd38ad8
TIMELINE
Jun 23 Reserved by EEF
Jul 7 EXPLOIT PUBLISHED — CVE-2026-56812 (phoenixframework phoenix). Public exploit reference added.
Jul 7 Published (CNA: EEF)
Description
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.
Lifecycle
Complete event history — 3 events, chronological
| Date | Event | Detail |
|---|
| June 23, 2026 | Reserved | Reserved by EEF |
| July 7, 2026 | EXPLOIT PUBLISHED | EXPLOIT PUBLISHED — CVE-2026-56812 (phoenixframework phoenix). Public exploit reference added. |
| July 7, 2026 | Published | Published (CNA: EEF) |
Affected
Affected products and packages — 3 rows
| Vendor | Product / Package | Ecosystem | Version introduced | Fixed |
|---|
| phoenixframework | phoenix | — | 1.2.0-rc.0 | — |
| phoenixframework | phoenix | npm | 1.2.0-rc.0 | — |
| phoenixframework | phoenix | — | 2270aaf21bd02c6a6a1022820564efb605a97655 | 7f7b971c1ea0994e3fbd1c11ddb05e780bd38ad8 |
About this page
This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2026-56812 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Wednesday, August 19, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.