Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff
Summary
| CVE | CVE-2026-56812 |
|---|---|
| State | PUBLISHED |
| Assigner | EEF |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-07-07 16:16:40 UTC |
| Updated | 2026-07-07 17:16:36 UTC |
| 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. |
Risk And Classification
Primary CVSS: v4.0 6.3 MEDIUM from 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Problem Types: CWE-754 | CWE-754 CWE-754 Improper Check for Unusual or Exceptional Conditions
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 4.0 | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | Secondary | 6.3 | MEDIUM | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/C... |
| 4.0 | CNA | CVSS | 6.3 | MEDIUM | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N |
CVSS v4.0 Breakdown
Attack Vector
NetworkAttack Complexity
LowAttack Requirements
PresentPrivileges Required
NoneUser Interaction
NoneConfidentiality
NoneIntegrity
NoneAvailability
LowSub Conf.
NoneSub Integrity
NoneSub Availability
NoneCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | Phoenixframework | Phoenix | affected 1.2.0-rc.0 1.5.15 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.6.0-rc.0 1.6.17 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.7.0-rc.0 1.7.24 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.8.0-rc.0 1.8.9 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.2.0-rc.0 1.5.15 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.6.0-rc.0 1.6.17 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.7.0-rc.0 1.7.24 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 1.8.0-rc.0 1.8.9 semver | Not specified |
| CNA | Phoenixframework | Phoenix | affected 2270aaf21bd02c6a6a1022820564efb605a97655 * git | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| github.com/phoenixframework/phoenix/commit/89a1c4be161e436241e12b2378a71... | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | github.com | |
| cna.erlef.org/cves/CVE-2026-56812.html | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | cna.erlef.org | |
| github.com/phoenixframework/phoenix/security/advisories/GHSA-63mc-hw7g-86rr | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | github.com | |
| github.com/phoenixframework/phoenix/commit/b90b22521465ece00eb5a19d5aa2b... | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | github.com | |
| github.com/phoenixframework/phoenix/commit/7f7b971c1ea0994e3fbd1c11ddb05... | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | github.com | |
| github.com/phoenixframework/phoenix/commit/beffc4da1e787e572121f68902c63... | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | github.com | |
| osv.dev/vulnerability/EEF-CVE-2026-56812 | 6b3ad84c-e1a6-4bf7-a703-f496b71e49db | osv.dev | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: Peter Ullrich (en)
CNA: Jonatan Männchen (en)
CNA: José Valim (en)
CNA: Steffen Deusch (en)
Additional Advisory Data
Workarounds
CNA: Reject or sanitize presence keys that collide with JavaScript Object.prototype member names (for example __proto__, constructor, prototype, toString, hasOwnProperty) before calling Phoenix.Presence.track, or namespace every presence key with a fixed prefix so that no key can equal a prototype member name. Alternatively, derive presence keys from server-controlled, validated values instead of raw user input.
There are currently no legacy QID mappings associated with this CVE.