CVE-2026-56812
Fix: phoenixframework/phoenix@7f7b971CVE-2026-56812 is a CWE-754 vulnerability in phoenix. O3 Security confirms whether CVE-2026-56812 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-56812.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
phoenix💧phoenix💧phoenix💧phoenix📦phoenix📦phoenix📦phoenix📦phoenixReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Hex packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💧Hex | phoenix | ≥ 1.2.0-rc.0&&< 1.5.15 | 1.5.15 |
| 💧Hex | phoenix | ≥ 1.6.0-rc.0&&< 1.6.17 | 1.6.17 |
| 💧Hex | phoenix | ≥ 1.7.0-rc.0&&< 1.7.24 | 1.7.24 |
| 💧Hex | phoenix | ≥ 1.8.0-rc.0&&< 1.8.9 | 1.8.9 |
| 📦npm | phoenix | ≥ 1.2.0-rc.0&&< 1.5.15 | 1.5.15 |
| 📦npm | phoenix | ≥ 1.6.0-rc.0&&< 1.6.17 | 1.6.17 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phoenix. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update phoenix to 1.5.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-56812 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-56812 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-56812. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-56812 in your dependencies?
O3 detects CVE-2026-56812 across Hex, npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.