# Phoenix JavaScript presence client crashes on presence keys colliding with Object.prototype members in Presence.syncState/syncDiff

Advisory record · `GHSA-63mc-hw7g-86rr`

- **Identifiers:** `GHSA-63mc-hw7g-86rr` · `CVE-2026-56812` · `EEF-CVE-2026-56812`
- **State:** Confirmed
- **Severity:** CVSS 6.3 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-754](https://cwe.mitre.org/data/definitions/754.html)
- **Tags:** [CWE-754](https://agentprime.io/advisories/tag/cwe-754.md)
- **Published:** 7 Jul 2026
- **Updated:** 24 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 7 Jul 2026, 15:22 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `phoenix` (Hex): affected `>=1.2.0-rc.0, <1.5.15 || >=1.6.0-rc.0, <1.6.17 || >=1.7.0-rc.0, <1.7.24 || >=1.8.0-rc.0, <1.8.9`; fixed in `1.5.15`, `1.6.17`, `1.7.24`, `1.8.9`
- `phoenix` (npm): affected `>=1.2.0-rc.0, <1.5.15 || >=1.6.0-rc.0, <1.6.17 || >=1.7.0-rc.0, <1.7.24 || >=1.8.0-rc.0, <1.8.9`; fixed in `1.5.15`, `1.6.17`, `1.7.24`, `1.8.9`

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.3 Medium, CVSS 4.0: `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`, from CNA (EEF)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0080, the 56th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

This record came from the full history of the GitHub Advisory Database, published 7 Jul 2026. New records show when each source confirmed them.

## Sources

- [GitHub Advisory Database: GHSA-63mc-hw7g-86rr](https://github.com/advisories/GHSA-63mc-hw7g-86rr)
- [OSV: GHSA-63mc-hw7g-86rr](https://osv.dev/vulnerability/GHSA-63mc-hw7g-86rr)
- <https://github.com/phoenixframework/phoenix/security/advisories/GHSA-63mc-hw7g-86rr>
- <https://cna.erlef.org/cves/CVE-2026-56812.html>
- <https://github.com/phoenixframework/phoenix/commit/2270aaf21bd02c6a6a1022820564efb605a97655>
- <https://github.com/phoenixframework/phoenix/commit/7f7b971c1ea0994e3fbd1c11ddb05e780bd38ad8>
- <https://github.com/phoenixframework/phoenix/commit/89a1c4be161e436241e12b2378a719904b9bd96f>
- <https://github.com/phoenixframework/phoenix/commit/b90b22521465ece00eb5a19d5aa2b9465b209c85>
- <https://github.com/phoenixframework/phoenix/commit/beffc4da1e787e572121f68902c63daf4fe7d9c2>
- <https://hex.pm/packages/phoenix>
- <https://www.npmjs.com/package/phoenix>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-63mc-hw7g-86rr>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-63mc-hw7g-86rr/>. Everything for agents: <https://agentprime.io/llms.txt>.