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Medium, CVSS 6.3

Phoenix: Presence keys colliding with Object.prototype members break existence checks

CVE-2026-56812GHSA-63mc-hw7g-86rr

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (phoenix 1.8.9, 1.7.24, 1.6.17, 1.5.15), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm phoenix

    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
    1.6.17, 1.8.9, 1.7.24, 1.5.15

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.3 · 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 · score source
EPSS
  • CVE-2026-56812: 0.80% estimated exploitation activity in the next 30 days; percentile 56.51 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The Phoenix JavaScript presence client (assets/js/phoenix/presence.js) tests whether a presence already exists using a bare truthiness check (state[key]) rather than an own-property check. Because applications commonly track presences under a client-supplied username or id, the presence key can be attacker-controlled. A user who joins a channel and picks a key that names an Object.prototype member (proto, constructor, toString, hasOwnProperty, and similar) makes the lookup return the inherited Object.prototype object instead of undefined, which is truthy. The code then reads .metas.map(...) off it and throws an uncaught TypeError, breaking presence sync for every viewer of that channel topic. Any authenticated channel participant can trigger it. Details The victim is any browser subscribed to a presence channel. When it receives the server's presencestate message, it invokes Presence.syncState, which iterates the incoming presences and checks whether each one already exists…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-63mc-hw7g-86rr

Agent brief

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# Patch Next advisory brief: CVE-2026-56812

Advisory: Phoenix: Presence keys colliding with `Object.prototype` members break existence checks
IDs: CVE-2026-56812, GHSA-63mc-hw7g-86rr
Record: https://patchnext.com/advisories/cve-2026-56812 (revision 1, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: phoenix 1.8.9, 1.7.24, 1.6.17, 1.5.15. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm phoenix: 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 1.6.17, 1.8.9, 1.7.24, 1.5.15

## Suggested inspection (read-only)
- `npm ls phoenix --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on phoenix.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-63mc-hw7g-86rr (GitHub-reviewed advisory GHSA-63mc-hw7g-86rr lists affected ranges: npm phoenix >=1.2.0-rc.0 =1.6.0-rc.0 =1.7.0-rc.0 =1.8.0-rc.0 <1.8.9)

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