Vulnerability in phoenix (CVE-2026-56812, CVSS 7.5)

Summary

Vulnerability in phoenix (CVE-2026-56812). Risk of unauthorized operations or information disclosure. Exploitable via Object.prototype. Mitigation: upgrade to 1.8.9 or later.

Original description

The description as published by the source.

Summary

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 sam…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-56812 has been found in phoenix.
Attackers can target a specific entry point like Object.prototype over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade phoenix to 1.8.9 or later.
If unsure, ask your IT team or search "phoenix CVE-2026-56812" on the vendor's site.

CVE-2026-56812 (phoenix) — CWE-754 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Object.prototype / __proto__ / constructor / toString
Patched: 1.8.9 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-754) exists in phoenix. Attackers reach the vulnerable code path via Object.prototype without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of phoenix. Attack surface: Object.prototype / __proto__ / constructor / toString.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 1.8.9.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'phoenix' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

1
Identify exposure identify
```
grep -r 'phoenix' . | grep -v node_modules
```
リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で phoenix を grep し、稼働しているサービス・バージョンを把握する。
2
Apply patch patch
```
Upgrade phoenix to 1.8.9
```
ステージング環境で 1.8.9 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
3
Post-deployment verification verify
```
Confirm patched version is live in production
```
パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

erlang phoenix

[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.8.9"}]}]

npm phoenix

[{"type":"SEMVER","events":[{"introduced":"1.2.0-rc.0"},{"fixed":"1.5.15"}]},{"type":"SEMVER","events":[{"introduced":"1.6.0-rc.0"},{"fixed":"1.6.17"}]},{"type":"SEMVER","events":[{"introduced":"1.7.0-rc.0"},{"fixed":"1.7.24"}]},{"type":"SEMVER","events":[{"introduced":"1.8.0-rc.0"},{"fixed":"1.8.9"}]}]

Hex phoenix

[{"type":"SEMVER","events":[{"introduced":"1.2.0-rc.0"},{"fixed":"1.5.15"}]},{"type":"SEMVER","events":[{"introduced":"1.6.0-rc.0"},{"fixed":"1.6.17"}]},{"type":"SEMVER","events":[{"introduced":"1.7.0-rc.0"},{"fixed":"1.7.24"}]},{"type":"SEMVER","events":[{"introduced":"1.8.0-rc.0"},{"fixed":"1.8.9"}]}]

References

advisory github.com
advisory nvd.nist.gov
package github.com
package hex.pm
package npmjs.com
patch github.com
patch github.com
patch github.com
patch github.com
web cna.erlef.org
web osv.dev
web github.com
web github.com