Socket.IO: Prototype Pollution via Unsafe Client Session Lookup

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.

Source published

29 Sep 2026 15:23:15 UTC

Source modified

29 Sep 2026 16:28:28 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.

Source published

Sep 29, 2026, 3:23:15 PM (timezone not stated)

Source modified

Sep 29, 2026, 4:28:28 PM (timezone not stated)

Observed here

11 Nov 2026 05:29:12 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.

Source published

29 Sep 2026 16:17:06 UTC

Source modified

30 Sep 2026 19:38:27 UTC

Observed here

25 Oct 2026 04:17:26 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### Impact This is a **prototype pollution / improper client lookup** vulnerability in `@socket.io/cluster-engine`. Servers using `@socket.io/cluster-engine` may be impacted when attacker-controlled session IDs are processed in clustered deployments. A malicious client could use special property names such as `__proto__`, `constructor`, or other inherited object keys as a session identifier, causing the server to read properties from the object prototype chain instead of only real connected clients. The impact is **denial of service** through process crash. Applications not using `@socket.io/cluster-engine` are not affected by this specific issue. Affected versions: * `@socket.io/[email protected]` ### Patches The issue was fixed in: * `@socket.io/[email protected]` ### Workarounds If upgrading immediately is not possible, users can reduce exposure by: - Rejecting or sanitizing suspicious session IDs before they reach the cluster engine. - Running the cluster engine behind trusted infrastructure that prevents arbitrary clients from crafting raw Engine.IO session-related requests. These workarounds are defense-in-depth only. Upgrading to a patched version is recommended.

Source published

5 Oct 2026 23:44:21 UTC

Source modified

6 Oct 2026 00:00:08 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH