Socket.IO: Engine.IO Protocol Revision Mismatch DoS

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. From Engine.IO 6.6.0 until 6.6.10, a transport upgrade can independently derive an Engine.IO protocol revision from the upgrade request instead of enforcing the revision stored on the existing session. A malicious client can establish a valid session and send an upgrade request with a different or omitted EIO query parameter, attaching a transport with incompatible parser and heartbeat behavior. A crafted heartbeat packet can then trigger an uncaught exception and terminate the Node.js process, including when Engine.IO v3 compatibility is disabled because an omitted EIO value is interpreted as revision 3 on the affected path. This issue is fixed in Engine.IO 6.6.10.

Source published

13 Oct 2026 19:05:10 UTC

Source modified

17 Oct 2026 00:39:46 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. From Engine.IO 6.6.0 until 6.6.10, a transport upgrade can independently derive an Engine.IO protocol revision from the upgrade request instead of enforcing the revision stored on the existing session. A malicious client can establish a valid session and send an upgrade request with a different or omitted EIO query parameter, attaching a transport with incompatible parser and heartbeat behavior. A crafted heartbeat packet can then trigger an uncaught exception and terminate the Node.js process, including when Engine.IO v3 compatibility is disabled because an omitted EIO value is interpreted as revision 3 on the affected path. This issue is fixed in Engine.IO 6.6.10.

Source published

Oct 13, 2026, 7:05:10 PM (timezone not stated)

Source modified

Oct 17, 2026, 12:39:46 AM (timezone not stated)

Observed here

9 Nov 2026 05:03:28 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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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: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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. From Engine.IO 6.6.0 until 6.6.10, a transport upgrade can independently derive an Engine.IO protocol revision from the upgrade request instead of enforcing the revision stored on the existing session. A malicious client can establish a valid session and send an upgrade request with a different or omitted EIO query parameter, attaching a transport with incompatible parser and heartbeat behavior. A crafted heartbeat packet can then trigger an uncaught exception and terminate the Node.js process, including when Engine.IO v3 compatibility is disabled because an omitted EIO value is interpreted as revision 3 on the affected path. This issue is fixed in Engine.IO 6.6.10.

Source published

13 Oct 2026 19:17:49 UTC

Source modified

17 Oct 2026 02:17:08 UTC

Observed here

25 Oct 2026 04:17:26 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

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 A denial-of-service vulnerability exists in Engine.IO / Socket.IO servers that allow transport upgrades. The Engine.IO protocol revision is negotiated during the initial handshake and stored on the session, but a newly-created transport, including a WebSocket upgrade transport, could independently derive a different protocol revision from the upgrade request query parameters. The server did not verify that the protocol revision of an upgrade request matched the protocol revision of the existing session. A malicious client could exploit this mismatch by establishing a valid Engine.IO session and then sending an upgrade request with a different, or omitted, `EIO` query parameter. This could cause the server to attach a transport using a parser and heartbeat behavior inconsistent with the session. Under some conditions, a crafted heartbeat packet could trigger an uncaught exception and terminate the Node.js process. Servers using the default Engine.IO v4 protocol are impacted. The issue can be triggered even when Engine.IO v3 compatibility is disabled, because an omitted `EIO` parameter is interpreted as protocol v3 on the affected transport path. The impact is **denial of service** through process crash. Affected versions: - `engine.io >= 6.6.0, < 6.6.10` ## Patches The issue was fixed in: - **engine.io 6.6.10** Users should upgrade to `[email protected]` or later. If using Socket.IO packages that depend on Engine.IO, users should update to a Socket.IO release that includes the patched Engine.IO version. ### Workarounds If upgrading is not immediately possible, users can reduce exposure by disabling transport upgrades: ```javascript const io = new Server(httpServer, { allowUpgrades: false }); ``` or by allowing only a single transport, for example WebSocket only: ```javascript const io = new Server(httpServer, { transports: ["websocket"] }); ``` These mitigations avoid the vulnerable upgrade path, but they may affect client compatibility and connection behavior. As an additional temporary mitigation, deployments may reject Engine.IO requests for an existing `sid` when the `EIO` query parameter is missing or does not match the protocol revision used during the initial handshake. This is best done at the application proxy or middleware layer only if the deployment can reliably track the session’s negotiated protocol. Upgrading remains the recommended fix.

Source published

29 Sep 2026 23:44:33 UTC

Source modified

30 Sep 2026 00:00:03 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