GHSA-r635-g3xr-vw7x is a high-severity (CVSS 7.5) CWE-404 vulnerability in engine.io. O3 Security confirms whether GHSA-r635-g3xr-vw7x is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Socket.IO: Engine.IO Polling Transport Connection Exhaustion
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.
engine.ionpmDescription
Impact
An unauthenticated remote attacker can cause a denial of service in affected versions of engine.io by opening Engine.IO polling sessions and sending an invalid binary POST request with:
Content-Type: application/octet-stream
against an Engine.IO protocol v4 polling transport.
In the vulnerable code path, the server reports a transport error but does not properly close the HTTP response associated with the malformed request. As a result, the underlying HTTP connection may remain open, consuming one server-side socket/resource per crafted request.
An attacker can repeat this with many sessions to exhaust available HTTP connections, sockets, file descriptors, or related server resources, potentially preventing legitimate clients from connecting.
Patches
The issue was fixed in:
- engine.io
6.6.7
The fix ensures that invalid binary polling POST requests are explicitly rejected with an HTTP response and closed properly.
Users should upgrade to:
npm install engine.io@^6.6.7
or a later fixed version.
If using Socket.IO through the monorepo/packages, update to a Socket.IO release that depends on a fixed engine.io version.
Workarounds
If upgrading immediately is not possible, possible mitigations include:
- Block or reject polling
POSTrequests withContent-Type: application/octet-streamfor Engine.IO protocol v4 at a reverse proxy, load balancer, WAF, or application middleware. - Disable HTTP long-polling if your deployment can use WebSocket-only transport.
- Enforce strict request/connection timeouts at the HTTP server, reverse proxy, or load balancer.
- Apply per-IP rate limits and connection limits for Engine.IO endpoints.
- Restrict access to the Socket.IO/Engine.IO endpoint where feasible.
Example Socket.IO configuration to disable polling, if compatible with your clients:
const io = new Server(httpServer, {
transports: ["websocket"],
});
References
- Fix commit: https://github.com/socketio/socket.io/commit/fc11285e14964c2132d122164bf130c355f60671
- engine.io changelog entry for
6.6.7: https://github.com/socketio/socket.io/blob/main/packages/engine.io/CHANGELOG.md#667-2026-04-27 - socket.io repository: https://github.com/socketio/socket.io
- engine.io package: https://www.npmjs.com/package/engine.io
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | engine.io | ≥ 4.1.0&&< 6.6.7 | 6.6.7 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for engine.io. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update engine.io to 6.6.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-r635-g3xr-vw7x is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-r635-g3xr-vw7x is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-r635-g3xr-vw7x. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-r635-g3xr-vw7x in your dependencies?
O3 detects GHSA-r635-g3xr-vw7x across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.