CVE-2026-59725 is a high-severity (CVSS 7.5) CWE-404 vulnerability in engine.io. A fix is available for engine.io — see the affected versions and patch details below.
Socket.IO: Engine.IO Polling Transport Connection Exhaustion
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-59725.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-59725 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
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 | 6.6.7npm install engine.io@6.6.7 |
Affected Products
engine.iosocketDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for engine.io, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update engine.io to 6.6.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59725 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw has been rated as Important. An unauthenticated remote attacker can exploit a vulnerability in the Engine.IO protocol v4 polling transport of Socket.IO by sending malformed binary POST requests. This can lead to server-side connection exhaustion and a denial of service, impacting the availability of…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Hardened Images | dotnet8-0-main-8.0.128-1.1.hum1 | RHSA-2026:37577 |
| Red Hat Hardened Images | dotnet8-0-main-8.0.129-2.hum1 | RHSA-2026:42986 |
Frequently Asked Questions
Is CVE-2026-59725 in your dependencies?
Find it across npm, including transitive dependencies.