CVE-2026-59725
ADVISORY - githubSummary
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
Common Weakness Enumeration (CWE)
Improper Resource Shutdown or Release
GitHub
3.9
CVSS SCORE
7.5high| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| engine.io | npm | - | - | >=4.1.0,<6.6.7 | 6.6.7 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is no loss of confidentiality.
There is no loss of trust or accuracy within the impacted component.
There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component.