CVE-2026-102600 in Socket.io
Summary
by MITRE • 09/29/2026
Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability identified in versions of @socket.io/cluster-engine prior to 0.1.1 stems from a fundamental flaw in how the library handles property lookups on objects within clustered Node.js deployments. Socket.IO facilitates bidirectional, low-latency communication across various platforms, and when operating in a cluster mode, it relies on this engine to manage session state distribution among multiple worker processes. The core technical issue arises because the code utilizes standard JavaScript object property access methods that do not strictly distinguish between an objects own properties and those inherited from its prototype chain. This behavior is inherent to how native JavaScript objects function by default, where accessing a key like _proto_ or constructor triggers traversal up the prototype hierarchy rather than checking for a direct match on the instance itself.
In a clustered environment, session IDs are used as keys to identify specific connected clients and route messages appropriately. When an attacker controls the input for these session identifiers, they can exploit this inheritance behavior by supplying special property names such as _proto_, constructor, or hasOwnProperty. Instead of failing to find a matching client connection because no such ID exists in the cluster state map, the lookup mechanism resolves through the object prototype chain. This incorrect resolution path leads to unexpected data retrieval or null pointer dereferences within the internal logic that manages these sessions. The immediate operational impact is severe instability, as this misdirection causes the Node.js process handling the request to crash unexpectedly.
This flaw results in a denial of service condition for applications relying on socket.io/cluster-engine. Since the worker processes terminate abnormally due to unhandled exceptions or memory corruption resulting from the prototype chain traversal error, active connections are dropped and new connection attempts may fail until the cluster is restarted or recovered. It is critical to note that this vulnerability only affects deployments explicitly utilizing the socket.io/cluster-engine package for managing state across multiple workers. Applications using Socket.IO in a single-process mode or with alternative clustering strategies remain unaffected by this specific implementation flaw, although they should still adhere to general input validation best practices.
From a classification perspective, this issue aligns with CWE-470, which describes use of externally-controlled reference without proper verification, specifically manifesting as prototype pollution-like behavior in property lookups. In the context of the MITRE ATT&CK framework, this vulnerability facilitates availability impact through process termination, mapping to techniques that exploit software vulnerabilities for denial of service rather than direct data exfiltration or privilege escalation. The attack vector is remote if the Socket.IO server accepts connections from untrusted sources and allows them to specify session identifiers in a manner that reaches the vulnerable lookup logic.
To mitigate this vulnerability, organizations must upgrade @socket.io/cluster-engine to version 0.1.1 or later, where the developers have implemented safeguards to ensure property lookups are restricted to own properties only. This typically involves using methods like Object.prototype.hasOwnProperty.call() or utilizing Map objects which do not suffer from prototype chain inheritance issues in key lookup operations. For environments that cannot immediately upgrade, implementing strict input validation on session IDs can provide a partial defense by rejecting inputs containing known dangerous prototype keys such as _proto_, constructor, and isPrototypeOf before they reach the vulnerable code path. However, relying solely on input filtering is less robust than patching the underlying library logic to enforce correct object property access semantics.