CVE-2026-33151: Socket.IO denial of service via unbounded binary attachments

CVE-2026-33151 · Severity: medium · CVSS 4 · Published 2026-03-18

Socket.IO is a library that enables real-time bidirectional communication between web clients and servers. A vulnerability in its packet parser allows an attacker to send a specially crafted message with an unbounded number of binary attachments, causing the server to buffer them indefinitely and exhaust available memory. This can render the application unavailable and disrupt services for legitimate users.

Vendors: Socket.IO.

Quick answers

Is CVE-2026-33151 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-33151 being exploited in the wild.
How severe is CVE-2026-33151?
Medium severity, with a CVSS score of 4, published 18 March 2026.
Which products and versions are affected?
Socket.Io-Client 4.x <4.2.6, 2.x <3.3.5.
Is there a fix for CVE-2026-33151?
A patch was published on 2026-03-18: Patches released: socket.io-parser 3.3.5, 3.4.4, 4.2.6 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-33151 attacked?
It is attacked over the network.
Am I affected by CVE-2026-33151?
Check whether you run Socket.Io-Client at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is an improper input validation issue (CWE-20, CWE-754) in socket.io-parser's handling of binary attachments in Socket.IO protocol messages. A crafted packet can declare a very large number of binary attachments without actually sending them, forcing the server to wait and buffer them indefinitely. An unauthenticated attacker on the network can exploit this by sending malicious packets to any socket.io server, leading to memory exhaustion and denial of service. Patches are available in socket.io-parser versions 3.3.5, 3.4.4, and 4.2.6, with corresponding fixes in socket.io itself.

Affected products

  • Socket.IO socket.io-parser <3.3.5, >=3.4.0 <3.4.4, >=4.0.0 <4.2.6
  • Socket.IO socket.io 4.x <4.2.6, 2.x <3.4.4 (via socket.io-parser)
  • Socket.IO socket.io-client 4.x <4.2.6, 2.x <3.3.5

Identifiers

  • CVE-2026-33151
  • GHSA-677m-j7p3-52f9

Timeline

  • 2026-03-18: disclosed: Advisory published on GitHub and OSV
  • 2026-03-18: patched: Patches released: socket.io-parser 3.3.5, 3.4.4, 4.2.6

References

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