CVE-2026-59724 Socket.IO: Engine.IO WebTransport SID DoS
Attackers can cause a denial of service in Socket.IO servers by sending crafted session IDs during WebTransport upgrades, requiring WebTransport to be enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59724?
CVE-2026-59724 is a high-severity denial of service vulnerability in socketio socket.io, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Socket.IO enables bidirectional and low-latency communication for every platform. From 6.5.0 before 6.6.7, Engine.IO servers with WebTransport enabled can resolve a crafted session ID such as __proto__ through an inherited property of the clients object during WebTransport upgrade handling, causing a TypeError and denial of service. This issue is fixed in version 6.6.7.
Weakness: CWE-20
Which versions are affected by CVE-2026-59724?
2 open source packages ship an affected version, including engine.io. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
engine.io npm | From 6.5.0 | 6.6.7 |
engine.io npm | From 6.5.0 | 6.6.7 |
Products include Socket Engine.io.
How do I fix CVE-2026-59724?
Upgrade engine.io to 6.6.7 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59724 being exploited?
There is no known exploitation of CVE-2026-59724 in the wild and no public exploit code.
Other vulnerabilities in socketio socket.io
Sources
Also known as CLSA-2026-1792193356, GHSA-gr94-w7qr-f4j3, MINI-c3rp-5fq7-xqx8, MINI-c667-m673-hpvc, ROOT-APP-NPM-CVE-2026-59724.