Summary
websocket-driver-ruby: Denial of service via malformed Host header
Workarounds
No known workarounds exist.
Acknowledgements
This issue was discovered and reported by Pranjali Thakur, DepthFirst Security Research Team.
Impact
If this library is used to implement a WebSocket server on top of a TCP server, by using the WebSocket::Driver.server() method, then a client can cause the server to crash by sending a Host header that is not a valid host[:port] string. When this happens, a URI::InvalidURIError exception is raised which is not caught, and this can cause the server process to crash if the application does not catch the error from the parse() method itself.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
The issue has been patched in version 0.8.2 by making the request parser catch URI::InvalidURIError and enter an error state if the Host header is malformed. This means the request is considered invalid and should not establish a WebSocket connection.
Frequently Asked Questions
- What is CVE-2026-61666? CVE-2026-61666 is a high-severity security vulnerability in websocket-driver (rubygems), affecting versions < 0.8.2. It is fixed in 0.8.2.
- Which versions of websocket-driver are affected by CVE-2026-61666? websocket-driver (rubygems) versions < 0.8.2 is affected.
- Is there a fix for CVE-2026-61666? Yes. CVE-2026-61666 is fixed in 0.8.2. Upgrade to this version or later.
- Is CVE-2026-61666 exploitable, and should I be worried? Whether CVE-2026-61666 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-61666 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-61666? Upgrade
websocket-driverto 0.8.2 or later.