What’s Vulnerable › Vulnerabilities
CVE-2026-55517: Deno: Denial of service via non-ASCII bytes in WebSocket response headers
- Severity: medium (CVSS 4.3 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.3% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-06-23 · updated 2026-10-25
- Weakness: CWE-248
Description
Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.7.5, a Deno program that opens a client WebSocket connection could be crashed by the remote server. While handling the WebSocket handshake response, Deno parsed the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions response headers in a way that assumed their bytes were always printable ASCII. A response header containing non-visible-ASCII bytes (0x80-0xFF) caused a panic that aborted the entire Deno process. This vulnerability is fixed in 2.7.5.
Affected products
- denoland deno: before 2.7.5 — fixed in 2.7.5
How to fix
Upgrade to a fixed version: denoland deno 2.7.5. Check the vendor advisory for mitigations.
References
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