CVE-2026-55517CWE-248

Deno: Denial of service via non-ASCII bytes in WebSocket response headers

Medium · published June 23, 2026

CVSS v3.1
4.3
EPSS
0%
Percentile
23.2
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v3.1
4.3 · MEDIUM
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
CVSS v4.0
Not supplied
EPSS
0.00315 · 23.2th percentile
Weakness
CWE-248 · Uncaught Exception
Published
2026-06-23T17:24Z
EPSS history
Timeline
  • 23 JUN 17:24Z
    Deno: Denial of service via non-ASCII bytes in WebSocket response headers
    cvelistv5