CVE-2026-55517
MediumCVSS 4.3Exploitation Probability (EPSS)
Low risk8th percentile - higher than 8% of all known CVEs
Summary
A vulnerability in Deno before version 2.7.5 allows a remote WebSocket server to crash the Deno process by sending a response with Sec-WebSocket-Protocol or Sec-WebSocket-Extensions headers containing non-printable ASCII bytes (0x80-0xFF). The flaw is due to improper parsing of these headers, causing a panic that terminates the entire process.
Risk Assessment
A malicious WebSocket server can cause a denial of service (DoS) by crashing the Deno application, potentially disrupting critical services.
Recommendation
Upgrade Deno to version 2.7.5 or later immediately, which includes a fix for this vulnerability.
Other vulnerabilities in Deno
See all- CVE-2026-22864High
Deno is a JavaScript, TypeScript, and WebAssembly runtime. Before 2.5.6, a prior patch aimed to block spawning Windows batch/shell files by returning an error when a spawned path’s extension matched .bat or .cmd. That check performs a case-sensitive comparison against lowercase literals and therefore can be bypassed when the extension uses alternate casing (for example .BAT, .Bat, etc.). This vulnerability is fixed in 2.5.6.
- CVE-2026-22863High
Deno is a JavaScript, TypeScript, and WebAssembly runtime. Before 2.6.0, node:crypto doesn't finalize cipher. The vulnerability allows an attacker to have infinite encryptions. This can lead to naive attempts at brute forcing, as well as more refined attacks with the goal to learn the server secrets. This vulnerability is fixed in 2.6.0.
- CVE-2026-49983Medium
In Deno prior to 2.8.1, the process.loadEnvFile() function does not honor environment permission restrictions (--deny-env). It allows writing variables from a .env file into the process even when environment access is denied.
- CVE-2026-49860Medium
In Deno prior to 2.8.1, when a WebSocket connection was opened, only the destination hostname was checked against --deny-net rules, but the IP addresses that hostname resolved to were not re-checked. An attacker-controlled script could use a specially crafted domain name that passes the hostname check yet resolves to a denied IP, bypassing the network restriction entirely.
- CVE-2026-49859Medium
In Deno prior to version 2.8.1, the fetch() function checked the destination hostname against --deny-net rules but did not re-check the resolved IP addresses. An attacker-controlled script could use a specially crafted domain name that passes the hostname check yet resolves to a denied IP, bypassing network restrictions entirely.
Original NVD description (English source)
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.