Medium severity · published Jun 23, 2026
CVE-2026-55517
Deno: Denial of service via non-ASCII bytes in WebSocket response headers
4.3
CVSS
0.3%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Confidentiality
None
Integrity
None
Availability
Low
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 24% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
DE
deno · denoland
< 2.7.5 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-55517
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-55517
Mitigation
https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf
More from deno
9.8
CVE-2026-32260
Command Injection via incomplete shell metacharacter blocklist in node:child_process (bypass of CVE-2026-27190 fix)
9.8
CVE-2026-27190
Deno has a Command Injection via Incomplete shell metacharacter blocklist in node:child_process
9.8
CVE-2026-22864
Deno has an incomplete fix for command-injection prevention on Windows — case-insensitive extension bypass
9.8
CVE-2023-33966
Deno missing "--allow-net" permission check for built-in Node modules
9.8
CVE-2023-28445
Deno improperly handles resizable ArrayBuffer