CVE-2026-55517

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

MEDIUM

4.3

CVSS Score

Published: June 23, 2026 at 05:24 PM UTC

Updated: October 25, 2026 at 11:10 PM UTC

2 products affected across 2 vendorsDenoDeno

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Severity

MEDIUM4.3

Exploit prediction

EPSS 0.3%

SSVC

No known exploit

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

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Deno · Denocargo

95%

v < 2.7.5

Affected range stops below 2.7.5, so it looks safe — our inference, not a vendor statement

Deno · Denoland

90%

v < 2.7.5

Affected range stops below 2.7.5, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf

References (1)

x_refsource_CONFIRMMitigationVendor Advisory

Source: cverecord

https://github.com/denoland/deno/security/advisories/GHSA-x2qc-cmh9-f4hf

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

4.3MEDIUMGitHub_M

Attack Vector: NETWORK
Complexity: LOW
Privileges: NONE
User Interaction: REQUIRED
Scope: UNCHANGED
Confidentiality: NONE
Integrity: NONE
Availability: LOW

Vector?

Quick Info

StatusANALYZED

CWE

CWE-248

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