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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-55517: 0.32% estimated exploitation activity in the next 30 days; percentile 24.49 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Details When establishing a client WebSocket connection, Deno read the Sec-WebSocket-Protocol and Sec-WebSocket-Extensions headers from the server's 101 Switching Protocols response and converted them to strings without handling the failure case. HeaderValue::tostr() returns an error for any value containing bytes outside the visible-ASCII range, so a header carrying such bytes triggered an unrecoverable error during conversion. Because the client initiates the outbound connection, the handshake response is fully controlled by the server. A server that returns bytes such as…

Published by the source

17 Jun 2026, 18:48 UTC

Last modified by the source

17 Jun 2026, 18:48 UTC

Source last checked

26 Oct 2026, 15:42 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-x2qc-cmh9-f4hf

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-55517

Advisory: Deno: Denial of service via non-ASCII bytes in WebSocket response headers
IDs: CVE-2026-55517, GHSA-x2qc-cmh9-f4hf
Record: https://patchnext.com/advisories/cve-2026-55517 (revision 1, source checked 26 Oct 2026, 15:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: deno 2.7.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io deno: affected <= 2.7.4; fixed in 2.7.5

## Suggested inspection (read-only)
- crates.io deno: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on deno.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-x2qc-cmh9-f4hf (GitHub-reviewed advisory GHSA-x2qc-cmh9-f4hf lists affected ranges: crates.io deno >=0 <=2.7.4)

Commands are suggestions: review each one before running it.