Deno: fetch() API sandbox bypass via missing DNS resolution check

Exploitation

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

CVSS

5.2 · CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-49859: 0.14% estimated exploitation activity in the next 30 days; percentile 3.44 (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 When fetch() was called, Deno checked the destination hostname against --deny-net rules but did not re-check the IP addresses that hostname resolved to. 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. Impact Code running under --deny-net could reach hosts that the user intended to block. In practice this means network isolation rules — for example, blocking access to localhost or internal services — could be silently circumvented by a malicious or compromised dependency. A companion advisory covers the same class of issue in the WebSocket API. Who is affected Users who: - run untrusted or third-party code with deno run, and - rely on --deny-net to restrict which hosts that code can reach. If you do not use --deny-net, or if you only run fully trusted code, you are not affected. Workaround No workaround is available short of upgrading. If upgrading…

Published by the source

16 Jun 2026, 19:02 UTC

Last modified by the source

20 Jul 2026, 21:01 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-cpgj-f7g3-2pp2

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# Patch Next advisory brief: CVE-2026-49859

Advisory: Deno: `fetch()` API sandbox bypass via missing DNS resolution check
IDs: CVE-2026-49859, GHSA-cpgj-f7g3-2pp2
Record: https://patchnext.com/advisories/cve-2026-49859 (revision 1, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: deno 2.8.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io deno: affected <= 2.8.0; fixed in 2.8.1

## 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-cpgj-f7g3-2pp2 (GitHub-reviewed advisory GHSA-cpgj-f7g3-2pp2 lists affected ranges: crates.io deno >=0 <=2.8.0)

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