---
id: CVE-2026-49859
title: 'Deno: `fetch()` API sandbox bypass via missing DNS resolution check'
summary: 'Deno: `fetch()` API sandbox bypass via missing DNS resolution check'
severity: medium
cvss: 5.2
cwe:
  - CWE-693
  - CWE-918
vendor: deno
product: deno
ecosystem: rust
affected:
  - deno <= 2.8.0
patched:
  - deno 2.8.1
published: '2026-06-16'
updated: '2026-06-16'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-cpgj-f7g3-2pp2'
references:
  - url: 'https://github.com/denoland/deno/security/advisories/GHSA-cpgj-f7g3-2pp2'
  - url: 'https://github.com/advisories/GHSA-cpgj-f7g3-2pp2'
tags:
  - ghsa
  - rust
epss: 0.00141
epssPercentile: 0.02779
ingestedAt: '2026-06-29T14:31:47.715Z'
---

## Overview

## 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 immediately is not
possible, avoid granting `--allow-net` to untrusted code that also has
`--deny-net` restrictions you depend on for security.

## Fix

The `fetch()` DNS resolver now performs a post-resolution check on every IP
address before passing it to the HTTP connector, consistent with how
`Deno.connect` already behaved.

## Affected packages

- `deno <= 2.8.0`

## Remediation

Upgrade to a patched release:

- `deno 2.8.1`