CVE-2026-49859: Deno: `fetch()` API sandbox bypass via missing DNS resolution check

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.