Deno is a JavaScript, TypeScript, and WebAssembly runtime. Prior to 2.8.1, 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. This vulnerability is fixed in 2.8.1.

Affected: crates.io: deno

## 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.