CVE-2026-49859
Deno: `fetch()` API sandbox bypass via missing DNS resolution check
Description
## 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.
How to fix CVE-2026-49859
To remediate CVE-2026-49859, upgrade the affected package to a fixed version below.
- —upgrade to 2.8.1 or later
Is CVE-2026-49859 being exploited?
Low — EPSS is 0.1%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 2.8.1