GHSA-CPGJ-F7G3-2PP2

Vulnerability from github – Published: 2026-06-16 19:02 – Updated: 2026-06-16 19:02
VLAI
Summary
Deno: `fetch()` API sandbox bypass via missing DNS resolution check
Details

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.

Severity
5.2 (Medium)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.8.0"
      },
      "package": {
        "ecosystem": "crates.io",
        "name": "deno"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.8.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-49859"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-693",
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-16T19:02:55Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nWhen `fetch()` was called, Deno checked the destination hostname against\n`--deny-net` rules but did not re-check the IP addresses that hostname\nresolved to. An attacker-controlled script could use a specially crafted domain\nname that passes the hostname check yet resolves to a denied IP, bypassing the\nnetwork restriction entirely.\n\n## Impact\n\nCode running under `--deny-net` could reach hosts that the user intended to\nblock. In practice this means network isolation rules \u2014 for example, blocking\naccess to `localhost` or internal services \u2014 could be silently circumvented by\na malicious or compromised dependency.\n\nA companion advisory covers the same class of issue in the WebSocket API.\n\n## Who is affected\n\nUsers who:\n\n- run untrusted or third-party code with `deno run`, and\n- rely on `--deny-net` to restrict which hosts that code can reach.\n\nIf you do not use `--deny-net`, or if you only run fully trusted code, you are\nnot affected.\n\n## Workaround\n\nNo workaround is available short of upgrading. If upgrading immediately is not\npossible, avoid granting `--allow-net` to untrusted code that also has\n`--deny-net` restrictions you depend on for security.\n\n## Fix\n\nThe `fetch()` DNS resolver now performs a post-resolution check on every IP\naddress before passing it to the HTTP connector, consistent with how\n`Deno.connect` already behaved.",
  "id": "GHSA-cpgj-f7g3-2pp2",
  "modified": "2026-06-16T19:02:55Z",
  "published": "2026-06-16T19:02:55Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/denoland/deno/security/advisories/GHSA-cpgj-f7g3-2pp2"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/denoland/deno"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Deno: `fetch()` API sandbox bypass via missing DNS resolution check"
}


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