CVE-2026-49859: Deno `fetch()` API Sandbox Bypass Vulnerability

MODERATE
Advisory Identifier: GHSA-cpgj-f7g3-2pp2
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CVSS 5.2
CWE-693 CWE-918
Published
Advisory Summary

CVE-2026-49859 is a MODERATE (CVSS 5.2) `fetch()` api sandbox bypass vulnerability (CWE-693: Protection Mechanism Failure) affecting Deno libraries in the crates.io ecosystem. The issue involves `fetch()` api sandbox bypass via missing dns resolution check. Remediation is available by upgrading affected packages to version 2.8.1.

CVE-2026-49859 at a Glance

CVE Identifier CVE-2026-49859
GHSA Identifier GHSA-cpgj-f7g3-2pp2
Severity MODERATE
CVSS v3.1 Base Score 5.2
CWE Classification CWE-693: Protection Mechanism Failure
Affected Product Deno
Package Ecosystem crates.io
Published Date
Last Updated

Security Impact

CVSS: 5.2

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N).

Attack Vector Local (AV:L)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-49859

To remediate this vulnerability, upgrade all affected Deno dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

deno fixed: 2.8.1 
Upgrade deno to 2.8.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for deno

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-49859, inspect your project dependencies for Deno:

inspect dependencies for deno

Analyze the terminal output to locate any version of Deno in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.8.1), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-693: Protection Mechanism Failure, where the product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. In Deno, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## 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.

CVE-2026-49859 Affected Deno Versions

1 component(s)
Fixed Versions:
2.8.1
Product / Package Ecosystem Affected Versions Fixed Version
deno
 crates.io
< 2.8.1
2.8.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/denoland/deno/security/advisories/GHSA-cpgj-f7g3-2pp2
NVD — CVE-2026-49859 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-49859
Deno Source Repository REPOSITORY
https://github.com/denoland/deno

Frequently Asked Questions

What is CVE-2026-49859?
CVE-2026-49859 is a security vulnerability in Deno involving `fetch()` API sandbox bypass via missing DNS resolution check under CWE-693: Protection Mechanism Failure.
Which Deno versions are affected?
Affected versions include releases prior to 2.8.1 across affected packages: deno.
What version fixes CVE-2026-49859?
Upgrading to 2.8.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is deno affected?
Yes, deno is affected in releases prior to 2.8.1. Upgrade your dependency to 2.8.1 or later.
What is the CVSS score of CVE-2026-49859?
CVE-2026-49859 has a CVSS v3.1 base score of 5.2 (MODERATE) with vector string CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2026-49859?
CVE-2026-49859 is classified under CWE-693: Protection Mechanism Failure (The product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product.).
How can I check crates.io dependencies for CVE-2026-49859?
Run the dependency tree command: 'inspect dependencies for deno'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-cpgj-f7g3-2pp2",
  "modified" : "2026-07-20T21:01:14Z",
  "published" : "2026-06-16T19:02:55Z",
  "aliases" : [ "CVE-2026-49859" ],
  "summary" : "Deno: `fetch()` API sandbox bypass via missing DNS resolution check",
  "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 — for example, blocking\naccess to `localhost` or internal services — 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "deno"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.8.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.8.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/denoland/deno/security/advisories/GHSA-cpgj-f7g3-2pp2"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-49859"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/denoland/deno"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-693", "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-16T19:02:55Z",
    "nvd_published_at" : "2026-06-23T18:18:03Z"
  }
}