CVE-2026-49859: Deno network restriction bypass in fetch API

CVE-2026-49859 · Severity: medium · CVSS 5.2 · Published 2026-06-23

Deno is a software runtime used to execute JavaScript and TypeScript applications. A security flaw allowed malicious or compromised scripts to bypass network security restrictions intended to block access to specific internal servers or sensitive local addresses. This could allow an attacker to access private data or internal services that were supposed to be isolated from the application.

Technologies: deno (crates.io). Vendors: crates.io.

Quick answers

Is CVE-2026-49859 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-49859 being exploited in the wild.
How severe is CVE-2026-49859?
Medium severity, with a CVSS score of 5.2, published 23 June 2026.
Which products and versions are affected?
deno (crates.io) <= 2.8.0, SEMVER:introduced=0,fixed=2.8.1.
Is there a fix for CVE-2026-49859?
Yes. deno (crates.io): upgrade to 2.8.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-49859 attacked?
It is attacked with local access, in a software library.
Am I affected by CVE-2026-49859?
Check whether you run deno (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • deno (crates.io): upgrade to 2.8.1 or later
    cargo update -p deno

Technical details

A protection mechanism failure (CWE-693) and Server-Side Request Forgery (SSRF) vulnerability exists in Deno's fetch() implementation. While Deno checks the initial hostname against --deny-net blocklists, it fails to perform a post-resolution check on the resulting IP addresses. An attacker can use a DNS rebinding-style attack or a specially crafted domain that resolves to a restricted IP (such as 127.0.0.1 or internal metadata services) to bypass the sandbox. This allows code running with restricted network permissions to reach unauthorized destinations. The issue is resolved in version 2.8.1 by implementing mandatory IP-level validation after DNS resolution.

Affected products

  • denoland Deno < 2.8.1

Identifiers

  • CVE-2026-49859
  • GHSA-cpgj-f7g3-2pp2

Timeline

  • 2026-05-27: advisory: GitHub advisory published by maintainers
  • 2026-06-23: disclosed: NVD publication date
  • 2026-06-23: patched: Fix confirmed in version 2.8.1

References

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