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DenoApplication

CVE-2026-49859

MEDIUM · 5.2 CVSS v3.1 Published 2026-06-23
Fix available
A fix is available. Upgrade to 2.8.1 or later.
See remediation →
54/100
Remediation priority · Moderate
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Deno's fetch() function only validated hostnames against --deny-net rules but did not re-check the resolved IP addresses, allowing attackers to bypass network restrictions by using domains that pass hostname validation but resolve to blocked IPs via DNS manipulation.

MitigationUpgrade Deno to version 2.8.1 or later which implements proper IP address validation after hostname resolution. If upgrading is not immediately possible, implement supplementary network filtering at the infrastructure level.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
DenoApplication
Affected:< 2.8.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine installed Deno version
    Run 'deno --version' and note the version number
    Affected if Version is earlier than 2.8.1 (e.g., 2.8.0, 2.7.0, etc.)
  2. Identify if --deny-net is configured
    Search startup scripts, deployment configs, and command-line arguments for '--deny-net' or 'Deno.permissions.net' deny rules
    Affected if --deny-net restrictions are in place but only hostname validation is performed
  3. Locate fetch() usage in the codebase
    Search source code files for 'fetch(' calls, especially those handling user-controlled URLs or external domain names
    Affected if fetch() is used to request URLs where the hostname passes allowlist but resolved IP could be manipulated
  4. Check for IP validation after hostname resolution
    Review custom fetch wrapper code or network request handling to see if resolved IP addresses are validated against blocklists after DNS resolution
    Affected if No secondary IP address validation exists after hostname resolution completes

You are affected if running Deno version earlier than 2.8.1 with --deny-net rules while using fetch() for network requests, since the resolved IP addresses are not validated against the network restrictions.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 2.8.1 or later
Fixed in 2.8.1
Interim mitigation

Upgrade Deno to version 2.8.1 or later which implements proper IP address validation after hostname resolution. If upgrading is not immediately possible, implement supplementary network filtering at the infrastructure level.

Recommended fix High confidence

Deno 2.8.1

  1. Check the current Deno version by running `deno --version`
  2. Upgrade Deno to version 2.8.1 or later using the official installation method: `deno upgrade` or by downloading from https://deno.land/
  3. Verify the upgrade was successful by running `deno --version` and confirming the version is 2.8.1 or higher
  4. Test that fetch() now properly validates resolved IP addresses against --deny-net rules

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Deno Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-49859 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data