dbcve.org
UndiciApplication · Node.js

CVE-2026-9675

HIGH · 7.5 CVSS v3.1 Published 2026-06-17
Fix available
A fix is available. Upgrade to 8.5.0 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges 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
Impact: The undici WebSocket client enforces maxPayloadSize per-frame but does not enforce the cumulative size of fragmented uncompressed messages. A malicious WebSocket server can stream many small fragments that each pass per-frame validation but collectively exceed the configured limit, causing unbounded memory growth in the client process. The result is memory exhaustion and a denial of service. Affected applications are those using the undici WebSocket client (new WebSocket(...)) that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint. This is a regression specific to undici 8.1.0. The 6.25.0 line shipped the equivalent cumulative check from the start and is unaffected. The 7.x line never had the maxPayloadSize feature and is also unaffected. Patches: Upgrade to undici >= 8.5.0. Workarounds: No workaround is available. The fix must be applied through an upgrade.

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

The undici WebSocket client in version 8.1.0 validates maxPayloadSize per-frame but fails to track cumulative size across fragmented uncompressed messages. An attacker-controlled server can send many small fragments that individually pass validation but collectively exceed the configured limit, causing unbounded memory growth and denial of service.

MitigationUpgrade undici to version 8.5.0 or higher to receive the cumulative fragment size validation fix. No workarounds exist; the vulnerability requires library upgrade.

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
UndiciApplication
Affected:>= 8.0.0, < 8.5.0

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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

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. Identify undici installation and version
    Run 'npm list undici' in your project directory, or inspect your package.json dependencies for the undici entry
    Affected if The installed version is 8.0.0 through 8.4.x (any version >= 8.0.0 and < 8.5.0)
  2. Confirm WebSocket client usage
    Search your codebase for imports of WebSocket from undici, such as 'import { WebSocket } from "undici"' or require statements
    Affected if Your application imports or uses the undici WebSocket client
  3. Check for maxPayloadSize configuration
    Search your codebase for WebSocket instantiation with maxPayloadSize option, for example: new WebSocket(url, { maxPayloadSize: <value> })
    Affected if You configure maxPayloadSize when creating WebSocket connections (the vulnerability exploits this setting by sending fragments that individually pass the limit)
  4. Verify fragmented message handling
    Review WebSocket implementation to confirm your server can send fragmented WebSocket messages (multiple small fragments forming one logical message)
    Affected if Your undici WebSocket client connects to servers that send fragmented messages and maxPayloadSize is set

You are affected if your project uses undici version 8.0.0 through 8.4.x with the WebSocket client and has maxPayloadSize configured, allowing a malicious server to send fragmented messages that exceed the cumulative limit.

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 8.5.0 or later
Fixed in 8.5.0
Interim mitigation

Upgrade undici to version 8.5.0 or higher to receive the cumulative fragment size validation fix. No workarounds exist; the vulnerability requires library upgrade.

Recommended fix High confidence

undici >= 8.5.0

  1. Identify the package manager used in your project (npm, yarn, or pnpm)
  2. Update the undici dependency in package.json to version 8.5.0 or higher using npm install undici@^8.5.0 (or equivalent for your package manager)
  3. Run the package manager install command to update dependencies
  4. Verify the installed version meets the requirement: npm list undici
  5. Test your application's WebSocket functionality to ensure compatibility with the new version
Caveat Review undici changelog for any breaking changes between your current version and 8.5.0

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

Fix this in Undici Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

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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-2026-9675 in production — separate from our analysis above.

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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