CVE-2026-9675: Undici Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-38rv-x7px-6hhq
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CVSS 7.5
CWE-400 CWE-770
Published
Advisory Summary

CVE-2026-9675 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Undici libraries in the npm ecosystem. The issue involves undici websocket client vulnerable to denial of service via cumulative fragment bypass. Remediation is available by upgrading affected packages to version 8.5.0.

CVE-2026-9675 at a Glance

CVE Identifier CVE-2026-9675
GHSA Identifier GHSA-38rv-x7px-6hhq
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Undici
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 7.5

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

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-9675

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

undici fixed: 8.5.0 
npm install undici@8.5.0
Verify Direct and Transitive Dependencies

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

npm ls undici

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-9675, inspect your project dependencies using npm:

npm ls undici

Analyze the terminal output to locate any version of Undici in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 8.5.0), your application is affected.

Note: The npm ls undici command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Undici, 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

## 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.

CVE-2026-9675 Affected Undici Versions

1 component(s)
Fixed Versions:
8.5.0
Product / Package Ecosystem Affected Versions Fixed Version
undici
npm/undici
npm
≥ 8.0.0
< 8.5.0
8.5.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/nodejs/undici/security/advisories/GHSA-38rv-x7px-6hhq
NVD — CVE-2026-9675 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-9675
Undici Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
Undici Source Repository REPOSITORY
https://github.com/nodejs/undici

Frequently Asked Questions

What is CVE-2026-9675?
CVE-2026-9675 is a security vulnerability in Undici involving undici WebSocket client vulnerable to denial of service via cumulative fragment bypass under CWE-400: Uncontrolled Resource Consumption.
Which Undici versions are affected?
Affected versions include releases prior to 8.5.0 across affected packages: undici.
What version fixes CVE-2026-9675?
Upgrading to 8.5.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is undici affected?
Yes, undici is affected in releases prior to 8.5.0. Upgrade your dependency to 8.5.0 or later.
What is the CVSS score of CVE-2026-9675?
CVE-2026-9675 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-9675?
CVE-2026-9675 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check npm dependencies for CVE-2026-9675?
Run the dependency tree command: 'npm ls undici'. 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-38rv-x7px-6hhq",
  "modified" : "2026-06-18T14:28:10Z",
  "published" : "2026-06-18T14:28:10Z",
  "aliases" : [ "CVE-2026-9675" ],
  "summary" : "undici WebSocket client vulnerable to denial of service via cumulative fragment bypass",
  "details" : "## Impact\n\nThe 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.\n\nAffected applications are those using the undici WebSocket client (`new WebSocket(...)`) that can be induced to connect to an attacker-controlled or compromised WebSocket endpoint.\n\nThis 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.\n\n## Patches\n\nUpgrade to undici >= 8.5.0.\n\n## Workarounds\n\nNo workaround is available. The fix must be applied through an upgrade.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "undici"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0"
      }, {
        "fixed" : "8.5.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nodejs/undici/security/advisories/GHSA-38rv-x7px-6hhq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-9675"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nodejs/undici"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-18T14:28:10Z",
    "nvd_published_at" : "2026-06-17T17:17:28Z"
  }
}