CVE-2026-66062: SvelteKit Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-29g2-3rmr-qm68
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CVSS 5.3
CWE-1333
Published
Advisory Summary

CVE-2026-66062 is a MODERATE (CVSS 5.3) denial of service vulnerability (CWE-1333: Inefficient Regular Expression Complexity) affecting SvelteKit libraries in the npm ecosystem. The issue involves redos (o(n^2)) in content negotiation — unauthenticated dos via the accept header. Remediation is available by upgrading affected packages to version 2.70.2.

CVE-2026-66062 at a Glance

CVE Identifier CVE-2026-66062
GHSA Identifier GHSA-29g2-3rmr-qm68
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-1333: Inefficient Regular Expression Complexity
Affected Product SvelteKit
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 5.3

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:L).

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 Low (A:L)

How to Fix CVE-2026-66062

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

@sveltejs/kit fixed: 2.70.2 
npm install @sveltejs/kit@2.70.2
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 @sveltejs/kit

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-66062, inspect your project dependencies using npm:

npm ls @sveltejs/kit

Analyze the terminal output to locate any version of SvelteKit in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.70.2), your application is affected.

Note: The npm ls @sveltejs/kit 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-1333: Inefficient Regular Expression Complexity, where the product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential. In SvelteKit, 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 SvelteKit is vulnerable to remote CPU-exhaustion DoS attacks via specifically-crafted `Accept` headers. The impact is mitigated by default header length limits on most platforms, but in the case of raised or absent limits a denial of service is possible. ### Patches The vulnerability is patched in `@sveltejs/kit` version 2.70.2.

CVE-2026-66062 Affected SvelteKit Versions

1 component(s)
Fixed Versions:
2.70.2
Product / Package Ecosystem Affected Versions Fixed Version
@sveltejs/kit
npm/@sveltejs/kit
npm
< 2.70.2
2.70.2

References

SvelteKit Source Repository REPOSITORY
https://github.com/sveltejs/kit
GitHub Advisory / Wiki ADVISORY
https://github.com/sveltejs/kit/security/advisories/GHSA-29g2-3rmr-qm68
Security Fix Commit FIX COMMIT
https://github.com/sveltejs/kit/commit/82712fc02c24b1dcf5b25d7a52129cd8455f04f5
SvelteKit Release RELEASE
https://github.com/sveltejs/kit/releases/tag/@sveltejs/kit@2.70.2

Frequently Asked Questions

What is CVE-2026-66062?
CVE-2026-66062 is a security vulnerability in SvelteKit involving ReDoS (O(n^2)) in content negotiation — unauthenticated DoS via the Accept header under CWE-1333: Inefficient Regular Expression Complexity.
Which SvelteKit versions are affected?
Affected versions include releases prior to 2.70.2 across affected packages: @sveltejs/kit.
What version fixes CVE-2026-66062?
Upgrading to 2.70.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is @sveltejs/kit affected?
Yes, @sveltejs/kit is affected in releases prior to 2.70.2. Upgrade your dependency to 2.70.2 or later.
What is the CVSS score of CVE-2026-66062?
CVE-2026-66062 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2026-66062?
CVE-2026-66062 is classified under CWE-1333: Inefficient Regular Expression Complexity (The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.).
How can I check npm dependencies for CVE-2026-66062?
Run the dependency tree command: 'npm ls @sveltejs/kit'. 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-29g2-3rmr-qm68",
  "modified" : "2026-08-07T16:50:02Z",
  "published" : "2026-08-07T16:50:02Z",
  "aliases" : [ "CVE-2026-66062" ],
  "summary" : "SvelteKit: ReDoS (O(n^2)) in content negotiation — unauthenticated DoS via the Accept header",
  "details" : "### Impact\nSvelteKit is vulnerable to remote CPU-exhaustion DoS attacks via specifically-crafted `Accept` headers. The impact is mitigated by default header length limits on most platforms, but in the case of raised or absent limits a denial of service is possible.\n\n### Patches\nThe vulnerability is patched in `@sveltejs/kit` version 2.70.2.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@sveltejs/kit"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.70.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.70.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/sveltejs/kit/security/advisories/GHSA-29g2-3rmr-qm68"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sveltejs/kit/commit/82712fc02c24b1dcf5b25d7a52129cd8455f04f5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/sveltejs/kit"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sveltejs/kit/releases/tag/@sveltejs/kit@2.70.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-07T16:50:02Z",
    "nvd_published_at" : null
  }
}