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CVE-2026-82259 – Denial of Service (Resource Exhaustion) – SvelteKit (@sveltejs/kit) 2.49.0–2.53.2

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Summary

CVE-2026-82259 is a denial-of-service vulnerability in SvelteKit’s experimental remote functions feature. When an application enables experimental.remoteFunctions and uses a form remote function that processes a submitted files array without validating its size, a small crafted request can expand into an enormous number of file entries, driving expensive server-side processing. The issue is rated HIGH severity and requires no authentication or user interaction.

Technical details

  • Root cause: SvelteKit’s binary form deserializer (used for application/x-sveltekit-binary-form submissions to remote form functions) trusts declared metadata about array/file counts without verifying it against the actual size of the submitted payload.
  • Trigger conditions: The application must have experimental.remoteFunctions enabled and define a form remote function that accesses a files array and performs processing on it without checking files.length or individual file sizes.
  • Attack vector: Network-based; an unauthenticated attacker submits a small, specially crafted request to a public-facing form endpoint.
  • Impact: The crafted metadata expands into a very large in-memory array of file objects, exhausting CPU/memory during processing and causing denial of service. No confidentiality or integrity impact.

Affected software

  • @sveltejs/kit (npm) versions 2.49.0 through 2.53.2, when experimental.remoteFunctions is enabled and a vulnerable form function handling files is present.
  • Fixed in 2.53.3 and later.

Severity

  • CVSS v4.0 Base Score: 8.7 (High) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
  • CVSS v3.1 Base Score: 7.5 (High) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Mitigation and recommended actions

  • Immediate: Upgrade @sveltejs/kit to 2.53.3 or later.
  • If immediate patching isn’t possible: Disable experimental.remoteFunctions if not required, or, for any form remote function that consumes a files array, explicitly validate files.length and individual file sizes before performing any processing on submitted data. Applying request-size and rate limits at the edge/CDN can also reduce exposure until patched.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw response body: /_app/immutable/entry/start.

References

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