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SvelteKit has deserialization expansion in unvalidated `form` remote function leading to Denial of Service (experimental only)GHSA-fpg4-jhqr-589c

Fix: sveltejs/kit@faba869

GHSA-fpg4-jhqr-589c is a Deserialization of Untrusted Data vulnerability in @sveltejs/kit. A fix is available for @sveltejs/kit — see the affected versions and patch details below.

Also known asCVE-2026-82259
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-fpg4-jhqr-589c.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk
0.00%0.34%0.69%1.03%0.4%0.5%0.5%0.5%Sep 26Nov 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

216other npm packages depend on this — each one inherits the vulnerability until it's patched upstream
@sveltejs/kitnpm
5.0Mdownloads / week

Description

SvelteKit versions from 2.49.0 through 2.53.2 (fixed in 2.53.3) contain a deserialization expansion issue in the experimental form remote function. When an application enables experimental.remoteFunctions and uses the form function to process the files array without validating files.length or individual file sizes, an attacker can submit relatively small inputs that expand into very large file arrays, leading to expensive processing and denial of service.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
📦npm@sveltejs/kit≥ 2.49.0&&< 2.53.32.53.3npm install @sveltejs/kit@2.53.3

Affected Products

1 product · 1 configurations
Application
sveltekitsvelte
≥ 2.49.0 && < 2.53.3
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @sveltejs/kit, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update @sveltejs/kit to 2.53.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fpg4-jhqr-589c is resolved across your whole dependency graph.

  3. Workarounds

    Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Frequently Asked Questions

Some relatively small inputs can cause very large files arrays in `form` handlers. If the SvelteKit application code doesn't check `files.length` or individual files' sizes and performs expensive processing with them, it can result in Denial of Service. Only users with `experimental.remoteFunctions: true` who are using the `form` function and are processing the `files` array without validation are vulnerable.
O3 Security · Impact-Aware SCA

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SvelteKit has deserialization expansion in unvalidated…