Svelte SSR vulnerable to cross-site scripting via spread attributes

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N · score source

EPSS

• CVE-2026-42599: 0.21% estimated exploitation activity in the next 30 days; percentile 10.57 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

When using spread syntax to render attributes from untrusted data, event handler properties are included in the rendered HTML output. If an application spreads user-controlled or external data as element attributes, an attacker can inject malicious event handlers that execute in victims' browsers. Note that this vulnerability only triggers if the user's browser has JavaScript enabled but Svelte's hydration mechanism does not reach the vulnerable element before the event fires. This is similar to but different from CVE-2026-27121.

Published by the source

14 May 2026, 20:19 UTC

Last modified by the source

9 Jun 2026, 18:45 UTC

Source last checked

26 Oct 2026, 12:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-pr6f-5x2q-rwfp

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-pr6f-5x2q-rwfp

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# Patch Next advisory brief: CVE-2026-42599

Advisory: Svelte SSR vulnerable to cross-site scripting via spread attributes
IDs: CVE-2026-42599, GHSA-pr6f-5x2q-rwfp
Record: https://patchnext.com/advisories/cve-2026-42599 (revision 3, source checked 26 Oct 2026, 12:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: svelte 5.55.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm svelte: affected <= 5.55.6; fixed in 5.55.7

## Suggested inspection (read-only)
- `npm ls svelte --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on svelte.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-pr6f-5x2q-rwfp (OSV record GHSA-pr6f-5x2q-rwfp lists affected ranges: npm svelte >=0 <5.55.7 (fixed))
- Verified: https://github.com/advisories/GHSA-pr6f-5x2q-rwfp (GitHub-reviewed advisory GHSA-pr6f-5x2q-rwfp lists affected ranges: npm svelte >=0 <=5.55.6)

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