Quick answers
- Is CVE-2026-42599 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-42599 being exploited in the wild.
- How severe is CVE-2026-42599?
- Medium severity, with a CVSS score of 4, published 9 June 2026.
- Which products and versions are affected?
- svelte (npm) <= 5.55.6, SEMVER:introduced=0,fixed=5.55.7.
- Is there a fix for CVE-2026-42599?
- Yes. svelte (npm): upgrade to 5.55.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-42599 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-42599?
- Check whether you run svelte (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- svelte (npm): upgrade to 5.55.7 or later
npm install svelte@5.55.7
Technical details
A Cross-Site Scripting (XSS) vulnerability exists in Svelte's Server-Side Rendering (SSR) when using spread attributes (e.g., {...data}). When an application spreads user-controlled or external data as element attributes, event handler properties (like 'onclick') are included in the rendered HTML output instead of being sanitized or restricted. This allows an attacker to inject malicious JavaScript event handlers. The vulnerability is specifically exploitable if the user's browser has JavaScript enabled but Svelte's hydration mechanism has not yet reached the vulnerable element before the malicious event is triggered. This issue is addressed in version 5.55.7.
Affected products
- Svelte Svelte <= 5.55.6
Identifiers
- CVE-2026-42599
- GHSA-pr6f-5x2q-rwfp
Timeline
- 2026-05-14: disclosed
- 2026-05-14: advisory
- 2026-05-14: patched: Fixed in version 5.55.7