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CVE-2026-42599: Cross-site scripting via spread attributes in Svelte SSR

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-09 · updated 2026-07-23
• Weakness: CWE-79

Description

Svelte is a performance oriented web framework. Prior to version 5.55.7, 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 issue has been patched in version 5.55.7.

Affected products

• sveltejs svelte: before 5.55.7 — fixed in 5.55.7

How to fix

Upgrade to a fixed version: sveltejs svelte 5.55.7. Check the vendor advisory for mitigations.

References

• https://github.com/sveltejs/svelte/releases/tag/svelte%405.55.7
• https://github.com/sveltejs/svelte/security/advisories/GHSA-pr6f-5x2q-rwfp

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