CVE intelligence and bounded remediation
CVE-2026-42599 — Svelte is a performance oriented web framework
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.
- Severity
- Medium
- CVSS
- 6.1 (3.1)
- Published
- 2026-06-09
- CISA KEV
- Not currently listed
- Ecosystem
- javascript/npm
- Weaknesses
- CWE-79
Affected products
- svelte / svelte
Matched remediation archetype
Cross-site scripting and unsafe browser output
This catalog composition supplies bounded fallback guidance. Explicitly reviewed curated workflows load with the complete record below.
Check exposure
- Trace reflected, stored, and DOM-derived untrusted values into HTML, attributes, URLs, styles, scripts, and client-side template sinks.
- Identify affected origins, authenticated user roles, sensitive browser capabilities, and where content is shared across tenants.
- Review framework escaping, rich-text sanitization, legacy templates, and client-side rendering paths.
Remediate safely
- Use context-aware framework output encoding and safe DOM APIs; keep untrusted data out of executable contexts.
- Sanitize intentionally supported markup with a maintained allowlist policy and validate URLs and attributes separately.
- Update affected rendering components and add tests for every output context using inert sentinel markup.
Authoritative sources
Complete CVE record and remediation plan
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