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SvelteApplication

CVE-2026-42599

MEDIUM · 6.1 CVSS v3.1 Published 2026-06-09
Fix available
A fix is available. Upgrade to 5.55.7 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

This is a DOM-based Cross-Site Scripting (XSS) vulnerability in Svelte framework versions prior to 5.55.7. When using spread syntax to render attributes from untrusted data, event handler properties can be included in the rendered HTML output, allowing attackers to inject malicious event handlers that execute in victims' browsers. The vulnerability triggers only when JavaScript is enabled but Svelte's hydration mechanism does not reach the vulnerable element before the event fires.

MitigationUpgrade to Svelte version 5.55.7 or later. Additionally, validate and sanitize all user-controlled or external data before using it with spread syntax on HTML elements.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
SvelteApplication
Affected:< 5.55.7

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Svelte version
    Check your package.json file for the 'svelte' dependency version, or run 'npm list svelte' or 'yarn list svelte' to see the installed version
    Affected if The version listed is less than 5.55.7 (for example, 5.55.6, 5.55.0, 5.0.0, etc.)
  2. Find attribute spreading in Svelte templates
    Search your codebase for the spread syntax pattern on HTML elements, such as '<element {...userData}>' or '<Component {...data}>' where data comes from props, API responses, or user input
    Affected if Code spreads objects containing untrusted or user-controlled data directly onto elements as attributes
  3. Identify sources of untrusted data
    Review any spread data that originates from external sources: URL parameters, API responses, user input fields, localStorage, cookies, or data parsed from JSON/untrusted files
    Affected if The spread data contains values from untrusted sources without sanitization before being applied to element attributes
  4. Check for event handler injection potential
    Examine if an attacker could craft malicious data that includes event handler names (like 'onclick', 'onerror', 'onload') within the spread object that gets rendered as attributes
    Affected if Untrusted data is spread onto elements and could contain attribute names that browsers interpret as event handlers

You are affected if your Svelte version is below 5.55.7 AND your codebase spreads untrusted or user-controlled data directly as element attributes using the spread syntax.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.55.7 or later
Fixed in 5.55.7
Interim mitigation

Upgrade to Svelte version 5.55.7 or later. Additionally, validate and sanitize all user-controlled or external data before using it with spread syntax on HTML elements.

Recommended fix High confidence

Svelte 5.55.7

  1. Upgrade Svelte to version 5.55.7 or later by running npm install [email protected] or yarn add [email protected]
  2. Verify the upgrade by checking the installed version with npm list svelte or yarn list svelte
  3. Review your codebase for any instances where untrusted or user-controlled data is spread onto HTML elements using the spread syntax (e.g., <div {...userData}>)
  4. Ensure that any data being spread onto elements does not contain event handler properties like onClick, onMouseOver, etc., especially if the data originates from external sources or user input
  5. Test the updated application to confirm the vulnerability is mitigated and the application still functions correctly

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Svelte Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-42599 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data