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SvelteApplication

CVE-2026-27121

MEDIUM · 5.4 CVSS v3.1 Published 2026-02-20
Fix available
A fix is available. Upgrade to 5.51.5 or later.
See remediation →
58/100
Remediation priority · Elevated
Remotely reachable

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 performance oriented web framework. Versions of svelte prior to 5.51.5 are vulnerable to cross-site scripting (XSS) during server-side rendering. 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. This vulnerability is fixed in 5.51.5.

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

Svelte versions before 5.51.5 are vulnerable to XSS during server-side rendering when using spread syntax with untrusted data. The framework incorrectly includes event handler properties (like onclick, onerror) in the rendered HTML output, allowing attackers to inject malicious scripts that execute in victims' browsers.

MitigationUpgrade to Svelte 5.51.5 or later. Review code that uses spread syntax with user-controlled or external data during SSR and ensure proper sanitization before the upgrade.

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:< 5.51.5

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
Low
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/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
    Run 'npm list svelte' or check package.json for the svelte dependency version number
    Affected if The version is less than 5.51.5
  2. Confirm SSR is in use
    Search for server-side rendering configuration: check for 'export const ssr = true' in svelte.config.js, or presence of @sveltejs/kit with server-side load functions, or usage of server.js/node server entry points
    Affected if SSR is enabled and the application renders HTML server-side
  3. Locate spread syntax usage in SSR-rendered components
    Search codebase for JSX-like spread syntax in .svelte files: look for '{...props}' or '{...data}' patterns, especially in components that receive external input
    Affected if Spread syntax is used to pass data to components that are rendered server-side
  4. Identify untrusted data in spread props
    Review the data being spread: check if props include data from HTTP requests, URL parameters, cookies, databases with user content, or API responses
    Affected if Untrusted or user-controlled data is passed via spread syntax to components during SSR
  5. Verify event handler attributes in SSR output
    Render a test page with spread props containing 'onclick', 'onerror', or similar event handler names and inspect the generated HTML server-side output (curl the endpoint or view page source)
    Affected if Event handler attributes like onclick or onerror appear in the raw HTML response from the server

You are affected if running Svelte version below 5.51.5, using SSR, and spreading untrusted data to components with the spread operator.

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 5.51.5 or later
Fixed in 5.51.5
Interim mitigation

Upgrade to Svelte 5.51.5 or later. Review code that uses spread syntax with user-controlled or external data during SSR and ensure proper sanitization before the upgrade.

Recommended fix High confidence

Svelte 5.51.5

  1. Identify your current Svelte version by checking package.json or running 'npm list svelte'
  2. Update your package.json to specify [email protected]
  3. Run 'npm install' or 'npm update' to install the fixed version
  4. Verify the installation by checking that the Svelte version is now 5.51.5
  5. Test your application thoroughly, especially any code using spread syntax with untrusted data during server-side rendering
  6. Ensure your build completes successfully and all existing tests pass
Caveat Review the Svelte 5.51.5 changelog for any breaking changes between your current version and 5.51.5; minor version upgrades typically have minimal breaking changes but test thoroughly

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

Fix this in Svelte Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
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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-2026-27121 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