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SvelteApplication

CVE-2026-27122

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. Prior to 5.51.5, when using <svelte:element this={tag}> in server-side rendering, the provided tag name is not validated or sanitized before being emitted into the HTML output. If the tag string contains unexpected characters, it can result in HTML injection in the SSR output. Client-side rendering is not affected. 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

In Svelte versions prior to 5.51.5, the <svelte:element this={tag}> component does not validate or sanitize the tag name parameter during server-side rendering. An attacker who controls the tag string value can inject arbitrary HTML into the SSR output by including unexpected characters (e.g., breaking out of the attribute context). Client-side rendering is unaffected. The fix is available in version 5.51.5.

MitigationUpgrade Svelte to version 5.51.5 or later. If immediate upgrade is not feasible, audit all uses of <svelte:element this={tag}> in SSR contexts and implement allowlist-based validation on the tag name input before passing it to the component.

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. Check installed Svelte version
    Run 'npm list svelte' or inspect node_modules/svelte/package.json to find the installed version number.
    Affected if The installed Svelte version is lower than 5.51.5.
  2. Confirm server-side rendering is in use
    Review your application configuration and code to determine if SSR (server-side rendering) is enabled. Check for SSR frameworks, server entry points, or configuration files that enable SSR.
    Affected if SSR is enabled and Svelte is rendering components on the server.
  3. Search for vulnerable component usage
    Search your codebase for instances of <svelte:element this={...}> syntax. Inspect each usage to see what values are passed to the 'this' attribute.
    Affected if The component <svelte:element this={tag}> is used in your codebase.
  4. Inspect tag parameter source and validation
    For each usage of <svelte:element this={tag}>, trace where the tag value originates. Determine if it comes from user input, external data, or untrusted sources. Check if any validation or sanitization is applied before passing to the component.
    Affected if The tag parameter accepts values from untrusted input without validation or allowlist filtering.

You are affected if your Svelte version is below 5.51.5, you use SSR, and you use <svelte:element this={tag}> with unvalidated tag values from untrusted sources.

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 Svelte to version 5.51.5 or later. If immediate upgrade is not feasible, audit all uses of <svelte:element this={tag}> in SSR contexts and implement allowlist-based validation on the tag name input before passing it to the component.

Recommended fix High confidence

Svelte 5.51.5

  1. Identify all projects using Svelte version less than 5.51.5
  2. Update the Svelte dependency in package.json to version 5.51.5 (or latest patch in the 5.x line)
  3. Run 'npm install' or 'yarn install' to update the dependency
  4. Verify the installed Svelte version is 5.51.5 or higher using 'npm list svelte' or 'yarn list svelte'
  5. Test the application focusing on server-side rendering functionality, particularly any usage of <svelte:element this={tag}>
  6. Ensure SSR output does not contain unexpected HTML tags or injection artifacts
Caveat Review Svelte 5.51.5 release notes for any SSR-related changes that may affect your application behavior

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

Fix this in Svelte Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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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-2026-27122 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