# CVE-2026-42567: Svelte ReDoS in svelte:element tag validation

Source: https://junglewise.ai/threats/cve-2026-42567-svelte-redos-in-svelte-element-tag-validation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-09. Updated: 2026-06-09  
License: CC BY 4.0 with a link back

CVE-2026-42567 · Severity: medium · CVSS 4 · Published 2026-06-09

Svelte is a popular framework used to build web user interfaces. A performance flaw in how it handles dynamic HTML elements could allow an attacker to crash a website or make it unresponsive by providing extremely long or complex tag names. This could lead to a service outage for users of the affected application.

Technologies: [svelte (npm)](https://junglewise.ai/threats/technologies/svelte). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-42567 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-42567 being exploited in the wild.

**How severe is CVE-2026-42567?** Medium severity, with a CVSS score of 4, published 9 June 2026.

**Which products and versions are affected?** svelte (npm) >= 5.51.5, <= 5.55.6, SEMVER:introduced=5.51.5,fixed=5.55.7.

**Is there a fix for CVE-2026-42567?** Yes. svelte (npm): upgrade to 5.55.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-42567 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-42567?** Check whether you run svelte (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- svelte (npm): upgrade to 5.55.7 or later

  ```sh
  npm install svelte@5.55.7
  ```


## Technical details

A Regular Expression Denial of Service (ReDoS) vulnerability exists in the Svelte runtime within the `<svelte:element>` tag validation logic. The internal regular expression used to validate the `this={tag}` attribute can exhibit exponential backtracking when processing unconstrained, specially crafted input strings. An attacker who can control the tag name passed to this component can cause the JavaScript execution thread to hang, leading to a denial of service. This vulnerability is mitigated if the application restricts tag names to a known-safe list or enforces strict length limits. The issue is fixed in Svelte version 5.55.7.

## Affected products

- sveltejs svelte >= 5.51.5, <= 5.55.6

## Identifiers

- CVE-2026-42567
- GHSA-9rmh-mm8f-r9h6

## Timeline

- 2026-05-14: disclosed
- 2026-05-14: advisory
- 2026-05-14: patched: Fixed in version 5.55.7

## References

- [https://api.github.com/users/Meltedd](https://api.github.com/users/Meltedd)
- [https://github.com/Meltedd](https://github.com/Meltedd)
- [https://api.github.com/users/Meltedd/gists%7B/gist_id%7D](https://api.github.com/users/Meltedd/gists%7B/gist_id%7D)
- [https://api.github.com/users/Meltedd/repos](https://api.github.com/users/Meltedd/repos)
- [https://avatars.githubusercontent.com/u/74914421?v=4](https://avatars.githubusercontent.com/u/74914421?v=4)
- [https://api.github.com/users/Meltedd/events%7B/privacy%7D](https://api.github.com/users/Meltedd/events%7B/privacy%7D)

## Related threats

- [Svelte XSS via spread attributes in SSR](https://junglewise.ai/threats/cve-2026-42599-svelte-xss-via-spread-attributes-in-ssr)
- [Svelte DOM clobbering leading to XSS in internal framework state](https://junglewise.ai/threats/cve-2026-42573-svelte-dom-clobbering-leading-to-xss-in-internal-framework-state)
- [Svelte XSS via insecure promise serialization in hydratable](https://junglewise.ai/threats/svelte-xss-via-insecure-promise-serialization-in-hydratable-3f504409)
- [Svelte XSS via HTML comment injection in SSR error boundary](https://junglewise.ai/threats/cve-2026-27902-svelte-xss-via-html-comment-injection-in-ssr-error-boundary)
- [Svelte XSS in SSR with contenteditable bind:innerText and bind:textContent](https://junglewise.ai/threats/cve-2026-27901-svelte-xss-in-ssr-with-contenteditable-bind-innertext-and-bind)