Executive Summary

CVE-2026-42567 is a Regular Expression Denial of Service (ReDoS) vulnerability in the Svelte web framework affecting versions 5.51.5 to 5.55.6.

The issue arises from an internal regular expression used in the `<svelte:element>` tag validation that can take exponential time to process when handling tags of unconstrained length.

This means that if an attacker can supply very long or specially crafted tag names, the regex evaluation can consume excessive CPU resources, leading to a denial of service.

The vulnerability was patched in version 5.55.7.

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Impact Analysis

This vulnerability can impact you by causing high availability disruption due to resource exhaustion.

If an attacker exploits the ReDoS vulnerability by providing tags of arbitrary length, the internal regex can consume excessive CPU time, potentially leading to denial of service conditions.

However, exploitation requires high privileges and specific conditions, and applications that restrict tags to a predefined list or limit tag length are not vulnerable.

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Detection Guidance

This vulnerability is related to a Regular Expression Denial of Service (ReDoS) in the Svelte framework versions 5.51.5 to 5.55.6, specifically in the <svelte:element> tag validation. Detection involves identifying if your system is running a vulnerable version of Svelte and if your application allows tags of arbitrary length that could trigger the exponential regex processing.

To detect the vulnerability on your system, you should first check the installed Svelte version. For example, if you use npm, you can run the following command in your project directory:

• npm list svelte

If the version is between 5.51.5 and 5.55.6 (inclusive), your system is potentially vulnerable.

Additionally, you can audit your codebase to check if <svelte:element> tags are used with unconstrained or arbitrary-length tag names, which are required to exploit this vulnerability.

There are no specific network detection commands or signatures mentioned for this vulnerability, as it is a runtime regex issue triggered by specific input patterns within the application.

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Mitigation Strategies

The primary mitigation step is to upgrade the Svelte framework to version 5.55.7 or later, where this vulnerability has been patched.

If upgrading immediately is not possible, you should restrict the use of <svelte:element> tags to a predefined list of allowed tags or ensure that tag names are trimmed and constrained in length to prevent triggering the vulnerable regex.

Additionally, review your application for any usage patterns that allow arbitrary-length tag names and modify them to enforce length limits or whitelist tags.

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Compliance Impact

The vulnerability CVE-2026-42567 is a Regular Expression Denial of Service (ReDoS) issue that primarily impacts the availability of applications using affected versions of the Svelte framework. It does not directly involve unauthorized access to personal data or confidentiality breaches.

Since the main impact is on availability through resource exhaustion, compliance concerns with standards like GDPR or HIPAA would relate to ensuring system availability and resilience. However, there is no explicit information indicating that this vulnerability leads to data breaches or privacy violations that would directly affect compliance with these regulations.

Applications that rely on Svelte versions prior to 5.55.7 and do not restrict tag lengths may be at risk of denial of service, which could indirectly affect compliance if system downtime impacts the ability to protect or process regulated data properly.

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