CVE-2026-27118: Svelte adapter-vercel cache poisoning via ISR query parameter

CVE-2026-27118 · Severity: medium · CVSS 4 · Published 2026-02-19

The @sveltejs/adapter-vercel library, used to deploy Svelte web applications to Vercel's hosting platform, contains a cache poisoning vulnerability that allows attackers to cause personalized user responses to be cached and served to other users. An attacker can exploit this by crafting a malicious link that tricks an authenticated victim into triggering the vulnerability, resulting in sensitive information being exposed across the user base. While Vercel's Web Application Firewall currently provides protection, developers should upgrade to patch this flaw immediately.

Vendors: npm, Svelte.

Quick answers

Is CVE-2026-27118 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-27118 being exploited in the wild.
How severe is CVE-2026-27118?
Medium severity, with a CVSS score of 4, published 19 February 2026.
Which products and versions are affected?
@sveltejs/adapter-vercel (npm) SEMVER:introduced=0,fixed=6.3.2; Svelte Adapter-Vercel <6.3.2.
Is there a fix for CVE-2026-27118?
Yes. @sveltejs/adapter-vercel (npm): upgrade to 6.3.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-27118 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-27118?
Check whether you run @sveltejs/adapter-vercel (npm) or Svelte Adapter-Vercel 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

  • @sveltejs/adapter-vercel (npm): upgrade to 6.3.2 or later
    npm install @sveltejs/adapter-vercel@6.3.2

Technical details

The vulnerability exists in @sveltejs/adapter-vercel versions prior to 6.3.2 and stems from improper handling of an internal query parameter used for Incremental Static Regeneration (ISR). This parameter, intended for internal use only, is accessible on all application routes, allowing an attacker to trigger caching of user-specific responses intended for different users. The attack requires user interaction—a victim must visit an attacker-controlled link while authenticated to the application. Upon successful exploitation, the attacker can cause sensitive responses (e.g., personalized data, authentication tokens, private information) to be cached and subsequently served to other authenticated users accessing the same route. The vulnerability is classified as cache poisoning (CWE-346) with a CVSS v4 score of 5.3. A patch is available in version 6.3.2.

Affected products

  • Svelte adapter-vercel <6.3.2

Identifiers

  • CVE-2026-27118
  • GHSA-9pq4-5hcf-288c

Timeline

  • 2026-02-18: disclosed
  • 2026-02-19: patched: Version 6.3.2 released

References

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