Mercurius: Incorrect Content-Type parsing can lead to CSRF attack

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.4 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

EPSS

• CVE-2025-64166: 0.18% estimated exploitation activity in the next 30 days; percentile 8.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A Cross-Site Request Forgery (CSRF) vulnerability was identified in Mercurius versions 16. The issue arises from incorrect parsing of the Content-Type header in requests. Specifically, requests with Content-Type values such as application/x-www-form-urlencoded, multipart/form-data, or text/plain could be misinterpreted as application/json. This misinterpretation bypasses the preflight checks performed by the fetch() API, potentially allowing unauthorized actions to be performed on behalf of an authenticated user. --- Impact An attacker could exploit this vulnerability by crafting a malicious request with a Content-Type that Fastify incorrectly parses as application/json. When such a request is made from a different origin, it bypasses the Cross-Origin Resource Sharing (CORS) protections, leading to a potential CSRF attack. This could result in unauthorized actions being performed on behalf of an authenticated user without their consent. --- Proof of Concept In the above…

Published by the source

5 Mar 2026, 18:18 UTC

Last modified by the source

5 Mar 2026, 20:46 UTC

Source last checked

26 Oct 2026, 12:42 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-v66j-6wwf-jc57

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v66j-6wwf-jc57

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# Patch Next advisory brief: CVE-2025-64166

Advisory: Mercurius: Incorrect Content-Type parsing can lead to CSRF attack
IDs: CVE-2025-64166, GHSA-v66j-6wwf-jc57
Record: https://patchnext.com/advisories/cve-2025-64166 (revision 2, source checked 26 Oct 2026, 12:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mercurius 16.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mercurius: affected <= 16.3.0; fixed in 16.4.0

## Suggested inspection (read-only)
- `npm ls mercurius --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mercurius.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-v66j-6wwf-jc57 (OSV record GHSA-v66j-6wwf-jc57 lists affected ranges: npm mercurius >=0 <16.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-v66j-6wwf-jc57 (GitHub-reviewed advisory GHSA-v66j-6wwf-jc57 lists affected ranges: npm mercurius >=0 <=16.3.0)

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