Improper Validation of Specified Type of Input Affecting fastify package, versions <5.12.1
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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- published19 Aug 2026
- disclosed18 Aug 2026
- creditvelgusgus599
Introduced: 18 Aug 2026
NewCVE-2026-18504 (opens in a new tab)How to fix?
Upgrade fastify to version 5.12.1 or higher.
Overview
fastify is an overhead web framework, for Node.js.
Affected versions of this package are vulnerable to Improper Validation of Specified Type of Input via the request validation path in lib/validation.js. An attacker can bypass body schema type enforcement by sending a root-level primitive JSON body such as "10" to a route that expects an integer. Fastify validates the coerced primitive value but leaves request.body as the original uncoerced string when the handler runs. Applications that trust the validated type can then apply business rules to the wrong type, breaking input limits and other request-body checks.
Workarounds
- Wrap the value in an object schema instead of using a root primitive body, for example accept
{ "value": 10 }and readrequest.body.value; this keeps coercion in-place and prevents the handler from seeing an uncoerced root string. - Re-check the type in the route handler before applying business rules; this prevents trust in a root primitive body value that may not match the validated type.