CVE-2026-84504 fastify vulnerable to request body replacement via an async validation result collision
An authenticated low-privilege user can replace the validated request body with nested data, bypassing schema authorization to modify or read unauthorized data.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84504?
CVE-2026-84504 is a high-severity vulnerability in fastify, published on 4 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the root, fastify replaces the entire request body with that property's value before the handler runs, so the handler receives a different object than the one that satisfied the schema. An authenticated low-privilege caller can use this to make nested data replace the validated body and trigger an operation the route schema did not authorize, leading to unauthorized state changes and data disclosure. Users should upgrade to fastify 5.12.2 or later.
Weakness: CWE-20
Which versions are affected by CVE-2026-84504?
2 open source packages ship an affected version, including fastify. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastify npm | All versions | 5.12.2 |
fastify npm | All versions | 5.12.2 |
Products include Fastify Fastify.
How do I fix CVE-2026-84504?
Upgrade fastify to 5.12.2 or later.
Is CVE-2026-84504 being exploited?
There is no known exploitation of CVE-2026-84504 in the wild and no public exploit code.
Other vulnerabilities in fastify
- CVE-2026-76169 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
- CVE-2026-84428 fastify vulnerable to header validation bypass via incomplete schema case normalization
- CVE-2026-84469 fastify vulnerable to request validation bypass via skipped boolean false schemas
- CVE-2026-33806 fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header
- CVE-2026-25223 Fastify's Content-Type header tab character allows body validation bypass
Sources
Also known as CLSA-2026-1790870980, CLSA-2026-1790937680, CLSA-2026-1790966173, CLSA-2026-1791028128, CLSA-2026-1791050166, CLSA-2026-1791052604 and 2 more.