GHSA-667r-xxjv-c9mm: Input Validation
Impact
Fastify runs a route's validator and, for a result shaped like { value, error }, unwraps it: an error becomes a validation failure and value replaces the request part. This convention is intended for synchronous custom compilers (for example Joi). A JSON Schema $async validator, however, resolves with the validated data itself, so Fastify applied the same unwrapping to it. If a request part validated by an $async schema contains a value property, Fastify replaced the whole request part with that nested value before the handler ran, so a value or error property in the payload was attacker-controlled. An application that dispatches operations from the validated request body could then act on data that never satisfied the route schema, leading to unauthorized state changes or disclosure. Reaching the vulnerable path requires the route to use an $async request schema.
Patches
Fastify no longer treats an asynchronous validation result as a { value, error } wrapper: an async validator's resolved value is used only to determine pass or fail, and it can no longer replace the request part or inject an error. The synchronous custom-compiler contract is unchanged. Patched in fastify 5.12.2 and 6.0.0.
Workarounds
If upgrading is not immediately possible, avoid $async request schemas, or perform the security-sensitive check in an onRequest or preHandler hook rather than relying on the schema-validated request part. Custom async validator compilers should signal failure by throwing (rejecting) rather than returning an { error } object.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/fastifyto a version that resolves this vulnerability.Fixed in 5.12.2 - Upgrade
Upgrade
fastifyto a version that resolves this vulnerability.Fixed in 5.12.2 - Upgrade
Upgrade
fastifyto a version that resolves this vulnerability.Fixed in 6.0.0 - Configuration
Avoid using $async request schemas; alternatively, perform security-sensitive checks in an onRequest or preHandler hook instead of relying on the schema-validated request part.
Fastify route schemas $async request schemas = disabled - Compensating control
For custom asynchronous validator compilers, signal failure by throwing or rejecting rather than returning an { error } object.
Event History
Frequently Asked Questions
Which routes are exposed to this issue?
Only routes that validate a request part with a JSON Schema using $async can reach the vulnerable path. Routes using synchronous validation are not described as affected by this behavior.
What must an attacker provide to exploit the issue?
The attacker must send a request to an affected route with a validated request part containing a value property. The application must then use the resulting request body to dispatch operations or otherwise act on fields that were expected to have been schema-validated.
Does exploitation require authentication or user interaction?
The supplied CVSS vector indicates network access, low privileges, and no user interaction are required. Impact depends on whether the affected application performs sensitive actions based on the altered request data.
How can I determine whether an application may be affected?
Review Fastify route schemas for $async validation on request parts, then identify whether handlers use the validated body to select or dispatch operations. Requests containing a value property are particularly relevant because the vulnerable behavior could replace the entire request part with that nested value.
What changes after applying the patch?
Async validator results are used only to determine whether validation passed or failed. They no longer replace a request part or inject an error based on value or error properties in attacker-controlled input.