CVE-2026-84428

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

Impact

Fastify lowercases header-schema property names before compiling the schema, because Node.js stores request header names in lowercase. That normalization was incomplete: it lowercased only top-level properties keys and the root required array, and did not lowercase the JSON Schema Draft 7 dependencies keyword (its trigger keys and dependent property names) or names in nested subschemas. As a result, a header schema that uses dependencies to require one header when another is present (for example X-Admin requiring X-Admin-Token) never matches the lowercased request headers, so the dependency assertion is silently skipped. An unauthenticated remote client can send the header that activates a privileged path while omitting the header the dependency was meant to require, bypassing a schema-enforced security control. The header schema is idiomatic, valid JSON Schema Draft 7, and no custom validator, malformed request, or misconfiguration is required.

Patches

Header-schema names are now normalized across all schema positions (properties, required, dependencies, dependentRequired, dependentSchemas, and nested subschemas). Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release. Header schemas referenced through an external shared $ref (registered with addSchema) are not reached by this normalization and now emit an FSTSEC002 startup warning; inline the header schema to keep case-insensitive assertions in effect.

Workarounds

If upgrading is not immediately possible, write header-schema names in lowercase so the dependencies and other case-sensitive assertions match Node's lowercased request headers, or enforce the cross-header requirement in an onRequest or preValidation hook instead of the schema.

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

Impact

Fastify lowercases header-schema property names before compiling the schema, because Node.js stores request header names in lowercase. That normalization was incomplete: it lowercased only top-level properties keys and the root required array, and did not lowercase the JSON Schema Draft 7 dependencies keyword (its trigger keys and dependent property names) or names in nested subschemas. As a result, a header schema that uses dependencies to require one header when another is present (for example X-Admin requiring X-Admin-Token) never matches the lowercased request headers, so the dependency assertion is silently skipped. An unauthenticated remote client can send the header that activates a privileged path while omitting the header the dependency was meant to require, bypassing a schema-enforced security control. The header schema is idiomatic, valid JSON Schema Draft 7, and no custom validator, malformed request, or misconfiguration is required.

Patches

Header-schema names are now normalized across all schema positions (properties, required, dependencies, dependentRequired, dependentSchemas, and nested subschemas). Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release. Header schemas referenced through an external shared $ref (registered with addSchema) are not reached by this normalization and now emit an FSTSEC002 startup warning; inline the header schema to keep case-insensitive assertions in effect.

Workarounds

If upgrading is not immediately possible, write header-schema names in lowercase so the dependencies and other case-sensitive assertions match Node's lowercased request headers, or enforce the cross-header requirement in an onRequest or preValidation hook instead of the schema.