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.

Impact

Fastify routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the preHandler declared in its setNotFoundHandler(). When the request method has no route in the main router, a malformed request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected.

Patches

Patched in fastify 5.12.2. Malformed URLs are now routed through the configured onBadUrl and onMaxParamLength handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed.

Workarounds

Reject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global onRequest authentication hook does not mitigate this, because the malformed-URL path skips it.

Impact

Fastify decided whether to validate a request part by checking its schema for JavaScript truthiness. JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance, but because false is falsy, a route that set body, querystring, params, or headers to false had that part left uncompiled: no validator was attached and the request reached the handler. An application that used false as a deny-all schema to make a route unreachable was therefore fully bypassed, and an unauthenticated remote client could reach the handler with any input. The same applied to the documented query alias for querystring. This is a complete bypass rather than a weak-schema issue, since false is the strongest JSON Schema assertion and must always fail.

Patches

Request-part schemas are now selected by an explicit presence check rather than truthiness, so a boolean false (or true) schema is compiled and enforced, including through the query alias. Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release.

Workarounds

If upgrading is not immediately possible, express a deny-all request schema with an always-failing object schema instead of the boolean false (for example { "not": {} }), or reject the request in an onRequest hook.

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.

Impact

The fix for CVE-2026-3635 (GHSA-444r-cwp2-x5xf) added a proxyFn(socket.remoteAddress, 0) guard on the X-Forwarded- reads in request.host, request.protocol, request.hostname, request.ip, and request.ips. That guard closes the IP, CIDR, and custom-function forms of trustProxy correctly because those forms compile to predicates that inspect the connecting address. The hop-count form (trustProxy: <number>) compiles to a predicate that structurally ignores the address argument, so the guard reduces to 0 < tp, always true for any tp >= 1.

Applications configured with trustProxy: <number> (documented as "behind N reverse proxies", trustProxy: 1 being the canonical single-proxy setting) remain vulnerable. An attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the request fields exactly as in the unpatched version. Impact class matches the parent CVE-2026-3635: host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie / CSRF-origin bypass, host-based routing and cache poisoning.

Patches

Patched in fastify 5.12.1. The numeric form of trustProxy is now disabled at runtime and removed from the TypeScript type union.

Workarounds

- Migrate to an IP / CIDR / custom-function trustProxy value that validates the connecting address. Custom functions must inspect the address argument, not only the hop index. - Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).

Impact

fastify before 5.12.1, when a route uses a root-level primitive body schema (for example an integer with a minimum and maximum) and the default type coercion, validates the coerced value but exposes the original, uncoerced value to the route handler. For example, a JSON body "10" is coerced to the number 10 and passes an integer 1 to 10 schema, but request.body stays the string "10". An application that trusts the validated type is handed a value that did not satisfy the schema, which can bypass limits the application enforces on that typed value. Object and array body schemas are not affected, they coerce their members in place.

Patches

Upgrade to fastify 5.12.1.

Workarounds

Until you can upgrade, avoid relying on the validated type of a root primitive body. Wrap the value in an object schema (object properties are coerced in place), for example accept { "value": 10 } and read request.body.value, or re-check the type in the handler.

Impact

A validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type.

For example, a request with Content-Type: application/json\ta will bypass JSON schema validation but still be parsed as JSON.

This vulnerability affects all Fastify users who rely on Content-Type-based body validation schemas to enforce data integrity or security constraints. The concrete impact depends on the handler implementation and the level of trust placed in the validated request body, but at the library level, this allows complete bypass of body validation for any handler using Content-Type-discriminated schemas.

This issue is a regression or missed edge case from the fix for a previously reported vulnerability.

Patches

This vulnerability has been patched in Fastify v5.7.2. All users should upgrade to this version or later immediately.

Workarounds

If upgrading is not immediately possible, user can implement a custom onRequest hook to reject requests containing tab characters in the Content-Type header:

javascript fastify.addHook('onRequest', async (request, reply) => { const contentType = request.headers['content-type'] if (contentType && contentType.includes('\t')) { reply.code(400).send({ error: 'Invalid Content-Type header' }) } })

Resources

- https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272 - https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125 - Fastify Validation and Serialization Documentation - https://hackerone.com/reports/3464114

Summary A validation bypass vulnerability exists in Fastify v5.x where request body validation schemas specified via schema.body.content can be completely circumvented by prepending a single space character (\x20) to the Content-Type header. The body is still parsed correctly as JSON (or any other content type), but schema validation is entirely skipped. This is a regression introduced by commit f3d2bcb (fix for CVE-2025-32442).

Details The vulnerability is a parser-validator differential between two independent code paths that process the raw Content-Type header differently. Parser path (lib/content-type.js, line ~67) applies trimStart() before processing: js const type = headerValue.slice(0, sepIdx).trimStart().toLowerCase() // ' application/json' → trimStart() → 'application/json' → body is parsed ✓

Validator path (lib/validation.js, line 272) splits on /[ ;]/ before trimming:

js function getEssenceMediaType(header) { if (!header) return '' return header.split(/[ ;]/, 1)[0].trim().toLowerCase() } // ' application/json'.split(/[ ;]/, 1) → [''] (splits on the leading space!) // ''.trim() → '' // context[bodySchema][''] → undefined → NO validator found → validation skipped!

The ContentType class applies trimStart() before processing, so the parser correctly identifies application/json and parses the body. However, getEssenceMediaType splits on /[ ;]/ before trimming, so the leading space becomes a split point, producing an empty string. The validator looks up a schema for content-type "", finds nothing, and skips validation entirely. Regression source: Commit f3d2bcb (April 18, 2025) changed the split delimiter from ';' to /[ ;]/ to fix CVE-2025-32442. The old code (header.split(';', 1)[0].trim()) was not vulnerable to this vector because .trim() would correctly handle the leading space. The new regex-based split introduced the regression.

PoC

js const fastify = require('fastify')({ logger: false });

fastify.post('/transfer', { schema: { body: { content: { 'application/json': { schema: { type: 'object', required: ['amount', 'recipient'], properties: { amount: { type: 'number', maximum: 1000 }, recipient: { type: 'string', maxLength: 50 }, admin: { type: 'boolean', enum: [false] } }, additionalProperties: false } } } } } }, async (request) => { return { processed: true, data: request.body }; });

(async () => { await fastify.ready();

// BLOCKED — normal request with invalid payload const res1 = await fastify.inject({ method: 'POST', url: '/transfer', headers: { 'content-type': 'application/json' }, payload: JSON.stringify({ amount: 9999, recipient: 'EVIL', admin: true }) }); console.log('Normal:', res1.statusCode); // → 400 FSTERRVALIDATION

// BYPASS — single leading space const res2 = await fastify.inject({ method: 'POST', url: '/transfer', headers: { 'content-type': ' application/json' }, payload: JSON.stringify({ amount: 9999, recipient: 'EVIL', admin: true }) }); console.log('Leading space:', res2.statusCode); // → 200 (validation bypassed!) console.log('Body:', res2.body);

await fastify.close(); })();

Output: Normal: 400 Leading space: 200 Body: {"processed":true,"data":{"amount":9999,"recipient":"EVIL","admin":true}}

Impact Any Fastify application that relies on <code>schema.body.content</code> (per-content-type body validation) to enforce data integrity or security constraints is affected. An attacker can bypass all body validation by adding a single space before the Content-Type value. The attack requires no authentication and has zero complexity — it is a single-character modification to an HTTP header. This vulnerability is distinct from all previously patched content-type bypasses:

CVE | Vector | Patched in 5.8.4? -- | -- | -- CVE-2025-32442 | Casing / semicolon whitespace | ✅ Yes CVE-2026-25223 | Tab character (\t) | ✅ Yes CVE-2026-3419 | Trailing garbage after subtype | ✅ Yes This finding | Leading space (\x20) | ❌ No

Recommended fix — add trimStart() before the split in getEssenceMediaType: js function getEssenceMediaType(header) { if (!header) return '' return header.trimStart().split(/[ ;]/, 1)[0].trim().toLowerCase() }

Fastify is a web framework with minimal overhead and plugin architecture. The attacker can use the incorrect Content-Type to bypass the Pre-Flight checking of fetch. fetch() requests with Content-Type’s essence as "application/x-www-form-urlencoded", "multipart/form-data", or "text/plain", could potentially be used to invoke routes that only accepts application/json content type, thus bypassing any CORS protection, and therefore they could lead to a Cross-Site Request Forgery attack. This issue has been patched in version 4.10.2 and 3.29.4. As a workaround, implement Cross-Site Request Forgery protection using @fastify/csrf'.

Summary

When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and X-Forwarded-Host headers from any connection — including connections from untrusted IPs. This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application.

Affected Versions

fastify <= 5.8.2

Impact

Applications using request.protocol or request.host for security decisions (HTTPS enforcement, secure cookie flags, CSRF origin checks, URL construction, host-based routing) are affected when trustProxy is configured with a restrictive trust function.

When trustProxy: true (trust everything), both host and protocol trust all forwarded headers — this is expected behavior. The vulnerability only manifests with restrictive trust configurations.

Description

Fastify incorrectly accepts malformed Content-Type headers containing trailing characters after the subtype token, in violation of RFC 9110 §8.3.1. For example, a request sent with Content-Type: application/json garbage passes validation and is processed normally, rather than being rejected with 415 Unsupported Media Type.

When regex-based content-type parsers are in use (a documented Fastify feature), the malformed value is matched against registered parsers using the full string including the trailing garbage. This means a request with an invalid content-type may be routed to and processed by a parser it should never have reached.

Impact

An attacker can send requests with RFC-invalid Content-Type headers that bypass validity checks, reach content-type parser matching, and be processed by the server. Requests that should be rejected at the validation stage are instead handled as if the content-type were valid.

Workarounds

Deploy a WAF rule to protect against this

Fix

The fix is available starting with v5.8.1.

Impact A Denial of Service vulnerability in Fastify’s Web Streams response handling can allow a remote client to exhaust server memory. Applications that return a ReadableStream (or Response with a Web Stream body) via reply.send() are impacted. A slow or non-reading client can trigger unbounded buffering when backpressure is ignored, leading to process crashes or severe degradation.

Patches The issue is fixed in Fastify 5.7.3. Users should upgrade to 5.7.3 or later.

Workarounds Avoid sending Web Streams from Fastify responses (e.g., ReadableStream or Response bodies). Use Node.js streams (stream.Readable) or buffered payloads instead until the project can upgrade.

References - https://hackerone.com/reports/3524779

Impact

In applications that specify different validation strategies for different content types, it's possible to bypass the validation by providing a slightly altered content type such as with different casing or altered whitespacing before ;.

Users using the the following pattern are affected:

js fastify.post('/', { handler(request, reply) { reply.code(200).send(request.body) }, schema: { body: { content: { 'application/json': { schema: { type: 'object', properties: { 'foo': { type: 'string', } }, required: ['foo'] } }, } } } })

User using the following pattern are not affected:

js fastify.post('/', { handler(request, reply) { reply.code(200).send(request.body) }, schema: { body: { type: 'object', properties: { 'foo': { type: 'string', } }, required: ['foo'] } } })

Patches

This was patched in v5.3.1, but unfortunately it did not cover all problems. This has been fully patched in v5.3.2. Version v4.9.0 was also affected by this issue. This has been fully patched in v4.9.1.

Workarounds

Do not specify multiple content types in the schema.

References Are there any links users can visit to find out more?

https://hackerone.com/reports/3087928

Fastify node module before 0.38.0 is vulnerable to a denial-of-service attack by sending a request with "Content-Type: application/json" and a very large payload.

A denial of service vulnerability exists in Fastify v2.14.1 and v3.0.0-rc.4 that allows a malicious user to trigger resource exhaustion (when the allErrors option is used) with specially crafted schemas.