Impact

An attacker can send an invalid Content-Type header that can cause the application to crash, leading to a possible Denial of Service attack. Only the v4.x line is affected.

(This was updated: upon a close inspection, v3.x is not affected after all).

Patches

Yes, update to >v4.8.0.

Workarounds

You can reject the malicious content types before the body parser enters in action.

  const badNames = Object.getOwnPropertyNames({}.__proto__)
  fastify.addHook('onRequest', async (req, reply) => {
    for (const badName of badNames) {
      if (req.headers['content-type'].indexOf(badName) > -1) {
        reply.code(415)
        throw new Error('Content type not supported')
      }
    }
  })

References

See the HackerOne report #1715536.

For more information

Fastify security policy