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Rate limiting

Last updated: June 21th, 2022

The Customs server is the service responsible for maintaining and implementing rate limiting in FxA. It was developed to help detect and deter fraud and abuse.

Currently, only it is used by the Firefox Accounts Auth Server.

The Customs server uses a sliding window to keep track of the current count of a specific type of action and trims the count as needed.

Policies​

There are two types of policies:

  • rate-limiting: slows down attackers by temporarily blocking requests for 15 minutes (see config.limits.rateLimitIntervalSeconds)
  • block / ban: stops attacks by temporarily blocking requests for 24 hours (see config.limits.blockIntervalSeconds)

We currently have the following policies in place:

  • rate-limiting when too many emails (config.limits.maxEmails defaults to 3) have been sent to the same email address in a given time period (config.limits.rateLimitIntervalSeconds defaults to 15 minutes)
  • rate-limiting when too many requests to look up account status by email address (config.limits.maxAccountStatusCheck) have been sent from the same ip address during
  • rate-limiting when too many sms (config.limits.smsRateLimit.maxSms) have been sent from the same ip address during period (config.limits.smsRateLimit.limitIntervalSeconds defaults to 60 minutes)
  • rate-limiting when too many sms (config.limits.smsRateLimit.maxSms) have been sent from the same email address during period (config.limits.smsRateLimit.limitIntervalSeconds defaults to 60 minutes)
  • rate-limiting when too many sms (config.limits.smsRateLimit.maxSms) have been sent to the same phone number during period (config.limits.smsRateLimit.limitIntervalSeconds defaults to 60 minutes)
  • rate-limiting when too many failed login attempts (config.limits.maxBadLogins defaults to 2) have occurred for a given account and IP address, in a given time period (config.limits.rateLimitIntervalSeconds defaults to 15 minutes)
  • rate-limiting too many attempts to verify randomly-generated codes (config.limits.maxVerifyCodes defaults to 10) have occurred for a given account and IP address, in a given time period (config.limits.rateLimitIntervalSeconds defaults to 15 minutes)
  • manual blocking of an account (see /blockEmail API call)
  • manual blocking of an IP address (see /blockIp API call)

The data that these policies are based on is stored in a memcache instance (keyed by email, ip or ip + email depending on the policy) and the code that implements them is split across these three files:

  • email_record.js handles blocking and rate-limiting based only on the email address
  • ip_email_record.js handles rate-limiting based on both the email and IP address of the request
  • ip_record.js handles blocking based only on the IP address

The rate-limiting and blocking policies are conveyed to the auth server via the block property in the response to /check.

Usage in Auth-server​

In the auth-server, we utilized a helper library to make calls to the custom server api. The request could contain email, ip, and the action being performed.

For example, the call customs.check(request, email, 'accountCreate') would increment the accountCreate event for the user coming from this ip address and email.

Below are the current actions supported:

// Actions that, if allowed, would allow an attacker
// to try a candidtate password against an account.
const PASSWORD_CHECKING_ACTION = {
  accountLogin: true,
  accountDestroy: true,
  passwordChange: true,
};

// Actions that, if allowed, would allow an attacker
// to try a guess at a randomly-generated security code.
// Code are higher entropy so we can allow more of these,
// but if you're doing it a lot, you're probably a baddie.
const CODE_VERIFYING_ACTION = {
  recoveryEmailVerifyCode: true,
  passwordForgotVerifyCode: true,
  verifyRecoveryCode: true,
  verifySessionCode: true,
  // not high entropy
  // changes at 60 seconds
  // limits by email
  verifyTotpCode: true,
};

// Actions that, if allowed, would allow an attacker
// to check whether an account exists or has certain
// properties for a particular user.
// Basically any unauthenticated endpoint that takes
// an email address as input.
const ACCOUNT_STATUS_ACTION = {
  accountCreate: true,
  accountLogin: true,
  accountDestroy: true,
  passwordChange: true,
  passwordForgotSendCode: true,
  accountStatusCheck: true,
  sendUnblockCode: true,
  recoveryKeyExists: true,
};

// Actions that send an email, and hence might make
// us look like spammers if abused.
const EMAIL_SENDING_ACTION = {
  accountCreate: true,
  createEmail: true,
  recoveryEmailResendCode: true,
  recoveryEmailSecondaryResendCode: true,
  passwordForgotSendCode: true,
  passwordForgotResendCode: true,
  sendUnblockCode: true,
};

// Actions that can send sms, and could make us
// very annoying to a user if abused.
const SMS_SENDING_ACTION = {
  connectDeviceSms: true,
};