Firebase rate limiting in security rules?

Question

I launched my first open repository project, EphChat, and people promptly started flooding it with requests.

Does Firebase have a way to rate limit requests in the security rules? I assume there's a way to do it using the time of the request and the time of previously written data, but can't find anything in the documentation about how I would do this.

The current security rules are as follows.

{
    "rules": {
      "rooms": {
        "$RoomId": {
          "connections": {
              ".read": true,
              ".write": "auth.username == newData.child('FBUserId').val()"
          },
          "messages": {
            "$any": {
            ".write": "!newData.exists() || root.child('rooms').child(newData.child('RoomId').val()).child('connections').hasChild(newData.child('FBUserId').val())",
            ".validate": "newData.hasChildren(['RoomId','FBUserId','userName','userId','message']) && newData.child('message').val().length >= 1",
            ".read": "root.child('rooms').child(data.child('RoomId').val()).child('connections').hasChild(data.child('FBUserId').val())"
            }
          },
          "poll": {
            ".write": "auth.username == newData.child('FBUserId').val()",
            ".read": true
          }
        }
      }
    }
}

I would want to rate-limit writes (and reads?) to the db for the entire Rooms object, so only 1 request can be made per second (for example).

Thanks!

Answer 1

The trick is to keep an audit of the last time a user posted a message. Then you can enforce the time each message is posted based on the audit value:

{
  "rules": {
          // this stores the last message I sent so I can throttle them by timestamp
      "last_message": {
        "$user": {
          // timestamp can't be deleted or I could just recreate it to bypass our throttle
          ".write": "newData.exists() && auth.uid === $user",
          // the new value must be at least 5000 milliseconds after the last (no more than one message every five seconds)
          // the new value must be before now (it will be since `now` is when it reaches the server unless I try to cheat)
          ".validate": "newData.isNumber() && newData.val() === now && (!data.exists() || newData.val() > data.val()+5000)"
        }
      },

      "messages": {
        "$message_id": {
          // message must have a timestamp attribute and a sender attribute
          ".write": "newData.hasChildren(['timestamp', 'sender', 'message'])",
          "sender": {
            ".validate": "newData.val() === auth.uid"
          },
          "timestamp": {
            // in order to write a message, I must first make an entry in timestamp_index
            // additionally, that message must be within 500ms of now, which means I can't
            // just re-use the same one over and over, thus, we've effectively required messages
            // to be 5 seconds apart
            ".validate": "newData.val() >= now - 500 && newData.val() === data.parent().parent().parent().child('last_message/'+auth.uid).val()"
          },
          "message": {
            ".validate": "newData.isString() && newData.val().length < 500" 
          },
          "$other": {
            ".validate": false 
          }
        }
      } 
  }
}

See it in action in this fiddle. Here's the gist of what's in the fiddle:

var fb = new Firebase(URL);
var userId; // log in and store user.uid here

// run our create routine
createRecord(data, function (recordId, timestamp) {
   console.log('created record ' + recordId + ' at time ' + new Date(timestamp));
});

// updates the last_message/ path and returns the current timestamp
function getTimestamp(next) {
    var ref = fb.child('last_message/' + userId);
    ref.set(Firebase.ServerValue.TIMESTAMP, function (err) {
        if (err) { console.error(err); }
        else {
            ref.once('value', function (snap) {
                next(snap.val());
            });
        }
    });
}

function createRecord(data, next) {
    getTimestamp(function (timestamp) {
        // add the new timestamp to the record data
        var data = {
          sender: userId,
          timestamp: timestamp,
          message: 'hello world'
        };

        var ref = fb.child('messages').push(data, function (err) {
            if (err) { console.error(err); }
            else {
               next(ref.name(), timestamp);
            }
        });
    })
}

Answer 2

The existing answers use two database updates: (1) mark a timestamp, and (2) attach the marked timestamp to the actual write. Kato's answer requires 500ms time-window, while ChiNhan's requires remembering the next key.

There is a simpler way to do it in a single database update. The idea is to write multiple values to the database at once using the update() method. The security rules validates the written values so that the write does not exceed the quota. The quota is defined as a pair of values: quotaTimestamp and postCount. The postCount is the number of posts written within 1 minute of the quotaTimestamp. The security rules simply rejects the next write if the postCount exceeds a certain value. The postCount is reset when the quotaTimestamp is staler than 1 minute.

Here is how to post a new message:

function postMessage(user, message) {
  const now = Date.now() + serverTimeOffset;
  if (!user.quotaTimestamp || user.quotaTimestamp + 60 * 1000 < now) {
    // Resets the quota when 1 minute has elapsed since the quotaTimestamp.
    user.quotaTimestamp = database.ServerValue.TIMESTAMP;
    user.postCount = 0;
  }
  user.postCount++;

  const values = {};
  const messageId = // generate unique id
  values[`users/${user.uid}/quotaTimestamp`] = user.quotaTimestamp;
  values[`users/${user.uid}/postCount`] = user.postCount;
  values[`messages/${messageId}`] = {
    sender: ...,
    message: ...,
    ...
  };
  return this.db.database.ref().update(values);
}

The security rules to rate limit to at most 5 posts per minute:

{
  "rules": {
    "users": {
      "$uid": {
        ".read": "$uid === auth.uid",
        ".write": "$uid === auth.uid && newData.child('postCount').val() <= 5",
        "quotaTimestamp": {
          // Only allow updating quotaTimestamp if it's staler than 1 minute.
          ".validate": "
            newData.isNumber()
            && (newData.val() === now
              ? (data.val() + 60 * 1000 < now)
              : (data.val() == newData.val()))"
        },
        "postCount": {
          // Only allow postCount to be incremented by 1
          // or reset to 1 when the quotaTimestamp is being refreshed.
          ".validate": "
            newData.isNumber()
            && (data.exists()
              ? (data.val() + 1 === newData.val()
                || (newData.val() === 1
                    && newData.parent().child('quotaTimestamp').val() === now))
              : (newData.val() === 1))"
        },
        "$other": { ".validate": false }
      }
    },

    "messages": {
      ...
    }
  }
}

Note: the serverTimeOffset should be maintained to avoid clock skew.

Answer 3

I don't have enough reputations to write in the comment, but I agree to Victor's comment. If you insert the fb.child('messages').push(...) into a loop (i.e. for (let i = 0; i < 100; i++) {...} ) then it would successfully push 60-80 meessages ( in that 500ms window frame.

Inspired by Kato's solution, I propose a modification to the rules as follow:

rules: {
  users: {
    "$uid": {
      "timestamp": { // similar to Kato's answer
        ".write": "auth.uid === $uid && newData.exists()"
        ,".read": "auth.uid === $uid"
        ,".validate": "newData.hasChildren(['time', 'key'])"
        ,"time": {
          ".validate": "newData.isNumber() && newData.val() === now && (!data.exists() || newData.val() > data.val() + 1000)"
        }
        ,"key": {

        }
      }
      ,"messages": {
        "$key": { /// this key has to be the same is the key in timestamp (checked by .validate)
           ".write": "auth.uid === $uid && !data.exists()" ///only 'create' allow
           ,".validate": "newData.hasChildren(['message']) && $key === root.child('/users/' + $uid + '/timestamp/key').val()"
           ,"message": { ".validate": "newData.isString()" }
           /// ...and any other datas such as 'time', 'to'....
        }
      }
    }
  }
}

The .js code is quite similar to Kato's solution, except that the getTimestamp would return {time: number, key: string} to the next callback. Then we would just have to ref.update({[key]: data})

This solution avoids the 500ms time-window, we don't have to worry that the client must be fast enough to push the message within 500ms. If multiple write requests are sent (spamming), they can only write into 1 single key in the messages. Optionally, the create-only rule in messages prevents that from happening.