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Protecting APIs against Replay Attack

Replay attacks are very popular against public APIs. 

What is a replay attack ?

When an attacker intercepts a valid HTTP request to your API and then replay the same request again and again tricking your API into thinking that it is a valid HTTP request coming from your user.

Protection

Here is a very simple method that I use to protect my APIs from Replay Attacks.

Client side steps

1. Generate a unique token on server for the device when app is used for the first time or if app requires login then generate token on successful login.

2. Whenever app makes a call to server it adds the following in HTTP headers

  • Token
  • 128 characters long random value (call it Random1)
  • Current Unix Timestamp of device
  • SHA256 of "Token+|||+Random1+|||+Timestamp" (call this result Random2)

Server side steps

When a HTTP request is received by any API it takes the following steps to verify the validity of request

  1. Check if Token is a valid token that exists in database (you can implement expiry mechanism to make it more secure)
  2. Check if Timestamp is valid
  3. Calculate SHA256 of "Token+|||+Random1+|||+Timestamp" and compare it with Random2 present in request headers.
  4. Check if this SHA256 result does not already exists in your database
  5. Save SHA256 result in database 

You can make it more secure by adding more random numbers. In my example you can see I have 3 pipes hardcoded twice (Token+|||+Random1+|||+Timestamp). You can make number of pipes random and then pass those random numbers in request headers.

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