TYKJS Middleware Gotcha When Base 64 Decoding Data

As mentioned in Unit Testing Your TYK (TYKJS) Middleware, I've recently been playing around with the TYK API Gateway and in this example I had been introspecting JSON Web Tokens (JWTs).

As seen in my post Pretty Printing JSON Web Tokens (JWTs) on the Command Line using Ruby, JWTs are actually just base64 encoded JSON objects. So I was doing something like:

var payload_str = b64dec(payload_encoded);
var payload = JSON.parse(payload_str);
if ('openid' == payload.scopes) {
  // ...
}

This was working in my unit tests for the middleware, but not when I was actually running it on an instance (running in Docker).

In some cases of the JWTs I was introspecting, I would receive an error similar to:

[Nov 28 22:17:18] ERROR jsvm: Failed to base64 decode: illegal base64 data at input byte 0

What was weird was how certain JWTs weren't being parsed, but others were.

As TYK provides the b64dec function to the JSVM on run, I had to fake it in my unit tests, using the below snippet from this StackOverflow question:

/*
 * Licensed under the Attribution-ShareAlike 3.0 Unported
 * https://stackoverflow.com/a/23097961/2257038
 */
function b64dec(str){
  return Buffer.from(str).toString('base64');
}

When googling for the error message, I found a StackOverflow post with a similar error, calling out the error coming from a Go program. This led to the realisation that instead of using the JavaScript implementation, would use the Go equivalent for (presumably) speed.

This meant that we were actually seeing a Go issue, not a Node one.

Looking in the mw_js_plugin.go file in the Tyk v2.7.4 source code, we can see that it does not ignore padding:

/*
 * NOTE: This file is licensed under the Mozilla Public License 2.0 (MPL-2.0)
 * which can be read in full at
 * https://github.com/TykTechnologies/tyk/blob/v2.7.4/LICENSE.md and the source
 * of this file can be seen at
 * https://github.com/TykTechnologies/tyk/blob/v2.7.4/mw_js_plugin.go#L421-L438
 */
j.VM.Set("b64dec", func(call otto.FunctionCall) otto.Value {
  in := call.Argument(0).String()
  out, err := base64.StdEncoding.DecodeString(in)
  if err != nil {
    log.WithFields(logrus.Fields{
      "prefix": "jsvm",
    }).Error("Failed to base64 decode: ", err)
    return otto.Value{}
  }
  returnVal, err := j.VM.ToValue(string(out))
  if err != nil {
    log.WithFields(logrus.Fields{
      "prefix": "jsvm",
    }).Error("Failed to base64 decode: ", err)
    return otto.Value{}
  }
  return returnVal
})

Because Tyk doesn't handle padding, we instead need to make sure that our middleware does this for us.

This fix was fairly straightforward - we would make sure to pad the base64 encoded payload to appease the Go code.

By padding the base64 string correctly, we now have the following:

+function pad(str) {
+  while(0 !== (str.length % 4)) {
+    str = str + '=';
+  }
+  return str;
+}
+
+var payload_encoded_padded = pad(payload_encoded);
-var payload_str = b64dec(payload_encoded);
+var payload_str = b64dec(payload_encoded_padded);
 var payload = JSON.parse(payload_str);
 if ('openid'' == payload.scopes) {
   // ...
 }

Finally, as a way to future-proof my testing, I also made sure that my b64dec fake in my tests would also throw an error to catch invalid padding, so in the future I'd be able to catch it at the test level:

 /*
  * Licensed under the Attribution-ShareAlike 3.0 Unported
  * Modified from https://stackoverflow.com/a/23097961/2257038
  */
 function b64dec(str){
+  if (0 !== (str.length % 4)) {
+    throw "Base64 string not padded"
+  }
   return Buffer.from(str).toString('base64');
 }

It appears that issue #1808 covers this, and although a fix was made, this has not yet been resolved. I've reached out about looking to see if I can raise a PR to fix it.

Written by Jamie Tanna on 10 December 2018, and last updated on 30 December 2018.

Content for this article is shared under the terms of the Creative Commons Attribution Non Commercial Share Alike 4.0 International, and code is shared under the Apache License 2.0.

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