After a discussion we had privately with gregory draperi (on PMC private ML) some time ago, I decided to apply his recommended changes in appropriate places.
I had a look to the source code and I would like to speak about how we use Random and Math.Random() java classes.
To give a little bit of context, these two functions are marked as unsecured because if you are able to know one output you are able to predict the future outputs.
As a short overview random java function relies on a linear congruential generator (a complex word for a simple thing). Reference http://franklinta.com/2014/08/31/predicting-the-next-math-random-in-java/
LCG works like this: (a * x + c) mod m where a,c and m are static values and x is the previous output (except for the first output where we determine x and it is called the seed)
So for example with a = 3, c = 5, m = 7 and we start with the seed x = 0,
then the next few random numbers generated will be: 5 = (3 * 0 + 5) mod 7,
6 = (3 * 5 + 5) mod 7, 2 = (3 * 6 + 5) mod 7, 4 = (3 * 2 + 5) mod 7
It means that if I know the last output (4) I can predict what will be the
(3 * 4 +5) mod 7 -> 3.
We use a lot of Math.Random which relies on Random() class and also Random() class itself especially in the LoginWorker class where it is used to generate the external link id. At that time I have no real proven vulnerability but that worries me for two things .
- If it is possible to retrieve the state of Math.Random() from another place in the code it will be possible to predict future links
- Even if the link is complex and almost impossible to guess by brute force we downgrade the security as the space of possible values for the link id is really lower than the space for session id. Example: 3EBA63DE1C0BE677A17CAB483689F9C vs EL708615493372
The topic is not urgent but I would recommend in the future using SecureRandom class and maybe UUID.randomUUID to generate the external link id
Because using SecureRandom comes with a cost, I have identified the places where it's reasonable to keep the non secured Random (like tests, internal sequences, etc.).
Using UUID.randomUUID to generate the external link id seems also the best possible solution.
Also, though there are no real proven vulnerabilities, I decided to backport as much as possible since it's now public.