Use crypto/rand instead of math/rand

[elithrar]

randomBytes in xsrf.go uses math.rand to generate CSRF tokens. This is unsafe/insecure, and because it is seeded with time.UnixNano, generates predictable results that would allow an attacker to bypass CSRF protection.

sessionID in session.go correctly uses crypto/rand to generate a session ID. You should split this code out and re-use it for both session ID generation and CSRF token generation.

(You're also welcome to import or repurpose https://github.com/gorilla/csrf for this as well, which has some additional mitigations against CSRF bypass)

[elithrar]

(I'll also gladly provide a PR to address this, but wanted to get buy in/explain it first!)

[dinever]

@elithrar Thanks for pointing this out. Please feel free to open a PR.

https://github.com/gorilla/csrf looks really nice. I would prefer to repurpose it and put a credit in the README. Thanks again!

[elithrar]

OK, great. To clarify, I plan on:

• Making sure crypto/rand is the only way tokens are generated throughout the framework (first and foremost)
• Import gorilla/csrf for generating CSRF tokens (second)

I'd strongly recommend that you import gorilla/csrf—which has a strict API stability guarantee, but would also be vendored by downstream users—rather than copying the code into your library so that any security patches or feature additions are kept in-sync. Ultimately that's up to you though.