Saturday, April 07, 2018

nginx reverse-proxy tricks for jwt and csrf

I've been doing devops work over the last few months at CDIS. One of the tasks I worked on recently was to extend the nginx configuration (on github as a kubernetes configmap here) for the gen3 data commons stack.

First, we added end-user data to the proxy log by extracting user information from the JWT token in the auth cookie. We took advantage of the nginscript (nginscript is a subset of javascript) to import some standard javascript code (also in the kubernetes configmap) to parse the JWT.

Next, we added a CSRF guard that verifies that if a CSRF cookie is present, then it matches a CSRF header. The logic is a little clunky (in github here), because of the restrictions on conditionals in nginx configuration, but it basically looks like the sample configuration below - where $access_token is the JWT auth token (either from a header from non-web clients, or a cookie for web browser clients), and an "ok-SOMETHING" $csrf_check is only required for browser clients. Finally - we do not enforce the HTTP-header based CSRF check in the proxy for endpoints that may be accessed by traditional HTML form submissions (that embed the token in the form body), or for endpoints that are accessed by third party browser frontends (like jupyterhub) that may implement a CSRF guard in a different way. Finally, there's also a little bit of logic that auto-generates a CSRF cookie if it's not present - a javascript client in the same domain can read the cookie to get the value to put in the X-CSRF header.

          set $access_token "";
          set $csrf_check "ok-tokenauth";
          if ($cookie_access_token) {
              set $access_token "bearer $cookie_access_token";
              # cookie auth requires csrf check
              set $csrf_check "fail";
          }
          if ($http_authorization) {
              # Authorization header is present - prefer that token over cookie token
              set $access_token "$http_authorization";
          }

          #
          # CSRF check
          # This block requires a csrftoken for all POST requests.
          #
          if ($cookie_csrftoken = $http_x_csrf_token) {
            # this will fail further below if cookie_csrftoken is empty
            set $csrf_check "ok-$cookie_csrftoken";
          }
          if ($request_method != "POST") {
            set $csrf_check "ok-$request_method";
          }
          if ($cookie_access_token = "") {
            # do this again here b/c empty cookie_csrftoken == empty http_x_csrf_token - ugh  
            set $csrf_check "ok-tokenauth";
          }
          ...
          location /index/ {
              if ($csrf_check !~ ^ok-\S.+$) {
                return 403 "failed csrf check";
              }
              if ($cookie_csrftoken = "") {
                add_header Set-Cookie "csrftoken=$request_id$request_length$request_time$time_iso8601;Path=/";
              }

              proxy_pass http://indexd-service/;
          }
          ...