Shopify Scripts, MRuby, and the $500,000 Release

April 7, 2017

Here’s a pretty good story about being a little too optimistic with technological implementation, and especially where applied to a language that lets you do whatever you want.

Last year Shopify released a product called Shopify Scripts that lets store owners run arbitrary Ruby code after one of their customers adds an item to their cart. This could be used to apply a product discount or activate a promotion for example. The product ran on MRuby, a lightweight version of the Ruby language which is designed to be more embeddable than its progenitor, and secured in a sandboxing engine of Shopify’s own design called MRubyEngine.

Shopify quickly found out the hard way that an extremely permissive language combined with a nascent C codebase wasn’t a recipe for secure code, and vulnerability reports started flowing in. A few examples:

It’s worth noting that as far as vulnerabilities go, these are all pretty run-of-the-mill bugs. Many security researchers work a lot harder to find problems that are worth a lot less.

Shopify did the only thing they could which was to put in kernel-level sandboxing around MRuby, but by then they must have already paid in close to $450k (the all time total as I’m reading it today is $479,300):

Update Dec 9, 2016: We wish to thank the researchers who have submitted vulnerability reports to the Shopify Scripts program. As of today, we have implemented technical mitigations (seccomp-bpf sandboxing and process isolation) on the application servers hosting Shopify Scripts. As a result, we expect most vulnerabilities will no longer be exploitable without additional bugs in the kernel or seccomp itself, and so we are lowering the payout amounts for our program to 10% of previous levels. Researchers who have submitted issues prior to today will receive payouts under the old structure, however any new reports received will be eligible for payouts within the structure below.

A few takeways come to mind:

  • Writing your own compiler from scratch might not be a good idea anymore, even if you’ve got 30 years of experience doing it (put another way, use LLVM).
  • Overly permissive languages introduce a lot of scary possibilities that are difficult to protect against. Consider something that’s not Ruby.
  • It’s probably not possible to have enough experience in C to be able to write it safely. Looking at some of these MRuby patches, the implementation’s code is so obscuring that it introduces plenty of opportunity for error. When you get one, it’s often a buffer overrun that leads to remote execution.

All of that said, Shopify Scripts is a great idea. I’m glad to see that an approach involving a more depth-wise defense seems to have been successful.

Shopify Scripts, MRuby, and the $500,000 Release was published on April 7, 2017.

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