Avoid using keywords as sentinel values in transducers


The use of keywords as sentinels in transducers could in rare circumstances expose some applications to bugs and potential security risks.

Ideally a private or local value that cannot be injected into the functions domain should be used instead, e.g. (Object.).


All environments


Rick Moynihan
January 13, 2018, 1:49 AM

Thanks for upping my privileges Alex.

I didn't mean to over egg the pudding, but perhaps I ended up doing it all the same!

My reasoning was that if you're using a transducer to map/reduce over values taken from user input, then an attacker could cause a loop to abort earlier than expected; which might then have other consequences. I did't have a specific attack vector in mind, but I'd seen also the sentinel ::halt and had imagined that a user could inject "clojure.core/halt" into a HTTP header. If ring has a middleware to keywordize keys then you have the sentinel value for a transducer injected by a user, and that sentinel could then potentially be used to shortcut the validation/encoding/escaping etc of other fields.

However I've looked a bit more at the implementation of halt-when and other transducers and it seems the key isn't ever mixed with user data; and that ::none is used to indicate the first pass over by some transducer functions. So it looks like I was mistaken. I doubt now that this is worth resolving.

Alex Miller
January 12, 2018, 1:53 PM

Rick - I added you to the necessary groups for edit privileges.

How is this a security risk?

Rick Moynihan
January 12, 2018, 12:39 PM

Also thanks to @reborg and @bronsa on #clojure-uk for sharing the above snippet.

Rick Moynihan
January 12, 2018, 12:24 PM

Apologies for messing up the formatting on the snippet, I also didn't mean to assign this as major. Though I can't seem to edit it, perhaps someone else can.

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Rick Moynihan



Affects versions