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It's a little obscure, but look at Embryo: http://docs.enlightenment.org/auto/embryo/

The interpreter was specifically designed with the assumption that Embryo scripts would come from untrusted sources. I believe that Enlightenment allows Embryo scripts to be embedded inside of theme files, for example.

Maybe Lua would be a good candidate? From the usages I have seen of Lua, it could match the use case.

OP, if you do go with Lua, this sandbox seems to work when combined with ulimit: https://github.com/jnwhiteh/luasandbox

If you test on Mac OS, you should be aware that ulimit is not capable of limiting memory in Mac OS.

Make it language neutral - communicate by HTTP. That would also greatly reduce security concerns.