One interesting LESS feature I found when we were evaluating it for letting some users use it for customization: it can evaluate arbitrary Javascript and open and embed files. Just in case you thought it was some pure transformative shorthand CSS syntax.

Of course, letting users upload arbitrary CSS served from your domain is an XSS-level risk, but let's say you let each user have his own domain and let them upload .less files.

Or you have a say, node.js APP that lets a user enter LESS and spit out the -- escaped -- CSS.

Now they can do:

    div.hack {
      background-image: data-uri("/etc/passwd");
    }

When compiled with lessc 1.4.2, this embeds the contents of that file as base64.

Also, with 1.3.x."@import" lets you open any filename and insert it, but it's required to be valid .less. I suppose you could try to @import /dev/random or urandom although I don't know if there's any practical attack surface for decreasing entropy. I don't think I could offhand find a valid file.

However in the theorethical web page evaluating .less, the error output will contain the contents of the file @imported.

Oh, and the evaluation of Javascript... I couldn't make it do much useful, as it seems there's some kind of limited JS environment that code executes in. The worst I could do was:

    div { 
      background: `process.kill(-1, "SIGKIL")`;
    }

The "fs" object does not seem to exist there, but I don't know enough about node.js/V8 environment to see whether there's equivalent of jailbreaks.

So, uh, don't compile arbitrary LESS files and send users their output, and LESS files someone sends you or checks into a project can kill your processes (at least).