Ctrl-F template strings... yep, there they are.<p>So if you didn't know, ES6 added template strings, which are these really awesome things because they have multiline and string interpolation capabilities (and they're safely far away from JSON, which in my opinion shouldn't have such capabilities). They are very pretty and incredibly handy; you write<p><pre><code> console.log(`
I'm so ${scared} in case ${ I.fall.off() } my chair
And I'm wonderin' how I'll get down the stair
`);
</code></pre>
and this gets converted into<p><pre><code> console.log("\nI'm so " + scared + " in case " + I.fall.off() +
" my chair\nAnd I'm wonderin' how I'll get down the stair\n");
</code></pre>
Except for one thing: they're called "template strings" because actually this is a sort of "default behavior" which can be metaprogrammed. There is a default interpreter which could be written:<p><pre><code> function interpret(text_segments, ...split_segments) {
var out = "";
for (var i = 0; i < split_segments.length; i++) {
out += text_segments[i] + split_segments[i];
}
return out + text_segments[i];
}
</code></pre>
but... you can write one of your own, if you want, and put it on the beginning. Therefore:<p><pre><code> > console.log(`abc ${[1,2,3]} def`)
abc 1,2,3 def
undefined
> console.log `abc ${[1,2,3]} def`
[ 'abc ', ' def' ] [ 1, 2, 3 ]
undefined
</code></pre>
Notice that the side effect of console.log has happened with the arguments given to it, allowing for code execution.<p>As for mitigation... add detection of backticks to whatever code was detecting parentheses. It's not a very widely used symbol in any context other than shell scripting and LaTeX anyways, so you're probably good to go if you just outlaw that character before calling eval() on the whole.