When the :preserve comments option is not enabled (which is the default behavior), Crass discards CSS comments by recursively consuming the next token. An attacker who provides a stylesheet containing a very large number of adjacent comments can cause excessive recursion and trigger a SystemStackError.

This is fixed in Crass 1.0.7, which no longer recurses when discarding comments.

Set the :preserve comments option to true if this is acceptable to your use case (recursion doesn't occur when comments are preserved).