GreaseMonkey: a two-edged sword

The core idea behind GreaseMonkey is simple: it allows the user to associate JavaScript programs with specific sites on the net. When one of the identified pages (as determined by a regular expression) is loaded, the script gets a chance to rewrite things before the page is displayed. GreaseMonkey is, in other words, a mechanism which enables readers to automatically rework web pages into the form they would have liked them to be in the first place.

The GreaseMonkey script repository shows that there is a demand for this capability. Scripts have been posted which:

• Remove articles or comments posted by specific users. Perhaps this would be a quick way to implement the comment filtering features occasionally requested for LWN.net.

• Rewrite web pages to get rid of intrusive navigation bars, interstitial ad pages, etc. For those who want more ads, there is a script which inserts Google ads into the handful of pages on the net which do not yet have them.

• Redirect SourceForge download links to skip the mirror selection page and simply get the requested files.

• Delete Michael Jackson stories from certain news sites ("Best. Userscript. Ever.").

• Rewrite Paul Graham's articles for better readability.

• Create cross links between Netflix and IMDB.

And so on; the list appeared to be growing as this article was being written.

The operators of various web sites will, beyond doubt, get upset if GreaseMonkey use takes off. To anybody who wishes to have a high degree of control over the appearance and use of their site, GreaseMonkey will be a threat. But GreaseMonkey is a clear expression of software freedom: we will control how things work on our own computers. Our tools are written to maximize that control, and there is little that can be done about it.

GreaseMonkey does, however, potentially threaten that control in a different way. A tool which encourages users to download and run scripts from random parts of the net would appear to be an open door for security problems. If the browser's sandboxing works properly, a script should not be able to affect the system outside of the browser. But even the mere ability to rewrite HTML is asking for some trouble: how long will it be until some phisher posts a script that, while perhaps doing something useful, also redirects links within financial sites? It is not entirely clear how that sort of problem can be addressed - the same capability which can redirect all New York Times links to the "printable" version can point a password submission form to a third-party site.

In other words, while GreaseMonkey is a cool and powerful tool, it should be used with great care. Install a minimum number of scripts, look them over first, and, preferably, write them yourself. As the GreaseMonkey community grows, there will certainly be exploit attempts. Firefox is a relatively secure web browser; it would be a shame to ruin that by inviting in random malware from the net.