New Malware Ruins Firefox

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Late last year, we read all the buzz about ChromeInject, a malicious DLL that was being billed as the first malware specifically targeting Firefox. It was interesting to see Installashunthat someone built a phishing Trojan for a different browser platform, but ChromeInject was also clearly an early phase in Firefox malware development: It was fairly obvious, and it was easy to eliminate, because it generated an entry in the Plugins menu called “Basic Example Plugin for Mozilla” which you could simply disable with a single mouse click.

Well now it looks like the bar’s been raised. In the past few weeks, we’ve seen malware writers up the ante in their bets against Firefox. Two new spies came across the transom in the past week, and easily managed to load themselves into a freshly installed copy of Firefox 3.0.7. I should note that this isn’t due to any problem or negligence on Mozilla’s part; once you execute malicious code on your PC, any application is vulnerable. Firefox just happens to be a big target.

The first is a malicious plugin that, structurally, looks like it might be a new variant of a spy we’ve seen before: DNSChanger (we sometimes call it Trojan-Downloader-Ruin), a browser hijacking tool. Unlike DNSChanger, which modifies the DNS settings in Windows itself, this plugin doesn’t add any discernible registry keys in order to do its job. The installer drops a DLL payload into the C:Program FilesMozilla Firefoxcomponents folder, and works a little juju; then when you next start Firefox, it runs in the background.

Firesox hooked into Firefox, as seen in Process Explorer

Firesox hooked into Firefox, as seen in Process Explorer

Like DNSChanger, it injects ads or modified results when it detects search query strings sent to sites like Google, Yahoo, MSN, Altavista, Teoma, Ask, Pricegrabber, and a whole mess of other sites both in the .com and the .ru top level domains. It sends queries through the same Ukrainian IP address space — the 85.255.x.x subnet — DNSChanger used to use. It even calls itself by a cutesy name: Firesox.

Strings dumped from Firesox

Strings dumped from Firesox

In the past, we saw DNSChanger used to help fraudulent advertising affiliates boost their numbers, and to direct unsuspecting users to rogue antimalware tools by generating bogus results. It remains to be seen whether this new variant will be as prolific as the old version.

The second is a piece of adware that only installs correctly with Firefox 3.x installed — it won’t install under Firefox 2.x. We got a copy of it bundled along with the installer for a third-party Firefox plugin called PlayMP3z. The PlayMP3z installer includes a lengthy EULA, which explicitly says that the software is ad-supported and spells out the atrocious terms if you install the music streaming plugin.

PlayMP3z installer dialog

But what looks like a concession to the interests of the end user, offered during installation, doesn’t turn out so rosy. During the installation, you’re given a choice to opt out of installing a well-known toolbar called Mirar Webband. We, of course, assume that Mirar is the adware client, and that if we choose not to install it, we won’t be saddled with ad popups.

But that’s misleading: Mirar is not the only adware the product installs; even if you deselect the Mirar checkbox, you still get saddled with something that calls itself Foxicle (installed under c:documents and settingsall usersdocumentsfoxicle), which itself generates popup and popunder ads.

PlayMP3z warning about uninstallation lists all the possible names its adware might use

PlayMP3z warning about uninstallation lists all the possible names its adware component might use

Neither Firesox, the DNSChanger clone, or Foxicle put an obvious entry in Firefox’s plugins dialog (to get there, click the Tools -> Add Ons menu items) that signal their presence. While not widely distributed, I suspect we’ll be seeing more of them.


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  1. [...] installiert – neben weiteren, heimlichen Bestandteilen, die ohnehin nicht abwählbar sind. Neue Schadsoftware installiert beispielsweise eine DLL-Datei in den Components-Ordner von Firefox und wird beim [...]