Careful... That Really Simple Syndication (RSS) feed may not just be serving you content, but malware, too.

The wildly popular RSS technology has become compulsory for most Websites and blogs today, but it can also provide attackers another way in. RSS Web feeds basically give users access to content from another site, without having to visit it, and provides Website operators a way to easily expand their content while also increasing their click statistics.

For attackers, RSS provides another conduit for launching cross-site scripting (XSS), cross-site request forgery (CSRF), and other Web-based exploits. "RSS is just another delivery mechanism for XSS and browser exploits," says Caleb Sima, CTO of SPI Dynamics.

"There's been a rush to RSS and it's a hot technology, so people with less experience are cobbling mashups that employ RSS and not thinking about security," Ray Dickenson, senior vice president of product management at Authentium. "That's what makes it ripe for exploitation."

Researchers at Authentium picking apart URLs inside malware have noticed the keyword "blog" more frequently lately than the old standbys of "free" and "triple XXX." "It's well-known that bad guys want to get their code executed in places good guys don't expect, and that starts the ball rolling," Dickenson says. "They want to lure people into clicking a bad link."

But RSS exploits thus far have been limited mostly to minor RSS readers. SPI Dynamics's Sima says these types of attacks should be on your radar screen, and these attacks are more likely to be targeted ones. "I see it as more of a targeted attack at a trusted [RSS] feed provider or popular blog."

And using RSS for an XSS attack is a bit more complicated than the standard Web XSS attack. For one thing, the attacker would need either a trusted blog, or the ability to manipulate one or its comment stream, Sima says. So if you subscribe to a security site that blindly feeds through RSS streams from Full-Disclosure or Bugtraq, for instance, an attacker could post XSS malware to one of those sites, knowing it would get picked up, he says.

"Looking ahead, this attack might get bigger because subscribing to RSS feeds via 'keywords' and doing filtering is becoming more common," with things like Yahoo Pipes, he says. "This allows XYZ Website to not be trusted but just fall into your keyword search for RSS feeds, and then exploit you via that route."

But your risk of that type of attack depends on the security of the RSS reader you use. When JavaScript is enabled in the reader, you're at risk. Luckily, many RSS reader providers are creating encoders to ensure that JavaScript isn't interpreted in the browser, Sim says.

"But I don't see many companies filtering their feeds for malicious data, though. In fact, I know of some well known security people in the industry that blindly feed out from some untrusted sources," he says.

Even worse is when an RSS reader not only executes JavaScript, but stores it. "Some RSS readers not only will execute JavaScript but will save the feed to a local HTML file and read it via the local machine, causing any code to have local privileges," Sim says.

How can you protect yourself from an RSS attack? Use an RSS reader that doesn't allow remote code execution of JavaScript, such as Bloglines, Sima says, adding a bit of perspective to the threat's risk. "RSS exploitation is not a common attack and I don't predict it will ever reach a massive scale."

— Kelly Jackson Higgins, Senior Editor, Dark Reading