Spammers have been exploiting a bug in McAfee's software-as-a-service (SaaS) antivirus software to turn PCs into spam relays. As a result, a number of McAfee's customers have had their emails blocked after their Internet protocol (IP) addresses were blacklisted by anti-spam services.

David Marcus, director of security research for McAfee Labs, posted a blog Wednesday detailing the two bugs being exploited to relay spam. Both bugs are in one of its products: McAfee SaaS Endpoint Protection Suite, formerly known as SaaS for Total Protection, which is a hosted anti-malware service.

McAfee, which is owned by Intel, has been actively developing and testing a patch for both bugs, which it plans to make live by Thursday. "Because this is a managed product, all affected customers will automatically receive the patch when it is released," said Marcus.

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Until the patch is ready, there are "mitigating factors already in place that reduce risk" for customers, according to Marcus. In addition, he said there was "no evidence of loss or compromise of any customer data in relation to either of these issues." But network managers who want to be extra safe can proactively disable the Rumor or McAfee Peer Distribution Service, and set external firewalls to block incoming requests to port 6515. (The Kaamar.com website contains detailed instructions for doing this.)

One of the SaaS Endpoint Protection Suite bugs involves an ActiveX control, which an attacker could misuse to execute arbitrary code. Marcus said that a patch it put in place in August 2011, to address a similar issue, had prevented attackers from exploiting the new vulnerability to access customer data.

The second bug involves McAfee's "Rumor technology", which uses peer-to-peer networking to distribute security updates inside a network. Due to the bug, attackers can use machines that run the SaaS Endpoint Protection Suite as open relays for sending large amounts of spam.

"Although this issue can allow the relaying of spam, it does not give access to the data on an affected machine," said Marcus. He said McAfee's forthcoming patch will block the spam-relaying capability.

Two McAfee SaaS Endpoint Protection Suite customers, Keith and Annabel Morgan, posted a blog Monday saying that they'd had emails blacklisted by spam services, since the IP addresses on which they host their own servers were the same ones exploited by spammers via the spam-relay hole in the McAfee product. "We found our IP addresses ... on several public blacklists that had detected the spamming activity passing through our open proxy during the few days it was open."

The couple said they first detected the problem on January 4, when an email was returned, undelivered, with a notice that all email from their IP was being blocked to protect people from spam. By the next day, they said, they'd disabled the Rumor technology and halted the spam relaying. "But [we] received a traffic data limit warning from our ISP that we were approaching our whole month's traffic in only a few days," they said. "At peak we had the equivalent of 10 months of our normal traffic in one day."

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