According to IT security vendor, Avira, the problem lasted around 105 minutes and was caused by the URL filter detecting the sites as potential phishing URLs, using a new technique that had appeared earlier in the week.
In a security posting, Dirk Knop, Avira's technical editor, said that the problem only affected a number of domains.
"In such a case, either update Avira AntiVir manually or just wait for the automatic update to take place, the issue is already fixed", he said, adding that the problem occurred due to a special combination of a new techniques used within spam which just recently has been seen.
According to the Softpedia news portal, although Avira AntiVir is one of the biggest providers of free anti-malware products in the world, with more than 140 million users of its software, the vast majority of its users are free users. The WebGuard software is a pay-for option.
Softpedia reports that the phishing emails that Avira was trying to block were using HTML-based tricks to advertise rogue online pharmacies.
"They also included hidden links for several major websites like Google, Yahoo, Amazon or AOL, in an attempt to evade spam filters", said the newswire, adding that there is probably an automated mechanism that adds URLs seen in spam emails to the WebGuard filter and unfortunately the URLs were legitimate.