Fifty-four zero-day vulnerabilities were discovered last year, according to a report released this morning by Symantec, more than double that of 2014, and the number of mega-breaches of more than 10 million records also hit a record high.

In fact, the number of newly-discovered vulnerabilities stayed between eight and 15 a year since 2006, then jumped to 23 in 2013 and 24 in 2014, leading researchers to hope that it had reached a new plateau.

Instead, last year's 125 percent increase in zero-days was a sign of the increasing professionalization of the industry.

"People figured out that they could make money by finding zero-day vulnerabilities and selling them to attackers," said Kevin Haley, director of security response at Symantec. "So there became a marketplace, and these things started to have value, and people started to hunt for them."

According to Symantec, attack groups exploit the zero-day vulnerabilities until they are publicly exposed, then move on to new ones.

The most attractive zero-days for attackers are those in widely-used software such as Internet Explorer and Adobe Flash.

Last year, four of the five most exploited zero-days were in Adobe Flash.

Overall, Symantec discovered a total of more than 430 million new unique pieces of malware total in 2015, up 36 percent from 2014.

In addition, according to the report, spear-phishing campaigns targeting individual employees increased 55 percent in 2015 and ransomware increased 35 percent.

But there was also some good news. The overall email spam rate last year was 53 percent, down from 60 percent in 2014 and 66 percent in 2013.

Generic phishing emails, those not targeted at specific individuals, accounted for just one in 1,846 emails, down dramatically from one in 965 in 2014 and one in 392 in 2013.

The overall email malware rate was one in 220 in 2015, down from one in 244 in 2014.

The number of bots also fell, from 2.3 million in 2013 and 1.9 million in 2014 to 1.1 million in 2015. According to Symantec, this is due to successful law enforcement activity against cyber criminals and increased cybersecurity awareness in general.

One significant exception was China. Even as the number of bots in the United States fell by 67 percent, the number of bots originating in China rose by 84 percent, so that China now accounts for 46 percent of the global population of bots, as a result of increased broadband penetration in that country.

More companies hiding breach data

The total number of personal identities exposed as a result of breaches rose 23 percent to 429 million, the second-highest year on record after 2013.

But 2015 also saw a record-setting nine mega-breaches, and the largest single data breach ever reported, at 191 million U.S. voter records.

More worrisome, however, is that the number of companies choosing not to report the number of records they have lost rose by 85 percent, from 61 to 113.

"More and more companies aren't actually revealing what was breached," said Haley. "They will say attackers came and stole from us, but not saying how many records were lost."

Not all companies have to disclose all the details of every breach, he explained -- the disclosure laws vary by location and industry.

Symantec estimates that the total number of records breached could be more than half a billion.