If you're using 'Password1,' change it. Now.

SAN FRANCISCO (CNNMoney) -- The number one way hackers get into protected systems isn't through a fancy technical exploit. It's by guessing the password.

That's not too hard when the most common password used on business systems is "Password1."

There's a technical reason for Password1's popularity: It's got an upper-case letter, a number and nine characters. That satisfies the complexity rules for many systems, including the default settings for Microsoft's (MSFT, Fortune 500) widely used Active Directory identity management software.

Security services firm Trustwave spotlighted the "Password1" problem in its recently released "2012 Global Security Report," which summarizes the firm's findings from nearly 2 million network vulnerability scans and 300 recent security breach investigations.

Around 5% of passwords involve a variation of the word "password," the company's researchers found. The runner-up, "welcome," turns up in more than 1%.

Easily guessable or entirely blank passwords were the most common vulnerability Trustwave's SpiderLabs unit found in its penetration tests last year on clients' systems. The firm set an assortment of widely available password-cracking tools loose on 2.5 million passwords, and successfully broke more than 2.1 million of them.

Verizon came up with similar results in its 2012 Data Breach Investigations Report, one of the security industry's most comprehensive annual studies. The full report will be released in several months, but Verizon (VZ, Fortune 500) previewed some of its findings at this week's RSA conference in San Francisco.

Exploiting weak or guessable passwords was the top method attackers used to gain access last year. It played a role in 29% of the security breaches Verizon's response team investigated.

Verizon's scariest finding was that attackers are often inside victims' networks for months or years before they're discovered. Less than 20% of the intrusions Verizon studied were discovered within days, let alone hours.

Even scarier: Few companies discovered the breach on their own. More than two-thirds learned they'd been attacked only after an external party, such as a law-enforcement agency, notified them. Trustwave's findings were almost identical: Only 16% of the cases it investigated last year were internally detected.

So if your password is something guessable, what's the best way to make it more secure? Make it longer.

Adding complexity to your password -- swapping "password" for "p@S$w0rd" -- protects against so-called "dictionary" attacks, which automatically check against a list of standard words.

But attackers are increasingly using brute-force tools that simply cycle through all possible character combinations. Length is the only effective guard against those. A seven-character password has 70 trillion possible combinations; an eight-character password takes that to more than 6 quadrillion.

Even a few quadrillion options isn't a big deal for modern machines, though. Using a $1,500 computer built with off-the-shelf parts, it took Trustwave just 10 hours to harvest its first 200,000 broken passwords.

"We've got to get ourselves using stuff larger than human memory capacity," independent security researcher Dan Kaminsky said during an RSA presentation on why passwords don't work.

He acknowledged that it's an uphill fight. Biometric authentication, smartcards, one-time key generators and other solutions can increase security, but at the cost of adding complexity.

"The fundamental win of the password over every other authentication technology is its utter simplicity on every device," Kaminsky said. "This is, of course, also their fundamental failing."