Editors' note: This story was originally published on Dec. 16

The aftermath of the hacking into Yahoo! (YHOO) subscriber accounts will resonate into 2017 and beyond.

Yahoo!'s latest revelation that 1 billion accounts were hacked in 2013, on top of the 500 million hijacked accounts in 2014, comes as CEO Marissa Mayer tries to close a $4.8 billion sale to Verizon Communications (VZ) - Get Report . The breaches underscore the growing importance of cyber security due diligence as the ambitions and capabilities of hackers grow.

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"Not that long ago privacy and cyber security were really on the radar screens for buyers only when they were looking for certain types of businesses-ecommerce, other B2C companies that collect a lot of information about consumers," Morrison & Foerster lawyer Christine Lyon said, referring to hacks that targeted wallets and identities and were more limited in scope than today's mega-attacks. The perception was the cyber security was about [protecting against] someone hacking into your systems, stealing credit card numbers or social security numbers and selling them."

M&A teams have taken note. In a Morrison & Foerster survey shortly after the disclosure in September of the 2014 Yahoo! attacks, 80% of bankers, lawyers and executives who work on transactions said they are putting more emphasis on cyber security when they conduct due diligence. Buyers can write legal protections into merger agreements and bring in experts from companies FireEye (FEYE) - Get Report or the Big Four accounting and consulting firms. Fundamentally, they must also decide how much risk they can stomach.

The Yahoo! 2014 breach sent investors and analysts to the sale agreement's material and adverse change clause, which can allow buyers to walk away from a deal if a problem causing long-term detriment to the target arose. With the newest announced hack Yahoo!'s case just became more complicated.

While Yahoo!'s hack does not include credit card numbers, it dwarfs other prominent breaches. The November 2013 attack of Target (TGT) - Get Report , which occurred months after Yahoo!'s first incursion, exposed 40 million credit cards. The hack of Home Depot (HD) - Get Report in November 2014, which occurred around the same time as the second Yahoo! data heist, involved 56 million credit cards and 53 million email accounts.

While MAC clauses offer protection in some cases, Morrison & Foerster lawyer Robert Townsend said that generally they are not the ideal defense from breaches at a target. "If a MAC is the only way you're protected with respect to cyber security breaches, the likelihood that you as a buyer would able to rely on that as a basis for not closing a transaction is low," Townsend said.

"The courts have found that in order to be able to use material adverse change clause to not consummate a transaction it has to be large, material and an ongoing detriment to the company that fundamentally impairs its value, as opposed to a one-time event that might over time not impact the value of the company as much," Townsend added. "You can see the legal challenges with showing that a cyber breach would constitute a MAC," Townsend said.

Other provisions in the representations and warranties section of the merger agreement may be more useful to a buyer of hacked operations. These representations, in which the seller makes pledges regarding the state of certain aspects of the business, face a lower standard than the MAC clause for a buyer.

Increasingly, attackers will lurk in networks without overt signs of a hack or detection by IT departments. So if a company can't tell that its own networks are clean, how can it know with certainty that a company it's buying is secure?

"One of the challenges you get into with the bidding process -- no different from buying a house -- is the diligence itself is going to be limited in nature," said Sean Curran, director of security and infrastructure at West Monroe Partners. "You get to walk around but you don't get to pull the drywall off to see if there are termites." West Monroe Partners provides due diligence on about 240 transactions a year and has its own cyber security team. It works primarily with PE-backed firms.

The merger of Yahoo! and Verizon is something akin to "two elephants dancing," he said, with buyer and seller containing vast portfolios of assets. Verizon would combine Yahoo!'s advertising technology, user base and other assets with its own digital holdings. Calculating the impact of the hacks is complex math.

After the costs that can logged into the company ledger, there is the drop in stock price over time and damage to the business and brand, which are difficult to asses. "Where do you draw the line and say this is the impact?" Curran asked.

Buyers companies need to evaluate the risk associated with their purchase against their investment thesis, he suggested. If a company is purchasing a peer to gain new technology or products it can bundle with its own offerings, the damage from a hack may be less than if it were paying up for a large group of subscribers that were compromised.

Hacks will continue, but so will acquisitions. "There are very few organizations who are saying if I found out there was a breach or I found out there are serious security holes I would walk away from a deal," Curran said.