10 things you need to know before the opening bell

Here is what you need to know.

The S&P 500 looks to close out 2017 with a bang. The benchmark index is on track to open up 0.27% near 2,695. It's gained about 20% this year.

Short volatility was the stock market's "trade of the year." Two exchange-traded products used to short volatility have surged almost 200% in 2017, dwarfing returns for even the best-performing stocks in major indexes.

Traders are betting billions the hottest stocks of 2017 will get crushed next year. Facebook, Apple, Netflix, and Google are among the most heavily shorted right now, according to data from financial analytics firm S3 Partners.

Ethereum's founder warns on crypto. "*All* crypto communities, ethereum included, should heed these words of warning," Ethereum founder Vitalik Buterin said on Twitter Thursday. "Need to differentiate between getting hundreds of billions of dollars of digital paper wealth sloshing around and actually achieving something meaningful for society."

Ripple hits a record high. Ripple's XRP trades up 26% at $1.5926 a coin, according to Markets Insider data. It's gained nearly 5,200% in 2017.

Apple apologizes for slowing down iPhones with older batteries. "We've been hearing feedback from our customers about the way we handle performance for iPhones with older batteries and how we have communicated that process," Apple said in a public letter. "We know that some of you feel Apple has let you down. We apologize."

Softbank is buying a big chunk of Uber. The Japanese investment firm agreed to buy a 15% stake in Uber for $48 billion, or a 30% discount to its recent $69 billion valuation. Softbank will also invest $1.25 billion in Uber directly.

Las Vegas tourism keeps sliding. Visitation fell 3.7% year-over-year in November, running the streak of consecutive monthly declines to six - the longest in eight years, the Las Vegas Review Journal says.

The People's Bank of China frees up cash for the Lunar New Year. China's central bank said in a statement out Friday that some banks will be allowed to lower their reserve requirement ratios by up to 200 basis points, for 30 days, Reuters reports.

Overseas markets end close out 2017. China's Shanghai Composite gained 13.71% in 2017 and Germany's DAX is on track to finish the year up 28.26%, Bloomberg data shows.