Corporate insiders are dumping stock in their companies at a rate not seen in 10 years.

With September not yet over, stock sales by company executives reached $5.7 billion, according to data from TrimTabs Investment Research -- the highest September in a decade. August's $10.3 billion in insider sales also reached a 10-year record.

At the same time, stock buybacks are roaring ahead, pumping up U.S. share prices to new heights. Companies this year have announced $827 billion in spending to purchase their own shares -- well above the buybacks that took place during all of 2007, which set the previous annual record.

Get Breaking News Delivered to Your Inbox

"Insiders have been committing lots of money for stock buybacks, and they're not doing buybacks because they think stocks are cheap. They're doing to it to pump up the stock so they can sell it," said David Santschi, director of liquidity research at TrimTabs.

Some investors like buybacks because they boost the value of the rest of the company's stock. They also bolster top executives' pay, much of which comes in the form of shares. But they're a purely financial play that don't improve a company's position in the long term, the way investing in equipment or in hiring might.

"Stock buybacks have seemed to be the main use of the corporate tax savings this year. Acquisitions haven't been particularly high, and we're not seeing much wage growth," Santschi added.

TrimTabs' figures underestimate the true amount of corporate insider activity, Santschi noted, because only certain insiders -- a company's highest-ranking executives or directors, as well as investors who hold more than 10 percent of the stock -- are required to disclose stock sales. So nonexecutive employees who sell stock aren't included in this measure.

Buybacks have come under scrutiny of late, with Securities and Exchange Commissioner Robert Jackson calling them out in a recent speech. "We give stock to corporate managers to convince them to create the kind of long-term value that benefits American companies and the workers and communities they serve," he said. "Instead, what we are seeing is that executives are using buybacks as a chance to cash out their compensation at investor expense." In June, a group of senators asked the SEC to review its buyback rules.

Said Santschi: "Insiders are doing one thing with their own money, and another thing with shareholders' money."