IBM has just sent a letter to Twitter charging it with infringement of at least three U.S. patents. They refer to "efficient retrieval of uniform resource locators [URLs]," a "method for presenting advertising in an interactive service" and "programmatic discovery of common contacts," Twitter disclosed in an SEC filing  on Monday.

Twitter isn't that worried, saying they are confident it would prevail. "Based upon our preliminary review of these patents, we believe we have meritorious defenses to IBM’s allegations."

The company added a caveat however that "there can be no assurance that we will be successful in defending against these allegations or reaching a business resolution that is satisfactory to us."

Twitter's IPO is set for either Nov. 6 or Nov. 7, according to reports.