Sources spoke to The Times, saying that Moonves disclosed the investigation to the board, "which then hired an outside law firm to investigate the matter, the knowledgeable people said."
The board hired an outside firm to dig into the allegations raised by an expose by Ronan Farrow in the New Yorker.
Board members have faced criticism from corporate governance experts, according to The Times.
In the wake of the #MeToo movement, the board's and Moonves' actions are being monitored closely.
Later Thursday, Moonves is expected to be on CBS' earnings call and will be handling questions about the allegations.
Moonves is still serving as the CEO of CBS at this time.
CBS did not immediately respond to requests for comment.
In morning trading, CBS was down 79 cents, at $51.76.