The world No.12 says it hasn't yet been determined whether the tumour is malignant or benign.
"I've gone through a lot of procedures so far, and with the process of elimination we are narrowing it down," said Kang, after the second round of the US Open. "It's going to take time."
Despite her diagnosis, she stood at three-over par through two rounds at Pine Needles Lodge & Golf Club in Southern Pines.
Kang withdrew from two tournaments earlier this season with back pain.
Her doctors do not know at this stage whether the tumour was the sole cause or a contributing factor to the pain.
She told reporters she didn't plan to share much about her situation, but addressed it because word "got out."
"Right now, it's not as simple as blaming the tumour to be the problem," Kang said.
"It's not just that I have an issue with my back, there is more to it. The scary part is that, I understand, and I didn't want to publicise it, but I know it got out.
"There's more to it. I just don't want to really discuss the details of what's going on in my back."
Kang has won six times on the LPGA Tour, including one major, the 2017 KPMG Women's PGA Championship.
Her most recent victory came in January at the LPGA's Tournament of Champions event.
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