Discharging sebaceous cyst on the neck of a 72 year old female patient. A sebaceous cyst is a sac-like bump under the skin filled with sebum, the oily product of the skin's sebaceous glands. It occurs when the glands becomes blocked. Most cysts heal on their own, but some require surgical removal. Anyone can get a skin cyst and it's not always known what causes them. a family member also has cysts (particularly pilar cysts). Older adults, women and those who have acne are at higher risk of developing them.