The size of the stake isn't known but is apparently fairly small, The Wall Street Journal reported, citing people familiar with the matter.
Starboard, among the most notable of the activist investment companies, has held talks with CVS management that have, so far, been amicable, according to the report.
Shares of CVS rose $1.27 to close at $76.58 in the regular session of trading, seeing a spike in prices shortly before the close as the report hit. Shares slipped 18 cents to $76.40 in after-hours trading.
CVS is a holding in Jim Cramer's Action Alerts PLUS member club.