** Shares of IT consulting firm Cognizant CTSH.O rise about 5% to $87.46 in extended trade
** Activist investor Mantle Ridge has built a stake of over $1 billion in CTSH, according to the Wall Street Journal
** Mantle Ridge believes Cognizant's shares undervalued; it started buying shares in the second half of 2022 - report
** Activist investor has been working behind the scenes at CTSH and isn't currently planning to nominate directors - WSJ
** CTSH up 9% so far this year
(Reporting by Juby Babu in Mexico City)
((Juby.Babu@thomsonreuters.com;))