https://www.comparitech.com/blog/information-security/data-b...
"Stocks of breached companies on average underperformed the NASDAQ by -3.2% in the six months after a breach disclosure"
That said, it's not clear what the long term impact is on stock price (if there is any).
Pick 118 random companies at 118 random points in time. It's vanishingly unlikely that the average returns of that group will exactly track the NASDAQ returns over the following 60 days. It might underperform, or it might overperform. An underperformance of 3.2% could easily just be the result of random chance, and have nothing to do with data breaches.
This wouldn't be that hard to test. I suspect that the breached companies underperformed in the six months before the breach as well as the six months after.