The release of French police officers' personal details was a security breach because it puts them and their families in danger. It is also a more personal breach to each of the officers. Sharing any persons information can be a danger to them and others.Them being police puts far more people at risk.
Anytime someone's personal information is released without their consent that is considered a breach. These officers are no different. They expect to keep their personal lives out of any type of light so that they and their families can be safe, no matter if they make mistakes. Releasing that information was not a way of protecting their security.
The recent attacks that have been going on should be cause for heightened security. Because their details have been released people who wish to do harm to them will be able to use those details against those police officers. There has been a security breach because the details were meant to be kept confidential.
The ranks of the French police force were rocked after many of their personal details were released. This should be considered a serious breach of the country's security because it makes these officers more exposed and vulnerable to people who may use those personal details for their own nefarious gain.