What to do in the first 24 hours of a security incident, and how does MDR help?
The first 24 hours after identifying a security incident define, in most cases, the size of the final damage. Companies that react quickly and in an organized way limit the damage. Companies that take too long to act, or that act without a clear process, usually see the problem spread before they can contain it. This is a practical guide to what to prioritize in these first hours.
The first hours: contain before investigating deeply
As soon as an incident is identified, priority number one is to contain the attack, not necessarily to understand everything that happened. This may mean isolating compromised machines from the network, revoking credentials and access keys that may be compromised, and blocking suspicious traffic identified in the first signs of the attack.
Investigating deeply before containing usually gives extra time for the attacker to advance. Fast containment, even if still incomplete, limits the reach of the problem while the investigation continues in parallel.
Preserve evidence from the first moment
While containment happens, it is important to preserve logs, screenshots, and records of what was observed, without deleting or overwriting information that will be essential to understand the origin and the real reach of the attack later. Rash decisions to reformat systems before collecting this evidence make the investigation harder and can hide important information about how the attacker got in.
Engage the right team immediately
A security incident should not depend on a single person trying to solve everything alone. Quickly engaging those with experience in incident response, whether an internal security team or a contracted MDR service, completely changes the speed and quality of the response in the first hours, which are usually the most critical.
Assess the real reach of the incident
After the initial containment, it is time to map which systems, data, and users were affected. This guides the next decisions, such as which credentials need to be changed, which systems need a deeper scan, and whether there is a legal obligation to notify customers or regulatory authorities about the incident.
Communicate internally with clarity
During the first 24 hours, internal communication needs to be clear and objective, avoiding unnecessary panic but also without minimizing the severity of the situation. Leadership teams and affected areas need to know what is happening, what has already been done, and what is still in progress.
Why does having an MDR completely change these first hours?
An MDR service is already monitoring the environment before the incident happens, which means faster detection, often before the attack causes significant damage. At the moment the incident is identified, the response team is already mobilized, without depending on someone internally to notice the problem and only then start looking for specialized help.
This drastically reduces the time between detection and containment, exactly the window that matters most in the first 24 hours of any security incident.
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