A SIEM (Security Information and Event Management) is a platform that collects the logs of many systems, normalises them and correlates them to detect suspicious activity, investigate incidents and keep evidence. It does not protect anything by itself: it is only useful if it receives good-quality data and someone acts on its alerts.
How it works
A SIEM receives events from sources such as servers, devices, firewalls, identity systems (for example Active Directory or Entra ID), email, applications and cloud services. It converts them to a common format, orders them in time and applies correlation rules: for example, many failed sign-in attempts followed by a successful one from another country.
When a rule matches, it raises an alert for a person to review. It also allows searching the history during an investigation and keeping logs for a defined period. Its value depends on which sources are connected, on clocks being synchronised and on rules being tuned to limit false positives. Because logs contain personal data (users, IP addresses), you also have to define who can access them and how long they are kept.
Example
An illustrative example, not a real case. A company connects its user directory, its firewall and its laptops to the SIEM. One night, a finance team member's account signs in from another country and, minutes later, an unusual volume of files is downloaded.
Neither event on its own is alarming, but the correlation rule links them and raises an alert. The person responsible reviews it, blocks the account, resets the credentials and uses the history to see what was accessed. Without centralised logs, that reconstruction would have meant checking each system separately.
Limits
A dashboard with no alerts may mean nothing happened, that a rule is unsuitable or that a source has stopped sending data: source health has to be monitored. A large number of events is not the same as coverage. And a SIEM needs people and a procedure: who reviews, with what priority and when to escalate.
It does not replace EDR, backups, patching or least privilege. Depending on the size of the organisation, it may make sense to start by centralising the most important logs well and defining which use cases matter before buying a platform.
When to call an expert
Ask for help when a rule, a customer or an audit requires logs to be kept and reviewed, when an incident could not be investigated for lack of logs, or when a SIEM already exists and nobody reviews or tunes its alerts.
Before choosing a tool, an assessment should answer which sources matter, which use cases are a priority, who responds to alerts and how long data is kept. At Impulso Tecnológico we work on diagnosis, technical controls and training: see our cybersecurity services or tell us about your case.
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Related glossary terms: SOC, EDR, MDR, XDR and IDS and IPS.
Official sources: the guide NIST SP 800-92 on security log management and the NCSC guidance Building a Security Operations Centre.