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Diversified Tech Solutions Diversified Tech Solutions Technology · Security · Compliance

What Is a SIEM (Security Information and Event Management)?

Definition

A SIEM (security information and event management) system collects logs from across a business, such as firewalls, servers, laptops, and cloud services, puts them in one place, and analyzes them to raise alerts on suspicious activity. NIST describes SIEM software as "a program that provides centralized logging capabilities for a variety of log types."

How It Works

A SIEM does four jobs:

  1. Collect: agents and connectors pull events from many sources into one store.
  2. Normalize: different log formats are converted so a sign-in from Microsoft 365 and one from a VPN can be compared.
  3. Correlate and alert: rules link related events, for example many failed sign-ins followed by a success from a new country, and raise an alert.
  4. Search and report: investigators query months of history in one place.

Joint guidance published in May 2025 by Australia's ASD with CISA and other partners, Implementing SIEM and SOAR Platforms, stresses that a SIEM is not "set and forget." It needs skilled staff to tune detections and review alerts. The guidance also notes that "most SIEM pricing models are based on the quantity of data the SIEM ingests," so cost depends on choosing the right logs, and it advises implementing a SIEM before adding SOAR automation. Central collection has a defensive side benefit: copies outside the attacked system are harder for an intruder to delete.

CISA built a no-cost tool for small and medium organizations, Logging Made Easy (LME), but its page now says LME "is no longer being maintained," so check the status of any free tool before you depend on it.

Real-World Example

In 2022, CISA's red team gained persistent access to a large critical infrastructure organization that CISA described as having "a mature cyber posture." CISA's advisory AA23-059A, published February 28, 2023, reports that the organization "did not detect the red team's activity throughout the assessment, including when the team attempted to trigger a security response." The defenders missed lateral movement, persistence, and command and control traffic in their intrusion detection systems, endpoint protection, web proxy logs, and Windows event logs. The data existed. What was missing was tuned correlation and alerting. CISA recommended creating alerts for Windows authentication events, especially on domain controllers.

Why It Matters for a Small Business

A small business does not have a security team watching a dozen consoles. Without a central view, an attack shows up as scattered, quiet events that nobody connects: a strange sign-in here, a new mailbox rule there. A SIEM, or a managed service that runs one, turns those into a single alert someone can act on. The trade-off is cost and attention. A SIEM that ingests everything gets expensive, and one that nobody tunes produces noise that staff learn to ignore. The right size for most small businesses is a focused set of high-value logs, a few well-tested detections, and a person responsible for reviewing them.

How to Spot It / How DTS Handles It

Signs you need one: logs sit on separate systems, no one would notice an admin sign-in at 3 a.m., or an investigation would start by asking each vendor for data.

DTS can help with choosing the logs worth collecting, sending them to a right-sized SIEM, and tuning alerts so the automated monitoring raises fewer, better alerts for a person to review. See our managed IT service, or our vCISO service for planning a logging and detection program.

Related Terms

  • Security logging: the raw material a SIEM uses.
  • SOAR: automation added after a SIEM.
  • SOC: the team that works SIEM alerts.
  • Alert fatigue: what an untuned SIEM causes.
  • False positive: alerts that turn out benign.
  • Log retention: how far back a SIEM can search.
  • EDR: endpoint telemetry a SIEM correlates.

Sources

  • ASD, CISA and partners, Implementing SIEM and SOAR Platforms: Executive Guidance (May 27, 2025): https://www.cisa.gov/resources-tools/resources/guidance-siem-and-soar-implementation
  • NIST CSRC Glossary, Security Information and Event Management (SIEM) Software (from SP 800-92): https://csrc.nist.gov/glossary/term/security_information_and_event_management
  • CISA, CISA Red Team Shares Key Findings to Improve Monitoring and Hardening of Networks, AA23-059A (February 28, 2023): https://www.cisa.gov/news-events/cybersecurity-advisories/aa23-059a
  • CISA, Logging Made Easy (archived service page): https://www.cisa.gov/resources-tools/services/logging-made-easy

Last updated October 14, 2026.