What Is SIEM? Security Information and Event Management for SMEs

What Is a SIEM and How Does It Work?

Security Information and Event Management (SIEM) is a category of cybersecurity technology that combines two functions: security information management (collecting and storing security logs from across the IT environment) and security event management (analysing those logs in real time to detect threats). A SIEM platform ingests log data from multiple sources — Windows event logs from endpoints and servers, authentication logs from Active Directory or Entra ID, firewall and network device logs, cloud service audit logs, email security alerts, and endpoint protection telemetry — and analyses this data continuously using correlation rules, threat intelligence, and increasingly machine learning.

The detection power of SIEM comes from correlation — the ability to identify patterns across multiple data sources that would be invisible when looking at any single source in isolation. A single failed login attempt is not an alert worth investigating. But a pattern of failed logins from multiple geographic locations, followed by a successful login from an unfamiliar IP address, followed by the creation of an inbox forwarding rule to an external email address — each individual event might appear innocuous in isolation; together they indicate a compromised account being actively exploited. SIEM's correlation engine identifies these multi-stage patterns across sources and time, generating a single actionable alert that a security analyst can investigate.

The need for this kind of cross-source visibility is significant. According to the DSIT Cyber Security Breaches Survey 2025, 43% of UK businesses experienced a cybersecurity breach or attack in the past twelve months. Many of these attacks involve multiple stages that span different systems — initial access through email, credential theft from an endpoint, lateral movement through identity systems, and data exfiltration through cloud services. Without a platform that correlates events across these sources, multi-stage attacks can proceed undetected.

How SIEM Works: The Technical Process

A SIEM operates through several interconnected processes. First, log collection agents or connectors gather event data from across the IT estate. This includes Windows Security Event logs recording login attempts and privilege usage, firewall logs showing network traffic patterns, Microsoft 365 audit logs tracking file access and sharing, Entra ID sign-in logs recording authentication events, endpoint detection and response (EDR) alerts, DNS query logs, and email gateway logs. The volume of data is substantial — even a small business with 50 users can generate millions of log events per day.

Second, the SIEM normalises this data into a consistent format. Different systems record events in different structures — a Windows login event looks nothing like a firewall connection log or a cloud service audit entry. Normalisation translates these disparate formats into a common schema so that correlation rules can operate across sources without needing source-specific logic for every rule.

Third, the correlation engine applies detection rules to the normalised data. These rules range from simple threshold alerts (more than five failed logins in one minute) to complex multi-stage detections (failed VPN login followed by successful cloud login from a different country within ten minutes, followed by bulk file download). Modern SIEM platforms supplement rule-based detection with User and Entity Behaviour Analytics (UEBA), which establishes baselines of normal behaviour for each user and device and flags significant deviations.

Fourth, when a detection rule triggers, the SIEM generates an alert with the correlated evidence — all the related events, the users and devices involved, and the timeline of activity. This alert must then be investigated by a security analyst to determine whether it represents a genuine threat or a false positive (legitimate activity that matches a detection pattern).

SIEM vs SOC: Understanding the Difference

A common source of confusion is the relationship between SIEM and SOC. A SIEM is a technology platform — it collects logs, applies correlation rules, and generates alerts. A SOC (Security Operations Centre) is a team of people — security analysts who monitor the SIEM, investigate alerts, and respond to confirmed threats. The SIEM generates the alerts; the SOC provides the human intelligence to interpret them and take action.

This distinction matters because deploying a SIEM without a SOC is like installing a burglar alarm that nobody monitors. The alerts will be generated, but without trained analysts to investigate them, genuine threats will be lost in a sea of notifications that accumulate without action. For large enterprises, an in-house SOC team monitors the SIEM around the clock. For SMEs, a managed SOC service provides this analyst capability without the need to hire and retain specialist security staff internally.

Only 14% of UK businesses have a formal incident response plan (DSIT Cyber Security Breaches Survey 2025). Without both a detection platform and the people to act on its output, security monitoring provides a false sense of confidence rather than genuine protection.

Why SIEM Is Challenging for SMEs

Traditional SIEM platforms — Microsoft Sentinel, Splunk, IBM QRadar, Elastic Security — are powerful but demand significant expertise and resources to operate effectively. The challenges for SMEs are substantial:

• Configuration complexity — Setting up log ingestion from all relevant sources, writing and tuning detection rules, and maintaining the platform requires dedicated security engineering expertise that most SMEs do not have in-house
• Alert fatigue — A poorly tuned SIEM can generate hundreds of alerts per day, the majority of which are false positives. Without the analyst capacity to investigate each alert, genuine threats are buried in noise
• Cost — SIEM platforms are typically priced by data ingestion volume. As log volumes grow, costs can escalate rapidly. Microsoft Sentinel charges per gigabyte of data ingested, with costs that can reach thousands of pounds per month for even moderate environments
• Ongoing maintenance — Detection rules must be continuously updated to address new attack techniques. Log source connectors break when systems are updated. False positive tuning is an ongoing process, not a one-time configuration
• Analyst requirement — Even a perfectly configured SIEM produces alerts that require human investigation. Without security analysts to review and act on alerts, the SIEM provides no security value

For most UK SMEs without dedicated security staff, operating a standalone SIEM in-house is not practical. A SIEM that generates hundreds of daily alerts that no one investigates provides no security benefit — it merely creates an audit trail that proves the organisation was alerted to threats it failed to act upon.

When SMEs Genuinely Need SIEM Capability

Despite the challenges, there are scenarios where SMEs need the capabilities that SIEM provides:

• Regulatory compliance — Some regulations (particularly in financial services, healthcare, and legal sectors) require centralised log retention and audit trail capability for specified periods. SIEM provides the centralised log storage and search capability needed to meet these requirements
• Cyber insurance requirements — Insurers increasingly require evidence of security monitoring as a condition of coverage. A managed SIEM or MDR service can satisfy this requirement
• Supply chain requirements — Larger clients or government contracts may require evidence of active security monitoring as part of supplier due diligence
• Post-incident investigation — After a security incident, historical log data is essential for understanding the full scope of a breach, identifying how the attacker gained access, and determining what data was affected. Without SIEM log retention, forensic investigation is severely limited
• Multi-source attack detection — Attacks that span multiple systems (email, identity, endpoint, cloud) can only be detected by correlating events across sources — the core capability of SIEM

The average cost of a data breach for UK organisations was £3.4 million (IBM Cost of a Data Breach Report, 2024). For businesses handling sensitive data or operating in regulated sectors, the cost of SIEM capability — whether operated directly or consumed as a managed service — is modest relative to the potential cost of an undetected breach.

SIEM for Microsoft 365 Environments

For businesses using Microsoft 365, Microsoft provides built-in security monitoring through the Microsoft 365 Defender portal. M365 Defender correlates signals across its component products — Defender for Business (endpoint), Defender for Office 365 (email), and Entra ID Protection (identity) — to detect multi-stage attacks within the Microsoft ecosystem. This provides a degree of cross-source correlation without requiring a separate SIEM platform.

Microsoft Sentinel, Microsoft's cloud-native SIEM, extends this capability by ingesting M365 Defender alerts alongside data from non-Microsoft sources — third-party firewalls, Linux servers, SaaS applications, and custom data sources. Sentinel provides the full SIEM experience — custom detection rules, UEBA, automated response playbooks, and long-term log retention — within the Azure cloud platform.

AMVIA uses Microsoft Sentinel as the SIEM layer for clients requiring full log correlation and retention capability. For most SME clients, AMVIA's managed detection and response service provides equivalent detection outcomes by combining M365 Defender's built-in correlation with AMVIA's AmviaIQ monitoring platform and human analyst investigation — delivering the security outcomes of SIEM without requiring clients to manage a full Sentinel deployment independently.

MDR vs SIEM: What SMEs Actually Need

The distinction between SIEM (a technology) and MDR (a service) is the most important concept for SME security planning. A SIEM tool generates alerts — humans must investigate and respond to them. An MDR service includes the human analysts who do the investigating and responding, using SIEM technology (or equivalent detection platforms) as the underlying engine.

For SMEs, the bottleneck is rarely the lack of a log collection platform. The bottleneck is the lack of expertise to configure detection rules, tune false positives, investigate alerts, and respond to confirmed threats. MDR addresses this directly by providing the analysts as a managed service. The MDR provider operates the detection technology, investigates every alert, filters out false positives, and escalates confirmed threats with clear remediation guidance — or takes direct containment action when an active attack is identified.

Most UK SMEs need MDR, not a standalone SIEM. AMVIA's managed detection and response service provides cross-source threat detection, expert alert investigation, and active incident response — the outcomes that SIEM enables, delivered as a managed service rather than a technology the business must operate and staff itself.

Cost Context for SMEs

SIEM costs vary significantly depending on the platform and data volume. Microsoft Sentinel is priced per gigabyte of data ingested, with costs that can range from a few hundred to several thousand pounds per month depending on the number of log sources and the volume of data. Splunk and other commercial SIEM platforms have similar volume-based pricing models. These costs are in addition to the analyst time required to operate the platform and investigate alerts.

Managed SIEM and MDR services bundle the technology and analyst costs into a predictable monthly fee, typically priced per user or per endpoint. For most SMEs, this managed approach is more cost-effective than attempting to build in-house capability — the combined cost of a SIEM licence, security analyst salaries, and ongoing training significantly exceeds the cost of a managed service that provides equivalent or superior detection outcomes.

Key Considerations for UK SMEs

• SIEM is a technology platform that requires people to configure, maintain, and investigate its output — without analysts, it provides no security value
• For businesses without dedicated security staff, MDR delivers SIEM-equivalent detection outcomes as a managed service with human analysts included
• Log retention requirements may be driven by compliance — some regulations require audit trail retention for 12 months or longer
• Microsoft 365 Defender provides meaningful cross-service correlation for M365 environments — a solid foundation before considering dedicated SIEM
• Assess detection maturity first — the question is not "do we have a SIEM?" but "are suspicious events being detected, investigated, and acted upon?"
• Consider the total cost of ownership — SIEM licence plus analyst time plus ongoing tuning versus managed MDR service with predictable monthly pricing

How AMVIA Can Help

AMVIA provides managed detection and response as a service that delivers SIEM-equivalent visibility and detection for UK SMEs. AMVIA's AmviaIQ platform aggregates security signals from Microsoft 365 Defender, endpoint security tools, and network monitoring — correlating events and escalating genuine threats for expert investigation. For clients requiring full SIEM log retention and advanced correlation capability, AMVIA can deploy and manage Microsoft Sentinel as part of a comprehensive managed SOC service. Contact AMVIA on 0333 733 8050 to discuss security monitoring requirements for your business.