SIEM (Security Information and Event Management) is a system that collects logs from servers, firewalls, email, cloud services and business applications in one place, correlates them to turn suspicious patterns into alerts, and retains them for investigation. Log management gathers and stores records; SIEM goes a step further and tells you that an attack may be under way.
The logs exist, but nobody reads them
Few companies lack logs. The firewall records every connection, the domain controller every sign-in, the mail server every message and the ERP every user transaction. The trouble is that these records sit on ten different devices, in ten different formats, and are often overwritten after a few days.
When an incident happens, the story is familiar. Someone signs in to the finance manager's account from abroad at midnight, and shortly afterwards hundreds of files are read from a file server. Both traces are recorded somewhere, but because nobody sees them together, the incident only surfaces weeks later when a supplier asks why they received an odd email from you.
The questions that follow are always the same: when did the attacker get in, which account did they use, and what data did they touch? Without complete, centralised logs the answers are guesswork. For companies operating in Türkiye, the clock on data breach notification within 72 hours under KVKK makes guesswork the most expensive answer of all.
What unmonitored logs cost you
The longer detection takes, the longer an attacker stays inside and the more damage they can do. IBM's Cost of a Data Breach Report 2025 found that the global average time to identify and contain a breach, including restoring services, was 241 days. Organisations that used AI and automation extensively in security operations shortened that lifecycle by 80 days on average.
According to Sophos' State of Identity Security 2026 survey of 5,000 IT and security leaders in 17 countries, only 24% of responding organisations continually monitor for unusual login attempts.
Where attacks begin also explains why monitoring matters. The Verizon 2026 Data Breach Investigations Report found that exploitation of vulnerabilities became the most common initial access vector, at 31% of breaches. An exploit attempt against a server that slipped through your patch management process shows up in firewall and web server logs; unread, it goes unnoticed.
There is a regulatory side as well. KVKK, Türkiye's Personal Data Protection Law (Law No. 6698), requires data controllers to take all necessary technical and administrative measures to ensure an appropriate level of security, and a company that cannot show who accessed personal data, and when, will struggle to evidence those measures. Separately, Law No. 5651, Türkiye's internet law, requires certain businesses, such as those offering internet access to guests or staff, to keep access logs. Who falls in scope and what must be kept is covered in our guide to Law 5651 log retention; this article focuses on turning logs into detection rather than on the legal duty.
What SIEM log management is, and how it differs from log collection
SIEM merged two older disciplines: security information management (collecting and retaining records) and security event management (real-time monitoring and alerting). It works in four stages: it ingests logs from sources, maps different formats onto common fields (normalisation), links events across sources using rules (correlation), and presents the result as alerts, dashboards and reports.
A single example shows why correlation matters. Five failed sign-ins alone are noise. Five failures, then a successful sign-in, then a new mailbox forwarding rule is the signature of a compromised account. Those three records live in three different systems, and SIEM is what puts them on one timeline.
Three often-confused approaches compare as follows:
| Aspect | Centralised log management | SIEM | SOC / managed monitoring |
|---|---|---|---|
| Core job | Collect, store and search logs | Correlate logs and raise alerts | Have people review alerts around the clock and respond |
| Real-time detection | Limited | Yes, through rules and behaviour analytics | Yes, with analyst judgement |
| Effort required | Low | Medium; rules need tuning and alerts need owners | High; an in-house team or a provider |
| Compliance and investigation | Good, if log integrity is protected | Good, with ready reports and timelines | Good, with incident reports |
| Best fit | First step; small, simple estates | Several critical systems, personal data, remote access | When alerts must be handled outside office hours |
The key lesson is that SIEM is a process more than a product. Deployed without someone to read alerts, tune rules and act on findings, it becomes an expensive log archive. We explain the general approach to spotting deviations with machine learning in our anomaly detection guide.
Which logs to collect first
Connecting every source on day one overwhelms both the budget and the team. The right question is not "what can we collect?" but "which attacks do we want to see, and where do they leave traces?" In practice, sources fall into three priority groups.
The first group is identity: domain controllers, the cloud identity provider, VPN and remote desktop sign-ins, and mailbox logins. Because so many breaches start with a compromised account, these records deliver the highest return. If you use single sign-on, every session arrives from one source; we cover the wider benefits in our guide to single sign-on for business.
The second group is the perimeter: firewall, web servers, email gateway and DNS. These reveal exploit attempts from outside and unusual data leaving the network; how to block that data leaving in the first place is covered in our data loss prevention (DLP) guide. The third group covers critical applications and endpoints, such as ERP, file servers, databases, antivirus or EDR, and SCADA servers in factories. Which production logs can be collected safely is covered in our article on OT and SCADA security.
An 8-step SIEM rollout plan
This sequence suits a small IT team. The first four steps come before choosing a product, and most failed projects fail precisely because they skip them.
- Write your use cases. Define 10–15 concrete scenarios, such as "admin sign-in outside office hours from abroad", "one user reading an unusual number of files in a short period" or "new mailbox forwarding rule". Note which log each one will be detected from.
- Inventory your sources. Which system produces which log, where is it kept and for how long? The inventory also exposes blind spots.
- Fix time and identity. Every device should use the same time source (NTP), or you cannot reconstruct the order of events. Replace shared accounts with personal ones, otherwise logs cannot answer "who?".
- Set retention and integrity rules. Decide in writing how long logs stay in fast, searchable storage and how long in archive, based on legal duties, contracts and investigation needs. Logs containing personal data belong in your data retention and disposal policy.
- Connect the first priority group. Start with identity and email; confirm that collection agents and network traffic do not slow production systems.
- Tune rules against noise. In the first weeks most alerts will be false positives. Improve or retire every rule; an alert nobody looks at is worse than no alert.
- Attach response steps to alerts. For every critical alert, one page should say who investigates, how quickly and which accounts get locked. For ransomware, these steps must work alongside your 3-2-1 backup and recovery plan.
- Test regularly. During a controlled penetration test, measure whether the SIEM saw each attack step. Every step it missed is a new rule or a missing log source.
These eight steps also produce direct evidence for the ISO 27001 logging and monitoring controls. If basic controls are not yet in place, begin with our SME cyber security checklist; watching an account without MFA being taken over costs more than protecting it.
Common mistakes
The most common mistake is "collect everything and work it out later". Licence and storage costs usually scale with data volume, so aimless collection drains the budget and buries the signal in noise. The second is deploying SIEM purely for audit and never writing an alert rule.
The third is treating log access as an ordinary permission. Logs contain usernames, IP addresses and sometimes email subjects, all of which can be personal data, so anyone with SIEM access sees that data. Restrict access by role and log the searches made in the SIEM itself.
Finally, do not forget leavers. A former employee's session on an account that was never closed is one of the easiest events for a SIEM to catch, yet it is often missed because no rule was written. We walk through the process in our guide to offboarding and email access.
How we approach this at Digital Bridge
We start by finding the gaps and attack paths, not by choosing a product. The penetration testing and vulnerability scanning in our cyber security consultancy rank findings by risk and give a remediation step for each; that report shows which attack paths deserve monitoring first and where your use cases should begin. In the same engagement we assess access and logging controls on the systems that hold personal data.
Through our data protection compliance work we build the data inventory, prepare the retention and disposal policy, switch on access logging for systems holding personal data and write an incident response plan that says who does what during a breach. How long logs are kept, and who may access them, becomes part of that policy.
For ERP and custom software that produce no logs, or produce them in non-standard formats, our system integration service builds data transfers that keep their own records. For deviations that rule-based alerts miss, we can connect anomaly detection models, which learn each user's and device's own normal behaviour, to your system logs. If your estate is moving to the cloud, access and security configuration is reviewed together with our cyber security team as part of our cloud migration and infrastructure consultancy.
When log data needs to move into a separate store for long-term analysis or reporting, we design it using the principles in our data warehouse and ETL guide. For more articles in this area, browse our cyber security and compliance guides.
Next step
Before deciding on SIEM, answer three questions. Where are the logs for your ten most critical systems today, and how long do they last? If an admin account were taken over tonight, who would notice, and when? Could you build an attacker's timeline after an incident?
If any answer is "we don't know", your starting point is clear. Get in touch to map your log sources and use cases together; together we can turn that into a prioritised list of what to monitor first.