Most security teams obsess over the perimeter and treat the inside of the network as a soft, trusted space. That worked fine when staff sat in offices and the firewall did the heavy lifting. It does not work today. Phishing succeeds often enough that an attacker on your internal network is the realistic starting position for any decent threat model, and treating the inside as friendly territory simply hands them the keys.
Insider Threats Are Not Always Malicious
The phrase insider threat conjures images of disgruntled employees stealing data on a USB stick. That happens, but it is the minority case. Far more commonly, the insider is a contractor with a stale laptop, a developer running outdated software, or a marketing manager whose personal device joined the corporate network three weeks ago. The threat is unintentional, but the impact is the same. Network controls need to assume that any device might be compromised at any time.
Active Directory Remains the Crown Jewels
Most internal networks still revolve around Active Directory, and most internal compromises end with domain admin. The attacks have not changed much in years. Kerberoasting, AS-REP roasting, NTLM relay, and abused trust relationships all keep working because the underlying defaults rarely change. Regular internal network penetration testing against an Active Directory environment finds these patterns before someone with bad intent does. The findings are rarely surprising. The surprise is usually how long they have been there.
Expert Commentary
Name: William Fieldhouse
Title: Director of Aardwolf Security Ltd
Comments: In nine out of ten internal engagements, I reach domain admin within a day. The path varies, but the underlying problem is consistent: too many privileged accounts with weak passwords, too much trust between systems, and not enough visibility into what is happening on the wire. The fix is not glamorous, but it works.
Segmentation Buys Time
If an attacker lands on a workstation in the marketing department, can they reach the database server directly? In too many networks the answer is yes. Flat networks make life easy for IT teams and even easier for attackers. Segmenting by function, blocking SMB and RPC between user subnets, and putting management interfaces on a separate VLAN all force an intruder to make noise. That noise gives your detection a chance to react before damage spreads.
Telemetry That Catches the Quiet Stuff
Endpoint detection and response tools are everywhere now, but configuration matters. Default rule sets miss a great deal. Watch for unusual parent-child process chains, PowerShell with encoded commands, and any tool reaching out to LDAP from an unusual workstation. Network telemetry helps too. A workstation enumerating shares across the entire estate has rarely got a good reason for doing so. Flag it and investigate quickly.
Where to Begin
Start with an honest assessment of where you stand. A proper internal review tells you which paths an attacker would actually use, not just which weaknesses theoretically exist. Pair that with a programme of incremental hardening and you build resilience over time rather than chasing the latest tool. The best penetration testing company for the job is one that will tell you what they did to get to domain admin and how long it took, in plain English the board can act on.