Signs of a Security Breach in a Corporate Network
- Neil Hare-Brown
- May 27
- 3 min read
If you’ve noticed something unusual on your network — strange logins, slow systems, or users getting locked out — you might be dealing with a security breach.
Early detection is critical: the faster you identify a breach, the more effectively you can contain it and limit the damage.
This guide outlines the key signs that your corporate network may have been compromised — and what to do next if you suspect an incident is underway.
1. Unusual Login Activity
One of the most common early signs of compromise is suspicious login behaviour, including:
Logins at odd hours or from unexpected locations
Multiple failed login attempts followed by success
Admin access used from unknown devices
Use of old, dormant accounts suddenly reactivated
Check your identity and access logs for patterns that don’t align with legitimate business activity.
2. Unexplained System or Network Slowdown
If systems are suddenly slow, unresponsive, or crashing unexpectedly, it may indicate:
Malware running in the background
Unauthorised data transfers
Network scanning or reconnaissance by an attacker
Rule out hardware issues — but if performance degradation affects multiple machines or departments, investigate further.
3. Security Tools Are Disabled or Triggering Alerts
Pay attention to your security systems. If antivirus, EDR, or firewalls are:
Being disabled or turned off
Triggering unusual alerts
Failing to update
…someone may be trying to bypass your defences or cover their tracks. Don’t ignore “minor” alerts — they could be part of a much larger issue.
Ensure your security tools are adequately configured to prevent and/or alert you to tampering.
4. Unknown Software or Services Running
Unexpected software, background tasks, or startup entries can signal malware or backdoor access. Check for:
New programs you don’t recognise
Tools commonly used by attackers (e.g. Mimikatz, PsExec, Cobalt Strike)
Suspicious scheduled tasks or startup scripts
Perform a full endpoint scan to validate all running processes.
5. Unusual Data Movement
Watch for signs of data being accessed or moved in unexpected ways:
Large data transfers outside working hours
Uploads to unknown IP addresses or cloud services
File access logs showing spikes in activity
This could indicate exfiltration — often part of ransomware or double-extortion attacks.
6. Users Reporting Strange Behaviour
Sometimes the first alert comes from someone in the business. Pay attention to reports like:
“My mouse moved on its own”
“I got logged out suddenly”
“I’m getting emails I didn’t send”
“There’s a pop-up I’ve never seen before”
These aren’t just annoyances — they could be signs that an attacker is already active inside your network.
7. Ransom Notes, Threat Messages or Suspicious Files
If you’ve discovered ransom demands, encrypted files, or folders with strange names, it’s a strong indication your network has been breached — likely by ransomware.
Do not delete the files. Isolate affected systems and contact your cyber incident response provider immediately.
You can find more information on how to handle these types of cyber incidents here.
What to Do If You Suspect a Breach
If you’ve identified one or more of the signs above:
Isolate the affected device(s) from the network
Preserve logs and evidence — don’t wipe or reimage yet
Escalate to your IT security team or managed provider
Begin your incident response plan if you have one
Report the issue to regulators (e.g. ICO) if personal data is involved
We can help you confirm, contain, and recover from a breach — including support with threat actor engagement if attackers are active.
You can also learn more about the types of cyber incidents that commonly affect businesses.
How STORM Guidance Can Help
✔ Live breach detection and investigation
✔ Threat hunting and forensic analysis
✔ Malware removal and secure system recovery
✔ Threat actor communication and ransomware response
✔ Regulatory and reputational support
Don’t Wait for Proof — Act on Suspicion
Most cyber attacks don’t start with fireworks.
They start quietly — with a strange login, a missed alert, or a user complaint. Spotting those early signs is your best chance to shut it down before damage is done.
STORM Guidance is here to help you investigate, respond, and recover — with confidence and control.
