Leaked credentials: testing resilience after account compromise

Leaked credentials are one of the most effective attack vectors. Whether obtained through a data breach, phishing or brute force against a poorly secured API, the result is often the same: direct access to internal systems. Organisations frequently underestimate both how easily credentials leak and how far an attacker can progress after gaining access. The danger lies not only in the leak itself, but in everything the attacker can do afterwards without being noticed.

Explanation

Millions of credentials are shared or traded daily through dark-web marketplaces and messaging channels. Common sources include large-scale breaches, credential harvesting through phishing kits, infostealer malware, credential stuffing and brute force, and exposure at suppliers and partners. Password reuse means a single leaked password may remain useful across several services for years.

Risk

With a valid account, an attacker can bypass many traditional defences. They authenticate through legitimate channels such as VPN, OWA or SaaS, move laterally through RDP, SMB or PsExec, escalate privileges through local administrator rights or misconfigurations, maintain persistence through scheduled tasks or cloud tokens and exfiltrate data through trusted channels. Because the access looks legitimate, conventional warning signs may be absent.

Points to check

• Password reuse across multiple platforms.
• Missing or bypassable MFA.
• Excessive access rights and weak privilege boundaries.
• Slow detection of access from unusual locations or devices.
• Limited detection of suspicious but authenticated activity.

Approach

A thorough penetration test should include a scenario based on leaked credentials. DeepBlue begins with assumed compromised access and evaluates permissions and privilege boundaries, internal segmentation, SIEM and SOC detection and logging, abuse of single sign-on and federated identity, and credential reuse between cloud and on-premises systems. This reveals not only whether an attacker can enter, but how far they can progress and where the defence fails.

Conclusion

Leaked credentials are a structural risk and the starting point for many targeted attacks. Testing from the moment access is already compromised validates detection capability, privilege boundaries and network architecture in a way conventional testing cannot. Assuming access has leaked tests resilience against the way modern attacks actually begin.

Contact

Want to understand the impact in your own environment? Contact DeepBlue Security & Intelligence at info@deepbluesecurity.nl or +31 (0) 70 290 6 290.