Trust is convenient, and that is precisely where the danger lies. In modern IT environments, companies are no longer isolated islands. Connections to external partners, service providers, or maintenance accounts are part of everyday life. But what happens when these very trusted access points are forgotten? When an old test account continues to exist or a maintenance account remains open, even though no one has actively used it for a long time? Such inconspicuous details seem harmless until they suddenly are no longer. For attackers, these are precisely the perfect entry points: inconspicuous, rarely monitored, and often granted more privileges than intended. A quiet access point that was never properly closed can quickly become the biggest vulnerability in the entire network.
This is exactly how a case recently discovered by the team at Certified Security Operations Center GmbH began. What initially appeared to be an unusual system login turned out, upon closer inspection, to be something entirely different: a series of suspicious connection attempts disguised as internal access. A seemingly familiar access point, and yet the beginning of a potentially critical security vulnerability.
It quickly became clear what was really behind it: Monitoring detected a conspicuous cluster of login attempts originating from a single IP address. Within a very short time, various systems and subnets were targeted—a typical pattern for so-called password spraying. You can find more information about password spraying here: https://www.csoc.de/password-spraying-auf-active-directory-konten/
Particularly suspicious was the use of the “ANONYMOUS-ANMELDUNG” account in combination with the outdated NTLMv1 protocol. What initially appeared to be internal traffic turned out, upon closer analysis, to be access via an existing connection to external IT partners. Although this was actually legitimate access, at that moment it served as the starting point for a large-scale scan and thus posed a serious security risk.
Initial Steps Taken
Immediately after the alert, our team of analysts worked together with the customer’s team, because every minute counts in such situations. As an immediate first step, the affected VPN tunnel was promptly disconnected to sever the connection and contain the potential source of the attack. At the same time, the customer contacted the connected service provider directly to clarify the situation as quickly as possible. The subsequent analysis provided clarity: The source of the suspicious activity was a test VPN account that had originally been set up for mobile error analysis. However, this account was not only still active but had also been compromised from the outside—apparently from abroad. The service provider reacted immediately, deactivated the affected account, and confirmed the incident as a compromised account. A seemingly harmless test account had thus turned out to be a critical vulnerability.
In-Depth Analysis
To gain absolute certainty, the team conducted a detailed analysis of the logs. The goal was to determine whether the attacker had done more than just “test logins”:
- Scanning Behavior: Various systems on the internal network were queried (reconnaissance).
- Successful login: An anonymous login via NTLMv1 was successful on an Exchange server.
- No anomalies: There were no indications of data exfiltration, lateral movement, or the creation of additional backdoors.
- Status quo: No data exfiltration was detected. The attack was stopped at the reconnaissance stage.
In a worst-case scenario, the compromised test account would have provided the attackers with a “bridge” directly into the customer’s IT network. Since the attacker was, logically speaking, already operating “within” the network, they might have been able to gain further access unnoticed or extract sensitive data from the Exchange server. The fact that this did not happen was not left to chance, but rather to the immediate intervention following the alert.
Our Recommendations
To avoid such scenarios in the future, we strongly recommend the following steps based on this incident:
- Proper housekeeping: Regularly review and delete temporary service or test accounts, especially on VPN gateways.
- Partner monitoring: Traffic from trusted partner networks (site-to-site VPN) should also be subject to the same strict security policies and monitoring rules as public internet traffic.
- Disabling legacy protocols: The NTLMv1 protocol used is considered insecure and should be disabled across the board wherever possible.
- Principle of Least Privilege: Test accounts should only be granted the absolutely necessary, temporary permissions and should not have blanket access to the network.
Conclusion
This incident serves as a clear warning: The risk of a security breach stems not only from direct external attacks but also from the exploitation of vulnerabilities in one’s own IT hygiene or in interfaces with partners. A “forgotten” test account is a golden key for an attacker if they know where it is. Thanks to continuous 24/7 monitoring, however, the scan was immediately detected as an anomaly before it could develop into a serious security incident. Security also means identifying the “blind spots” in one’s own infrastructure.