Decoding Security Alert: Port Scans, Lateral Movement, And T1110

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Decoding Security Alert: Port Scans, Lateral Movement, and T1110

Hey guys, let's dive into a security alert that popped up and sparked a bit of head-scratching. We'll break down what Potential Massive Port Scan from the local network means, how it relates to Lateral Movement, and the connection to T1110 (Brute Force Attacks). We'll also look at a real-world scenario involving a server, DNS over TLS, and why the alert might seem a bit off. This should help you understand these security concepts and how to approach them.

Understanding the Security Alert: The Basics

Alright, so the alert in question is tagged with the signature 3300317: Potential Massive Port Scan from local network - Possible Lateral Movement - T1110. The initial reaction of the user, Francois, is that this looks weird. Why? Because the alert flags a massive port scan, but it's also linked to T1110, which generally refers to brute-force attacks. The core of the issue is whether the alert accurately reflects what's happening. The rule is triggered when a large number of connection attempts (specifically, the threshold: type threshold, track by_src, count 150, seconds 1; part) are initiated from a single source. This high count within a short time period is what triggers the “potential massive port scan” label.

Let’s unpack this further: Port scanning involves systematically probing a server's ports to find open ones. Attackers do this to identify potential vulnerabilities. The Lateral Movement part suggests that if an attacker gained access to a system, they might use it to explore and compromise other systems on the network. T1110, or Brute Force, is a technique where an attacker repeatedly tries different passwords or usernames to gain access. The alert is trying to say that a large number of connection attempts (potentially from a brute-force attack) are happening from your local network. It is important to look at the context of the alert because the alert itself may not always match exactly what is happening.

Now, the alert itself came from a security rule, designed to catch suspicious activity. It's essentially a set of instructions that says, “If you see this type of behavior (a large number of connection attempts within a short timeframe), then raise an alert.” The alert's purpose is to draw your attention to something that might need investigating. This specific rule, and many others, are based on known patterns of malicious behavior (like port scans and brute force attempts), aiming to identify possible intrusions or reconnaissance activities on your network. However, as Francois pointed out, it is important to check the details.

Dissecting the Alert's Details

The details of the alert provide specific information about what triggered it. Let's look at the key data points:

  • timestamp: Indicates when the event occurred (e.g., 2025-11-07T20:40:22.223221+0100).
  • src_ip: The source IP address, which is the machine initiating the connection (e.g., 10.0.1.100 – a server on the LAN).
  • src_port: The source port used by the initiating machine (e.g., 10545).
  • dest_ip: The destination IP address, where the connection is heading (e.g., 1.1.1.1 – Cloudflare's DNS).
  • dest_port: The destination port, in this case, 853, which is used for DNS over TLS (DoT).
  • proto: The protocol used (e.g., TCP).
  • alert: This section contains information about the alert itself, including the signature ID, the message, and severity.
  • flow: Provides more detail about the network flow, such as the number of packets and bytes transferred.

From this, we see that the server at 10.0.1.100 is attempting to connect to 1.1.1.1 (Cloudflare DNS) on port 853 (DoT). This leads to Francois's main point: is this really a “massive port scan”? The traffic appears to be DNS over TLS which is perfectly normal. This is why it's critical to understand the context of your network traffic and alerts.

The Discrepancy: Port Scan vs. DNS over TLS

The crux of the problem lies in the mismatch between the alert's description (potential massive port scan) and the actual activity observed, which is DNS over TLS. Here's why this discrepancy is important:

  1. Understanding the Technology: DNS over TLS is a secure way to perform DNS queries. Instead of sending queries in plain text, they are encrypted, making them more private and secure. The use of TCP port 853 is specifically associated with DoT. When a server on the local network (10.0.1.100) is communicating with a DNS server (like Cloudflare at 1.1.1.1) over port 853, it is not inherently suspicious.
  2. The Nature of Port Scans: A port scan attempts to connect to multiple ports on a target to see which ones are open. In this scenario, the source is sending traffic to a specific port (853), not scanning a range of ports on a target. A genuine port scan would involve probing various ports to discover which services are active, not just using a known service (DoT) on a specific port.
  3. False Positives: Security alerts, especially those based on general rules, can sometimes generate false positives. This is when an alert triggers due to legitimate activity that resembles malicious behavior. The rule's intention is good – to identify potentially malicious port scanning activity – but it is getting triggered by a normal function (DoT).

Francois also provides additional data points that support the idea that this is normal traffic:

  • High Volume of DNS Queries: The log shows a high volume of DNS queries over port 853 (35,516 requests). This further supports the normal operation of DoT within the network.
  • Multiple DNS Servers: The server is communicating with multiple DNS servers, including Cloudflare (1.0.0.1, 1.1.1.1), and Google (8.8.8.8, 8.8.4.4). This is a common setup for redundancy and load balancing, which also doesn't indicate malicious intent.

Lateral Movement: A Quick Overview

Let’s briefly talk about Lateral Movement, which is also mentioned in the alert. Lateral movement is a tactic used by attackers after they have gained access to a network. The main goal is to move from one compromised system to another, gradually gaining access to more sensitive data or systems. Think of it like a chess game; once an attacker gets a foothold (a compromised machine), they try to move their pieces (access) across the board (the network) to capture the king (critical assets).

In the context of the alert, the potential for lateral movement means that if an attacker were to somehow compromise the server 10.0.1.100, they could then use this machine to try and access other systems on the network. The alert, in part, suggests that by observing a high volume of connection attempts, the security system is trying to identify reconnaissance – the attacker's attempts to map the network and find more vulnerable targets. Therefore, the connection with lateral movement comes from the alert's implication that reconnaissance could precede an attempt to move laterally through the network.

Conclusion: Evaluating and Tuning the Rule

Francois rightly questions the alert. While security rules are helpful, they aren’t perfect and can sometimes generate false positives. Here's a quick summary of what we’ve discussed and the actions you might take:

  • Context is Key: Always consider the context of the alert. In this case, the behavior (DNS over TLS) doesn’t align with the description (massive port scan). The alert is likely a false positive.
  • Understand Your Network Traffic: Knowing what normal network traffic looks like is very important. This helps you identify what is genuinely suspicious. In Francois's case, he recognizes that his network uses DoT.
  • Rule Tuning: You might need to adjust or disable the rule to reduce false positives. This could involve modifying the rule's criteria (e.g., the threshold for connection attempts) or simply disabling the rule if it’s consistently generating false alarms.
  • Investigate: If there are any doubts, it is always a good idea to dig deeper. Check logs, monitor network behavior, and make sure everything is in order. But if the details suggest everything is normal, then it could be a false positive.

In conclusion, understanding how security alerts work, and why they can sometimes be triggered by normal activity, is critical for effective network security. Always think about what the alert is trying to tell you, compare it with the actual network behavior, and adjust your rules accordingly. Great work on spotting the oddity, Francois!