Certified: The Security+ Prepcast

When it comes to understanding how an attack occurred or how a threat moved through your network, sometimes logs just aren’t enough. You need to see exactly what was transmitted, down to the byte level. That’s where packet capture analysis comes in. In this episode, we’re exploring how network traffic captures can be used to reconstruct incidents, identify malicious activity, and provide definitive answers during a cybersecurity investigation.
Packet capture, often shortened to “PCAP,” is the process of recording data packets as they travel across a network. Tools like Wireshark, tcpdump, and TShark allow analysts to capture, filter, and review this raw network data. Each packet includes headers and payloads that reveal where the traffic came from, where it was going, what protocol it used, and what data it carried. For investigators, this level of detail is invaluable. It’s like having a high-resolution video recording of everything that happened on the wire.
Let’s begin with how packet captures are used in investigations. Suppose your intrusion detection system flags that a user’s workstation is communicating with a suspicious domain. The alert tells you something’s wrong—but it doesn’t tell you what was actually transmitted. By reviewing a packet capture from the timeframe in question, you can see whether data was exfiltrated, whether the system downloaded malware, or whether it was just part of a harmless scan. This kind of visibility gives you confidence in your conclusions.
Here’s a real-world scenario. A manufacturing company experiences a sudden spike in outbound network traffic from a development server that rarely talks to external systems. Their firewall logs show the IP addresses involved, but not the content of the communication. The security team pulls packet captures for that interface and finds that the server is sending encrypted traffic to an unfamiliar host over port four four three. At first glance, this looks like normal HTTPS traffic. But a deeper look at the packets shows that the server is sending files in rapid bursts at regular intervals. The destination is associated with a known command-and-control server. The team quickly isolates the system, performs forensic analysis, and discovers malware configured to exfiltrate design files. Without packet captures, this activity might have looked like routine web traffic and gone unnoticed.
Packet capture is especially valuable for reconstructing the timeline of an incident. Let’s say an attacker gains initial access through a phishing email. With the right PCAP data, analysts can trace the original connection, identify the download of the payload, watch the beacon to the external host, and even see how the attacker moved through the network. Each step in that chain is made up of packets—and each packet tells part of the story.
This level of detail is also critical when trying to verify or refute claims during a dispute. Imagine a situation where a third-party vendor is accused of leaking confidential data. The vendor claims they never accessed the data. But the organization’s security team has a packet capture that shows the exact timestamp, destination IP address, and the file content being transmitted to an unauthorized endpoint. That capture becomes hard evidence that supports a disciplinary or legal action.
Now let’s look at how analysts identify malicious activity at the packet level. One of the first signs of trouble in a packet capture is traffic that doesn’t belong. This could be a device communicating on an unusual port, a protocol being used where it shouldn’t be, or an unexpected connection between two systems that have no reason to talk to each other.
For instance, if you see NetBIOS traffic on an internet-facing interface, that’s suspicious. If you find Secure Shell traffic going to an unknown server in a foreign country, that’s a red flag. And if you spot Domain Name System requests for random, algorithmically generated domain names—sometimes referred to as DGA domains—you may be looking at malware that’s trying to connect to its command-and-control infrastructure.
Another common technique involves searching for packet payloads that match known attack signatures. This is especially effective in detecting things like SQL injection attempts, cross-site scripting payloads, and malware download attempts. While most organizations rely on intrusion detection systems to alert on these patterns, packet capture allows you to manually verify what was transmitted and whether it succeeded.
Let’s consider another example. A university detects a sudden surge in failed login attempts to its central authentication system. The logs suggest brute-force activity, but the source addresses are changing constantly. The security team captures network traffic and analyzes the packet headers. They find that the User-Agent strings in the HTTP headers are identical for each request, indicating automation. Even though the IPs are rotating, the attacker is using the same tool for each login attempt. By identifying that pattern, the team blocks the requests using behavior-based firewall rules and shares the indicators of compromise with their peers.
In another scenario, packet captures are used to confirm that data was not exfiltrated. After detecting an unauthorized login to a cloud-hosted database, the security team fears that the attacker may have stolen sensitive information. The cloud service doesn’t retain deep logs, but the team has configured PCAP on their network gateway. After reviewing the packet captures, they see that the attacker connected to the interface but never issued any export or download commands. The only activity recorded is a few simple queries and logout. In this case, packet capture provides reassurance that while access was unauthorized, no data left the system.
One powerful feature of packet analysis is protocol dissection. Tools like Wireshark break down complex protocols like SMB, TLS, and HTTP into human-readable form. You can see DNS queries, TLS handshakes, HTTP GET and POST requests, and even credentials sent in plain text—if encryption isn’t used. This allows analysts to spot things like failed authentication attempts, malformed packets, and attempts to bypass inspection tools.
Of course, encryption does limit visibility. When Transport Layer Security is properly implemented, you won’t see the contents of a web session—but you can still analyze the metadata. The size of the packets, the timing of connections, the domain names involved, and the duration of sessions all provide clues. For example, malware using encrypted DNS over HTTPS may be visible in the form of frequent, short-lived connections to obscure content delivery networks.
To be effective, packet capture must be deployed strategically. It’s not practical—or even possible—to capture all traffic all the time. The volume would be overwhelming. Instead, organizations typically capture traffic at critical chokepoints, such as network egress points, high-value segments, or detection hotspots. They may also use rolling buffers—temporary captures that overwrite themselves after a set period unless an alert triggers preservation.
Retention and privacy are also key considerations. Packet captures often contain sensitive information, especially if encryption isn’t enforced. Storage must be secured, access must be restricted, and retention policies must comply with organizational and regulatory requirements. In some environments, packet capture is only used during active investigations or red team exercises.
To summarize, packet capture analysis is one of the most powerful tools in an analyst’s toolkit. It enables security teams to see exactly what happened on the wire, reconstruct attacker behavior, verify or refute activity, and identify malicious traffic that other tools might miss. While it’s not always the first tool you reach for, it’s often the one that provides the final answer when questions remain.
For the Security Plus exam, expect questions about when packet capture is appropriate, what kinds of information it provides, and how it supports investigations. You may be asked to differentiate between log-based detection and packet-level inspection, or to identify what tool would help confirm data exfiltration. Review terms like PCAP, Wireshark, protocol dissection, encrypted traffic, and command-and-control detection—they are all relevant and frequently tested.
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