Active vs. Passive Reconnaissance in Cyber Security

Every cyber attack starts the same way, but it’s not how you might think. You see, it's not malware or brute force that kicks it off. It happens long before that, in a phase most people never see.

Hackers gather intel first, searching for clues, weak spots, and unnoticed openings. Sometimes, they do it silently. Other times, they leave a trail.

So here’s the Million dollar question: Would you notice if someone was scouting your system right now? Most people wouldn’t. But if you could spot it early, you might stop an attack before it even begins.

In this guide, we’ll take a look at how reconnaissance works, why it’s important to know about, and why understanding it is the first step to real security.

Let’s dive in…

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With that out of the way, let’s get into this guide.

What is reconnaissance in Cyber Security?

Before a thief breaks into a building, they don’t just walk up and force the door open. They case the place first - checking for unlocked windows, security cameras, and weak spots. Hackers do the same thing, but in the digital world.

This information-gathering phase is called reconnaissance, and it’s where every cyberattack begins. The goal? Find the easiest way in before making a move.

Attackers look for:

Exposed entry points → Open ports and misconfigured services
Weak security → Outdated software with known vulnerabilities
Leaked credentials → Passwords from past data breaches

But here’s the twist: Cyber security professionals do the exact same thing. The difference? They do it to stop attacks, not launch them. Kind of like checking to see if the doors are locked.

Just like with hackers, a good security team:

Scans for exposed data before attackers find it
Tests for weak spots and patches them early.
Monitors for suspicious activity to detect attacks before they escalate

And this brings us to a key distinction in that not all reconnaissance works the same way. Some methods are completely passive, leaving no trace. Others are active, interacting with systems in ways that can be detected.

So let's break them down, how they work, and what to be aware of.

What is passive reconnaissance and how does it work?

In 2014, hackers pulled off one of the biggest financial breaches in history, stealing the records of 76 million households and 7 million small businesses from JPMorgan Chase.

No malware. No brute-force attack.

They didn’t even touch the bank’s systems until they already knew exactly where the weaknesses were.

How?

They mapped out JPMorgan’s network using publicly available information — data that was already out there, just waiting to be found. Then, they spotted a forgotten server that wasn’t protected by multi-factor authentication. That tiny gap was all they needed to slip in, escalate their access, and steal vast amounts of customer data.

And the worst part? JPMorgan had no idea this was happening.

This is passive reconnaissance. It's the quietest, stealthiest way for attackers to gather intelligence. No direct interaction with the target. No alarms triggered. Just research.

Why do hackers start with passive reconnaissance?

Because it’s undetectable. Attackers don’t want to tip anyone off before they even begin, so they collect as much intelligence as possible under the radar.

They dig through leaked credentials, hidden infrastructure details, and exposed company data — all without touching the target’s systems. The more they know in advance, the easier it is to find weak spots without raising a single alarm.

And sometimes? They don’t even need to hack anything. If security is weak enough, they might stumble upon an open door and walk right in without anyone noticing.

How do hackers gather passive reconnaissance?

What might blow your mind is that hackers don’t even need special access to get started. Just good research skills and the right tools.

Let’s break down the main methods they use.

Method #1. OSINT (Open-Source Intelligence)

A ridiculous amount of sensitive information is freely available online but companies just don’t realize it. This means that most of the time, attackers don’t need a breach when employees (or even the company itself) leak information by accident.

Where do they look?

Social media posts – A LinkedIn job listing mentioning “experience with Cisco ASA firewalls” tells an attacker exactly what security tech is in place
Company websites – Press releases, employee directories, and technical documentation often reveal internal details
Leaked credentials – If an employee’s password was exposed in a past data breach, it might still work

For example

In 2019, Hackers researched Airbus employees using public records, gathering emails and job titles. With this info, they crafted personalized spear-phishing emails that tricked employees into revealing login credentials.

Then, those same credentials were later used to infiltrate Airbus’ supply chain partners — giving attackers indirect access to internal systems.

Sneaky eh?

Method #2. WHOIS lookups

Every website you visit has a WHOIS record. This is basically, a digital fingerprint that can reveal:

Who owns the domain
Email contacts for IT staff
Internal subdomains that shouldn’t be public

Most companies don’t think twice about this information, but hackers? They use WHOIS lookups to find weak spots before launching an attack.

For example

A security researcher for a Fortune 500 company was checking WHOIS records, when they found an exposed test environment running outdated software.

If an attacker had found it first? They could’ve used it as a backdoor to pivot into the company’s main network.

Method #3. DNS enumeration

DNS records (the phone book of the internet) often leak valuable details about a company’s infrastructure.

By analyzing DNS records, attackers can:

Find hidden subdomains (e.g., dev.example.com, admin.example.com)
Identify email servers (MX records) which is helpful for phishing
Locate unsecured cloud storage which sometimes leads straight to sensitive data

For example

In 2020, security researchers used DNS enumeration to discover hundreds of misconfigured Amazon S3 buckets tied to major companies.

What was inside? Financial reports, customer databases, and internal documents — all completely exposed because of bad security settings.

Method #4. Metadata extraction

Ever downloaded a PDF or a Word document? Turns out, files store extra hidden details, and hackers know exactly where to look to get that information.

Metadata inside files can reveal:

Usernames of employees
Software versions that might have security flaws
Internal network paths exposing IT infrastructure

For example

In 2016, Hackers pulled metadata from publicly available military documents, uncovering usernames of defense contractors.

Doesn’t seem like a huge deal right? Until those same hackers launched targeted phishing attacks, broke into military systems, and stole classified information.

Method #5. Google dorking

All the methods so far seem complex, like you need a technical background. Well, you’d be surprised how simple this information gathering can be with just a basic knowledge of how to use Google.

By using ‘advanced’ search operators, attackers can:

Find internal reports accidentally uploaded to public folders
Uncover misconfigured Google Drive links exposing private company data
Locate admin panels and login pages that were never meant to be public

Heck, let’s be honest here - the searches are not even that advanced. If you’ve ever tried to search a website for something directly in Google, chances are you’ve done this already.

For example

This tells Google to only show PDF files that are publicly accessible on the website ‘example.com’.

site:example.com + filetype:pdf

While this searches for open directories on the same website. It's basically looking for folders full of files that might not have been meant for public access.

intitle:"index of" site:example.com

Super simple but it works more often that you would want it to!

For example

In 2021, Cyber Security researchers found thousands of misconfigured Google Drive links that had been indexed by Google by accident.

The files?

Financial reports, HR documents, private contracts. Basically everything an attacker could ever want. All sitting there, waiting to be stolen.

The users hadn’t made them public — Google Drive had! And all it took was a basic search to find them.

Scary to think about, right? And that’s just the easy stuff.

Even worse? Passive reconnaissance only scratches the surface. At some point, attackers need to take the next step, and that’s where active reconnaissance comes in.

What is active reconnaissance and how does it work?

Active reconnaissance is where an attacker stops watching from a distance and starts poking at the system to see how it reacts.

Unlike passive methods, active reconnaissance interacts directly with the target. This means it can be detected by security defenses, including:

Firewalls & IDS – Flag unusual scanning activity
SIEM tools – Correlate logs to detect suspicious behavior
Honeypots – Fake systems designed to trap and analyze attackers

That’s why attackers don’t start here. They always begin with passive reconnaissance first and then only switch to active methods when they’re ready to confirm weaknesses and find entry points.

And even then? They aren’t always looking to attack the core system directly.

Instead, they often search for backdoor opportunities such as a weaker system or misconfiguration that lets them slip in unnoticed.

For example

In 2013, hackers pulled off one of the largest data breaches in history, stealing 40 million customer credit card details from a major U.S. retailer.

They didn’t start by breaking into payment systems though.

First, they scanned the company’s external network, searching for open ports, weak firewalls, and outdated software — chipping away to find a nick in the armor.

And when they found a vulnerable third-party vendor’s system, they brute-forced their way in undetected. No major hacks, no systems going down, no mocking images on a screen. Just theft of millions of dollars before anyone noticed.

Once inside the weaker system, they mapped out the internal network, escalated privileges, and deployed malware — siphoning off payment card data for weeks before anyone caught on.

The most important thing of an attack like this though?

Before they could act, they needed to be sure. That’s why attackers rely on active reconnaissance — to test for weaknesses before making their move.

So let’s break down the most common techniques they use, so you can understand exactly how these attacks unfold - and possibly save yourself from falling victim to them.

Methods of active reconnaissance

Active reconnaissance isn’t about breaking in—it’s about testing what’s exposed. Here are the most common methods attackers use.

Method #1. Port scanning

A company’s network is like a building, and ports are its doors. Some need to be open, such as the main entrance, but others? They should be locked tight.

Port scanning allows attackers to check:

Which ports are open
What services are running on those ports
Whether those services are misconfigured or outdated

Because ports act as gateways to running applications, an open port could mean an opportunity to break in — especially if it’s tied to unpatched software or a forgotten service left running in the background.

For example

In 2021, attackers scanned the internet for Exchange email servers running an unpatched vulnerability (CVE-2021-26855).

Once they found an exposed system, they:

Ran a simple scan to detect the server
Checked the software version to confirm it was vulnerable
Launched the exploit, gaining remote access without needing a password

(It’s crazy how simple this can be).

From there, attackers were able to exfiltrate emails, deploy malware, and take full control of corporate email systems — all because of one exposed port running outdated software.

Important: Never scan a system you don’t own or have explicit permission to test. Running Nmap or any other scanning tool without authorization can be illegal, and companies actively monitor for this kind of activity. Always stick to systems you have permission to test.

Method #2. Network mapping – understanding the system’s structure

Once attackers find an opening, they need to understand the full layout of the system. Network mapping helps them do exactly that, revealing how systems connect and where security gaps might exist.

With the right tools, they can:

Uncover critical infrastructure – Servers, routers, and firewalls
Trace internal pathways – Finding direct or overlooked entry points
Identify security blind spots – Weakly secured devices or misconfigured access points

One tool attackers use is Traceroute, which tracks how data moves through a network. Every request — whether loading a webpage or sending an email — hops through multiple systems before reaching its final destination. Traceroute exposes these paths, mapping out security layers and revealing hidden vulnerabilities.

For example

In 2010, state-sponsored attackers used network mapping to find weak spots in Google’s infrastructure.

They first scanned internal systems to locate developer workstations
Then they mapped out connections between systems to find hidden pathways
Finally, they used zero-day exploits to break into high-value targets — stealing Google’s private source code

And because they had a detailed map of the network, they knew exactly where to strike and how to stay undetected for months.

Method #3. Service fingerprinting

Not all open ports are immediately useful, so attackers need to figure out what’s actually running behind them.

Service fingerprinting allows attackers to:

Identify specific software and versions running on a server
Check for misconfigurations or known vulnerabilities
See if any applications are outdated and exploitable

Why care?

Well, if an attacker sees that a web server is running Apache 2.4.48, they can check if that version has any public exploits and if it does, they know exactly how to attack it.

Method #4. Brute-force attacks

If an attacker finds a login page, VPN portal, or remote desktop connection, they may attempt a brute-force attack, where they try multiple common username-password combinations until they break in.

Heck sometimes they don’t even need to do that. They can simply do a search to see if any passwords belonging to the user have been breached in the past.

(Wonder why Gmail is always asking if it's you? It’s because of stuff like this).

For example

In 2023, hackers targeted corporate VPNs, brute-forcing weak credentials to gain access. Then, they bypassed multi-factor authentication (MFA) by using stolen session cookies, allowing them to move undetected inside corporate networks.

Time to find the gaps before attackers do!

Every cyber attack starts with reconnaissance. If you don’t know what’s exposed, you can’t defend against it. That’s why good security hygiene methods are so vital.

The best way to improve security? Think like a hacker and find the problems in your security before they do. Audit your own network, check for exposed data, and see what an attacker might find.

Not sure how?

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