How Hackers Use Reconnaissance – and How to Protect Against It

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Information gathering is often the starting point of a cyberattack. For many hackers, before attempting anything, they want to know who they’re dealing with, what vulnerabilities they might exploit, and whether they can operate stealthily or not.

During such reconnaissance operations, attackers collect relevant data about their victims, but it’s not without risks for them. Secured computer systems can use advanced detection tools to spot and block such suspicious activities and even catch adversaries.

It’s essential for companies and organizations to understand this initial step to prevent breaches and detect attacks early.

This article is sponsored by the nonprofit Center for Internet Security (CIS). The Center for Internet Security, Inc. (CIS®) is responsible for globally recognized best practices for securing IT systems and data including the CIS Benchmarks™ and CIS Controls®. CIS Hardened Images® provide secure, on-demand, scalable computing environments in the cloud. See the CIS blog for more on cybersecurity best practices and current cyber issues.

Active vs. Passive Reconnaissance

MITRE ATT&CK, a popular knowledge base for beginners and security professionals, defines reconnaissance as a fundamental tactic that leverages the “techniques that involve adversaries actively or passively gathering information that can be used to support targeting.”

That valuable information is then used to aid further actions, which might involve attacking a vulnerability or additional reconnaissance work.

Reconnaissance is a critical phase in the adversary cycle. It’s not uncommon for hackers to spend much more time on the pre-attack than on the actual attack. And wrong or incomplete reconnaissance can result in a massive failure for the attackers.

There are two approaches for reconnaissance: Active and passive. Pentesters often combine these two approaches to assess vulnerabilities and prevent harmful exploitation.

Ping probes, port scanning, or traceroute are practical examples of active reconnaissance. The attackers directly interact with the targeted machines to enumerate exploitable data.

Passive reconnaissance is the opposite. The attackers do not engage and instead collect data indirectly, using techniques such as physical observation around buildings, eavesdropping on conversations, finding papers with logins/passwords, Google dorks, open source intelligence (OSINT), advanced Shodan searches, WHOIS data, and packet sniffing.

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What Data Do Hackers Collect?

MITRE has identified a number of reconnaissance techniques used by attackers to collect actionable information, such as:

  • Active IP addresses, hostnames, open ports, certificates, server banners, gateways, and routers
  • Credentials, emails, employee names, roles, departments/divisions, and physical location
  • Antivirus and EDR tools, SIEM systems (security information and event management), security vendors, software, hardware, firmware, and operating systems
  • Domain names, subdomains, CDN, mail servers, and other hosts
  • Public WHOIS data such as DNS name servers, IP blocks, and contact information
  • Financial data and intellectual property
  • Purchased data from reputable sources or black markets

While many of these data types would likely require a direct interaction such as active scanning, it’s still possible to use the results of a previous active scan in a passive analysis, for example, by using a dedicated search engine such as Shodan.

Sometimes, hackers only need Google and motivation to find vulnerabilities to exploit.

Corporate websites or apps can reveal details about the business relationships, the supply chain, and other sensitive elements. Adversaries can also leverage social media to stalk victims and collect precious information.

The Top Reconnaissance Tools

You can combine premium products and open-source tools to conduct penetration tests, and security frameworks and scanners can collect data and monitor the targeted network.

The most popular reconnaissance-detection tools are probably the following:

  • Nmap, a popular enumeration software
  • Wireshark, an accurate sniffer (network analysis)
  • Metasploit or the Burp suite, very comprehensive attacking frameworks
  • BeEF, the Browser Exploitation Framework
  • Cobalt Strike

Indeed, there’s an extensive range of vendors that include modules for reconnaissance work or that can be used to collect specific data. Moreover, many of them are free and open source.

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Be Aware of the Attackers’ Timing and Goals

One of the most underappreciated aspects of hacking is the timing. It’s not uncommon to conduct reconnaissance work way before the attack.

Experienced hackers rarely strike just after gathering information. They often collect data and return several weeks or months later to see if the targeted system is still vulnerable, which greatly limits the risks of being caught.

Besides, attackers can scan and collect data from several targets over time, allowing them to determine which targets would be worth attacking and which can be removed from their list. When there’s money involved and easier targets elsewhere, there’s no use wasting resources on secured systems and cybersecurity-aware organizations.

How to Protect Against Reconnaissance

Unfortunately, not all targeted systems are challenging for attackers, allowing them to collect data, which can then be sold to competitors or other threat actors if it’s valuable enough.

After successful reconnaissance, all kinds of attacks become possible, including ransomware, sensitive data exfiltration, additional reconnaissance work, public disclosures, destructions, lateral movement, and more.

Simple security hygiene and awareness isn’t enough to prevent such bad outcomes. You should consider implementing advanced security strategies and solutions, such as endpoint detection and response (EDR), SIEM, breach and attack simulation, and pentesting.

Stay informed about the latest vulnerabilities outlined by MITRE ATT&CK, CISA, NIST and other sources, prioritize fixes, and patch them and apply other recommended mitigations. Between neglected patches and zero-day threats, organizations are no more than a few moves by a hacker from data disaster.

These steps might not spot and protect against all kinds of reconnaissance, though, as stealth scanning techniques such as decoy scans, zombie scans, spoofing, or bouncing can be used to bypass most detection tools.

In any case, you must take measures to mitigate reconnaissance, and modern security solutions are ever smarter, with special features such as deception technology to trap intruders.

Top EDR Solutions Options to Combat Reconnaissance


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2 Heimdal Security

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Heimdal Endpoint Detection and Response is a seamless EDR solution that consists of six of our top-of-the-line products working in unison to hunt, prevent, and remediate any cybersecurity incidents that might come your way. The products in question are Heimdal Threat Prevention, Patch & Asset Management, Ransomware Encryption Protection, Next-Gen Antivirus, Privileged Access Management, and Application Control.

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3 ManageEngine Desktop Central

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Using too many tools to manage and secure your IT? Desktop Central bundles different IT management and security tools in one unified view without cutting corners in end-user productivity and enterprise security. From keeping tabs on your enterprise devices, data, and apps to securing those endpoints against threats and attacks, Endpoint Central ticks all the boxes of a unified endpoint management solution. Try it for free on unlimited endpoints for 30 days.

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Julien Maury
Julien Maury
Julien Maury is a backend developer, a mentor and a technical writer. He loves sharing his knowledge and learning new concepts.

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