Packet Sniffing: What It Is, How It Works, and How to Stay Safe

Packet sniffing is the practice of intercepting and analyzing data packets as they travel across a network. It can be used legitimately by IT teams to monitor network performance, or maliciously by cybercriminals to steal passwords, credit card numbers, and other sensitive information. 

Understanding how packet sniffing works is the first step in protecting yourself from cybercriminals trying to steal your data. We recommend also using antivirus software to enhance your safety online.

What is packet sniffing?

Packet sniffing is the practice of intercepting and analyzing data packets as they travel across a network. It can be used legitimately by IT teams to monitor network performance, or maliciously by cybercriminals to steal passwords, credit card numbers, and other sensitive information. 

Using the internet is a little like playing a game of catch where your data is the ball. Every time you send a message, your computer throws data across a network and the website catches it, then sends back what you requested. 

Messages are broken into smaller chunks called packets so they travel faster. A packet sniffer intercepts those packets before they reach their destination.

But packet sniffing isn't always malicious. A company might use packet sniffing tools for legitimate network monitoring tasks such as:

• Reviewing network or data usage
• Identifying network problems
• Making sure the network activity is normal

Advertisers also use packet data to gather information for  targeted ads by looking at websites you visited, your search history, and your downloads.

Active sniffing vs. passive sniffing

There are two types of packet sniffing: active sniffing and passive sniffing. 

Active sniffing targets a switch. The sniffer sends fake data packets to the switch that interrupt data flows and redirects the original traffic to capture the original packets.

Passive sniffing targets the hub. The sniffer connects to the network at the hub and then monitors traffic without sending any data to the network.

What is a packet sniffing attack?

With a packet-sniffing cyber attack, criminals use packet sniffers to steal unencrypted sensitive data. When you send data across an insecure network, malicious actors can use sniffing tools to intercept the packets and capture the data.

Cybercriminals can use packet sniffing to steal personally identifiable information (PII), the sensitive information you transmit every time you log in or make a purchase. Examples includes:

• Usernames
• Passwords
• Credit card information

To pull this off, attackers typically exploit two specific devices on most networks:

• Switch: The network switch is a software-equipped piece of hardware that connects devices and controls how data flows, which ensures packets get to the intended destination.
• Hub: A hardware device that connects devices, usually to a local area network (LAN), with an Ethernet cord, which ensures packets get to the intended destination.

How packet-sniffing attacks happen

Cybercriminals use the following packet-sniffing attack techniques:

• Transmission Control Protocol (TCP) hijacking: TCP is the digital rule that computers, networks, and applications use to exchange data. With TCP hijacking, cybercriminals trick the network into thinking they’re the legitimate user and then collect network data.
• Domain Name System (DNS) poisoning: DNS is the technical standard used to translate websites from numbers, called IP addresses, into words. With DNS poisoning, also called DNS spoofing, cybercriminals make a fake website with a different IP address than the real one.
• Address Resolution Protocol (ARP) sniffing: ARP is the technical standard that maps a device to a location on the network. With ARP sniffing, also called ARP spoofing or ARP poison routing, cybercriminals fake both the device and network location information to redirect the traffic and data.
• Dynamic Host Configuration Protocol (DHCP) starvation: DHCP servers assign devices network configuration information so they can connect to a network. With DHCP starvation, cybercriminals overwhelm the server with fake packets so it redirects devices to different locations, usually ones the malicious actor owns.
• JavaScript sniffing: JavaScript is a coding language on websites that allows you to input information. With JavaScript sniffing, cybercriminals add code to a website so the website sends them the information instead of the real website owner.

3 types of sniffing attacks

Although cybercriminals use different techniques, the outcomes can be grouped into a few categories.

1. Packet sniffing attacks on Wi-Fi networks

This is the classic “don’t connect to the coffeehouse Wi-Fi” attack, or as security professionals call it, a man-in-the-middle (MitM) attack. Cybercriminals use a wireless network sniffer, which can be either hardware or software, to monitor your connection to an unencrypted, public Wi-Fi network. Their goal is to capture packets of data, such as credit card information or phone numbers.

2. Browser history packet sniffers

Your browser keeps a record of the websites you visit so it can autocomplete a web address when you return to a site. For example, when you do a search and click on a result, the link changes color because your browser “remembers” your visit.

Cybercriminals manipulate tracking code to trick the browser into sending data to both the real website and their fake website.

3. Password sniffing

Cybercriminals can steal passwords through two sniffing tactics:

1. Using a MitM attack on an unencrypted public Wi-Fi.
2. Using a JavaScript sniffer.

Websites use JavaScript for forms such as login pages. Cybercriminals use JavaScript sniffers to alter website forms and steal passwords.

Is packet sniffing illegal?

Packet sniffing is legal when used on a network you own or have explicit permission to monitor. IT teams do this routinely to troubleshoot performance issues and detect threats. 

It becomes illegal when used to intercept traffic without authorization. In the U.S., unauthorized packet sniffing violates the Electronic Communications Privacy Act (ECPA), and depending on what data is captured, could trigger additional federal charges.

So if you're an IT admin monitoring your company's network, you're fine. If a cybercriminal is using a packet sniffer to capture your data on public Wi-Fi, that's a federal crime.

How to protect yourself from packet sniffer attacks

In technology, as in life, knowledge is power. Although packet sniffing attacks sound complicated, you can take some easy steps to protect yourself and your information.

Install antivirus software

Cybercriminals can use malicious websites to install sniffer malware on your device. Antivirus software is important because it maintains a database of known malware and helps keep your data safe from packet sniffers by blocking malware installation.

Use a VPN on public Wi-Fi networks

A virtual private network (VPN) is an application that you can download to most devices, including your cell phone, tablet, laptop, or router. When you turn it on, the VPN encrypts the data you send and receive so cybercriminals can’t intercept or read it.

Practice online safety

Most online safety tips are about being aware of your digital surroundings. Some good practices include:

• Make sure a website link starts with HTTPS, which means the site is encrypted and secure.
• Hover over embedded links in emails to make sure you recognize the website in the actual URL.
• Never share personal data publicly on social media or other sites.
• Use a password manager so you can create a unique, strong password for every login.

Avoid social engineering and phishing attacks

Cybercriminals use phishing and social engineering attacks to infect devices with malware. In a social engineering attack, cybercriminals may try to manipulate your emotions to convince you to take an action that ultimately harms you.

Common signs of a phishing attack include:

• Be wary of links in emails, even if they seem to be from reputable sources.
• Check the sender's email address, not the name, if you think the message is suspicious.
• Look for misspelled words and grammar mistakes in the email’s text.

Edit your home router settings

You probably have two devices that help you connect to your home Wi-Fi: a modem and a router. The modem is how your internet service provider (ISP) delivers your internet connection. Your router is the device that sends Wi-Fi signals through the air.

To protect against packet sniffing attacks, you can edit your router’s settings by doing the following:

• Change your network encryption to Wi-Fi Protected Access 2 (WPA2).
• Change your router’s default password so no one can edit your settings.
• Update your router’s firmware so no one can use a vulnerability to change your router’s settings.

Bottom line

Packet sniffing isn’t good or bad. Whether packet sniffing is useful or dangerous depends on how the tools are used. If someone has malicious intentions, then packet sniffing is a danger to you and the safety of your personal information.

The good news is that, with some awareness, you can protect yourself online. From online awareness to antivirus software and ad blockers, you can keep yourself and your data safer.

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