VPN, privacy and security glossary

Plain definitions of the terms you'll meet on Tunnelcheck and across the security world. Each entry is short on purpose. Where a topic has its own page, the entry links to it.

A

• AES: Advanced Encryption Standard, the symmetric cipher standardized by NIST in 2001 and used by most VPNs, disk encryption and HTTPS. "AES-256" means it uses a 256-bit key.
• Ad blocker: A browser extension or network filter that stops ads and often the trackers that come with them from loading.
• Antivirus: Software that looks for known malicious files and suspicious behavior on a device. Built-in tools such as Microsoft Defender cover most home users.
• Authentication: Proving who you are to a system, usually with something you know (a password), have (a phone or security key) or are (a fingerprint).

B

• Botnet: A network of hijacked computers or smart devices controlled by an attacker, often used for spam or DDoS attacks. The Mirai botnet of 2016 was made mostly of insecure cameras and routers.
• Brute-force attack: Guessing a password or key by trying huge numbers of possibilities. Long passwords and rate limits make it impractical.
• Bug bounty: A program in which a company pays outside researchers for reporting security flaws responsibly.

C

• Cookie: A small file a website stores in your browser to remember you. Third-party cookies set by ad networks are used to track you across sites.
• Credential stuffing: Trying usernames and passwords leaked from one site on many other sites, which works because people reuse passwords.
• CVE: Common Vulnerabilities and Exposures, the public catalog that gives each known security flaw an ID such as CVE-2019-3568.

D

• Dark web: Websites reachable only through anonymity networks such as Tor. It hosts both legitimate privacy services and criminal markets.
• Data broker: A company that collects personal information from public records, apps and purchases and sells profiles of people. See our data broker removal guide.
• DDoS: Distributed denial of service: flooding a website or network with traffic from many machines so real users can't reach it.
• DNS: The Domain Name System, which turns names like example.com into IP addresses. DNS lookups can reveal which sites you visit.
• DNS leak: When your device sends DNS lookups outside the VPN tunnel, exposing the sites you visit to your ISP. A leak test checks for it.

E

• Encryption: Scrambling data with a key so only someone with the right key can read it.
• End-to-end encryption: Encryption where only the sender and recipient hold the keys, so the service carrying the message can't read it. Signal and WhatsApp messages work this way. See encrypted messaging apps.
• Exploit: Code or a technique that takes advantage of a specific vulnerability.

F

• Firewall: Software or hardware that allows or blocks network traffic based on rules.
• Fingerprinting: Identifying a browser from many small details, such as screen size, fonts and settings, without cookies. A VPN doesn't stop it.

G

• GDPR: The EU's General Data Protection Regulation, in force since May 2018, which sets rules for how organizations handle personal data. See our privacy explainer.
• Geo-blocking: Restricting content by the country your IP address appears to be in.

H

• Hash: A one-way function that turns data into a fixed-length fingerprint. Sites should store passwords as salted hashes, never in plain text.
• HTTPS: The secure version of HTTP. It encrypts the connection between your browser and a website using TLS.

I

• IKEv2: Internet Key Exchange version 2, the protocol that sets up IPsec VPN connections. See VPN protocols.
• IP address: The number that identifies a device on a network. Websites see your public IP address unless you use a VPN, proxy or Tor. Check yours with What is my IP.
• ISP: Internet service provider, the company that connects you to the internet.

K

• Kill switch: A VPN feature that blocks internet traffic if the VPN connection drops, so nothing leaks out unprotected. See VPN kill switch.

M

• Malware: Any software written to harm or exploit a device or its user, including viruses, worms, trojans, spyware and ransomware.
• Man-in-the-middle attack: An attacker secretly sits between two parties and reads or changes their traffic. HTTPS and VPNs are defenses.
• Multi-factor authentication (MFA): Logging in with two or more kinds of proof, such as a password plus a code or security key. Also called two-factor authentication (2FA).

N

• No-logs policy: A provider's promise not to record what users do. Its value depends on audits and track record. See no-logs policies explained.

O

• Obfuscation: Disguising VPN traffic so it looks like ordinary web traffic, used where VPNs are blocked.
• OpenVPN: A widely used open-source VPN protocol built on TLS.

P

• Passkey: A login method based on public-key cryptography that replaces passwords. The private key stays on your device, so there is nothing to phish.
• Password manager: An app that creates and stores a unique strong password for every account. See password managers.
• Patch: A software update that fixes bugs or security flaws.
• Phishing: Tricking people into giving up passwords, codes or money through fake emails, texts, sites or calls.
• Proxy: A server that forwards traffic for one app or browser. Unlike most VPNs, many proxies don't encrypt. See VPN vs proxy vs Tor.

R

• Ransomware: Malware that encrypts or steals an organization's files and demands payment to restore them or keep them private.

S

• SIM swapping: Persuading a mobile carrier to move a victim's phone number to the attacker's SIM card, so the attacker receives text-message login codes.
• Social engineering: Manipulating people rather than machines, for example by posing as IT support or a relative in trouble.
• Split tunneling: Sending some apps through the VPN and others directly to the internet. See split tunneling.
• Spyware: Malware that secretly monitors a device. Commercial spyware such as NSO Group's Pegasus has been used against journalists and activists.

T

• TLS: Transport Layer Security, the protocol behind HTTPS and many other encrypted connections.
• Tor: A free network that routes traffic through three volunteer relays to hide who is talking to whom. The Tor Browser is its main app.
• Tracker: Code embedded in websites and apps that reports your activity to analytics or ad companies.
• Trojan: Malware disguised as a legitimate program.

V

• VPN: Virtual private network: an encrypted tunnel from your device to a server that forwards your traffic. See what is a VPN.
• Vulnerability: A weakness in software or hardware that an attacker can use.

W

• WebRTC leak: When a browser's real-time communication feature reveals your real IP address despite a VPN. Try our WebRTC leak test.
• WireGuard: A modern, compact open-source VPN protocol, part of the Linux kernel since version 5.6.
• Worm: Malware that spreads from computer to computer by itself, without anyone opening a file. WannaCry in 2017 was a worm.

Z

• Zero-day: A vulnerability that attackers use before the vendor knows about it or has released a fix.
• Zero-trust: A security approach that checks every user and device on every request instead of trusting anything inside the company network.