attack (DDoS) is an attempt to make a computer resource unavailable to its intended users. It works by making so many service requests at once that the system is overwhelmed and becomes unable to process any of them. DoS may target cloud computing systems. According to business participants in an international security survey, 25% of respondents experienced a DoS attack in 2007 and another 16.8% in 2010. DoS attacks often use bots (or a botnet) to carry out the attack. Phishing Phishing targets online users in an attempt to extract sensitive information such as passwords and financial information. Phishing occurs when the attacker pretends to be a trustworthy entity, either via email or a web page. Victims are directed to web pages that appear to be legitimate, but instead route information to the attackers. Tactics such as email spoofing attempt to make emails appear to be from legitimate senders, or long complex URLs hide the actual website. Insurance group RSA claimed that phishing accounted for worldwide losses of $10.8 billion in 2016. Man in the middle A man-in-the-middle (MITM) attack is a type of cyber attack. Cybercriminals can intercept data sent between people to steal, eavesdrop or modify data for certain malicious purposes, such as extorting money and identity theft. Public WiFi is often insecure because monitoring or intercepting Web traffic is unknown. Application vulnerabilities Applications used to access Internet resources may contain security vulnerabilities such as memory safety bugs or flawed authentication checks. Such bugs can give network attackers full control over the computer. A widespread web-browser application vulnerability is the cross-origin resource sharing (CORS) vulnerability - for maximum security and privacy, make sure to adopt adequate countermeasures against it (such as the patches provided for WebKit-based browsers). Countermeasures Network layer security TCP/IP protocols may be secured with cryptographic methods and security protocols. These protocols include Secure Sockets Layer (SSL), succeeded by Transport Layer Security (TLS) for web traffic, Pretty Good Privacy (PGP) for email, and IPsec for the network layer security. Internet Protocol Security (IPsec) IPsec is designed to protect TCP/IP communication in a secure manner. It is a set of security extensions developed by the Internet Engineering Task Force (IETF). It provides security and authentication at the IP layer by transforming data using encryption. Two main types of transformation form the basis of IPsec: the Authentication Header (AH) and ESP. They provide data integrity, data origin authentication,