【正文】 s. privacy 15 ? Cookies are files that store identifying information about clients for the purposes of personalization. See The Cookie FAQ for more information. ? Malicious programs can read cookies to gain private information. Many sites do not store sensitive data in cookies. ? Cookies are not inherently bad, but it is wise to learn about them. Software exists that enables you to identify, manage, display, and eliminate cookies. See Cookie Crusher, and Cookie Pal. Cookies 16 ? Since many Web sites gather information about visitors to their sites, you are constantly giving away information such as your IP address. ? There are portals that allow you to surf the Web anonymously by visiting their portal first. ? Their site acts as a firewall, preventing any leaks in information. ? Example: Anonymous browsing 17 ? Malicious code is a program that causes damage to a system. ? Malicious code can affect both the server and the client. Typically servers engage in much more thorough detection and disinfection. ? Examples: ? Virus or worm ? Trojan horses ? Malicious mobile code in active content Client threats 18 ? Macro virus (Anna Kournikova) – 7580% of all viruses – Application specific – Spread through attachments ? Fileinfecting virus – Infects executable files (., .exe, .drv, .dll) – Spread through and file transfer ? Script viruses (ILOVEYOU) – Written in scripting languages (VBScript, JavaScript) – Activated by clicking a .vbs or .js file Viruses 19 ? Viruses are often bined with a worm. ? A worm is designed to spread from puter to puter rather than from file to file. ? A worm does not necessarily need to be activated by a user or program for it to replicate. ? Example: ILOVEYOU virus was both a script virus and a worm that propagated by sending itself to the first 50 people in a user’s Microsoft Outlook address book. Worms 20 ? Malicious active content may be embedded into a seemingly innocuous Web page. ? A Trojan horse is a program hidden inside another program or Web page that masks its true purpose. ? Origin of the name? Trojan horse programs 21 Active content ? Active content, programs embedded in Web pages, can be a threat to clients. ? Active content displays moving graphics, downloads and plays audio, places items into shopping carts, putes the total invoice amount, etc. ? Active content can be implemented in a variety of ways: – Java – JavaScript – ActiveX 22 ? Java is a highlevel, objectoriented programming language developed by Sun Microsystems. ? It was created for embedded systems, but its most popular use has been in Web pages where applets implement clientside applications. ? Java is platform independent. ? It reduces the load on servers by downloading work onto the client’s machine. Java 23 ? To counter security problems, a special security model called the Java sandbox was created. ? The Java sandbox confines Java applet actions to a set of rules defined by a security model. ? These rules apply to all untrusted Java applets, those that have not been proven to be secure. ? The sandbox prevents applets from performing file input or output and from deleting files. ? All applets from a local file system are trusted and have full access to system resources. Java sandbox 24 ? JavaScript is a scripting language developed by Netscape to enable Web page designers to build active content. ? When you download embedded JavaScript code it executes on your machine. It does not operate under the sandbox model. ? For this reason it can invoke privacy and integrity attacks by destroying your disk, copying credit card numbers, recording the URLs of pages you visit, etc. Secure connections do not help. ? JavaScript programs must be explicitly run. JavaScript 25 ? ActiveX is an object that contains programs and properties that Web designers place on pages to perform certain tasks. ? ActiveX controls only run on Windows machines. ? When embedded ActiveX controls are downloaded, they are run on the client machine. ? Examples: Flash, Shockwave ? Once downloaded, ActiveX controls have access to system resources, including the operating system. ActiveX controls 26 Graphics: ? Some graphics file formats have been designed to contain instructions on how the graphic is to be rendered. ? Code embedded into the graphic is a potential threat. Plugins: ? A browser plugin is a program that enhances the capabilities of the browser. They handle things like playing audio clips and displaying movies. ? Many plugins work by executing mands buried within the media they are displaying. Graphics and plugins 27 The primary task in protecting a client machine is the monitoring of active content. Each browser handles this in a different way. The primary issue is trust of the site providing the active content. One way to improve trust is through the use of digital certificates. Protecting client puters 28 ? A digital certificate, or digital ID, is an attachment to a Web page or message verifying the identity of the creator of the page/message. ? It identifies the author and has an expiration date. ? A page or message with a certificate is signed. ? The certificate is only a guarantee of the identity of the author, not of the validity of the page/code. ? Certificates are obtained from a Certificate Authority (CA) that issues them to an individual or an anization. Example: VeriSign ? Identification requirements vary. Digital certificates 29 ? Provides content warnings ? Reacts to ActiveX and Javabased content ? Uses Microsoft Authenticode technology that: – Verifies who signed the code – Checks if the code has been modified since it was