XYZ
XACML - Extensible Access Control Markup Language (Extensible Markup Language (XML)-based standard used to express security policies and access rights to assets provided through web services and other applications.)
XBL - Spamhaus Exploit Block List (A form of reverse-lookup program or service which provides information about website clients (visitors to a website) in the area of blacklists (real-time blacklist lookups (RBL)). A blacklist is a set of known-bad resources, such as software, executables, disallowed websites, specific types of data, IP addresses, domain names, or applications, which have been listed and explicitly not allowed on a network or host, or are not allowed to be accessed.)
XDR - External Data Representation (System developed by Sun Microsystems which allows a program running on one computer to execute code on a remote system using a mechanism called a Remote Procedure Call (RPC).)
XKMS - Extensible Markup Language (XML) Key-Management Specification (XKMS) (Is a key management cryptography mechanism which provides protocols for distributing and registering public keys for use with Extensible Markup Language (XML), a markup language for documents containing structured information.)
XML - Extensible Markup Language (A markup language which was developed beginning in 1996. A markup language is a way to, not only structure text and data sets, but also to dictate how they will be used, It has served as a foundational and universal standard which provides a structure for other independent markup languages to be built from while still allowing for interoperability.)
XOR - Exclusive OR (A binary mathematical function commonly used in binary mathematics and encryption methods.)
XP - Extreme Programming (A software development model intended to reduce coding errors and increase security at the program level. It is a form of code review during development that takes code reviews "to the extreme" by having them take place continuously, using an approach called "pair programming" in which one programmer dictates to another, who types in the code, allowing both team members to review for errors. XP also relies on writing a minimal amount of code, using a step process which relies on just passing individual pre-written tests, and no more, and then moving to the next code/step test.)
XSRF - cross-site request forgery (Type of attack in which an attacker attempts to steal authentication information from session cookies during the user's current browsing session. The user is usually tricked into visiting a malicious website through redirection or phishing, and the malicious website executes code that attempts to steal authentication information and impersonate the user. This attack requires that the user already be authenticated to the secure site, so that their session credentials are stored in the browser.)
XSS - cross-site scripting (Type of attack in which a victim's computer/browser executes malicious code which has been injected into a vulnerable web page. Usually a victim visits an infected web page and takes some action which allows a malicious script to execute. The most common locations where malicious code will be encountered are online forums, message boards, search boxes, social networking websites, and links embedded in e-mails.)
XST - cross-site tracing
XTACACS - Extended Terminal Access Controller Access Control System (Access control system which allows a remote user to connect and authenticate to a network via an intermediary TACACS server and also separates the authentication, authorization, and accounting (AAA) functions into separate processes, which can be handled by separate servers and technologies.)