X.509
X.509 is an ITU-T standard for public key infrastructure (PKI). X.509 specifies, amongst other things, standard formats for public key certificates and a certification path validation algorithm. X.509 was initially issued in 1988 and was begun in association with the X.500 standard and assumed a strict hierarchical system of certificate authorities (CAs) for issuing the certificates. The X.500 system has never been fully implemented, and the IETF's public-key infrastructure working group has adapted the standard to the more flexible organization of the Internet. In fact, the term X.509 certificate usually refers to the IETF's PKI Certificate and CRL Profile of the X.509 v3 certificate standard, as specified in RFC 3280, commonly referred to as PKIX.

XACML
XACML is a standard for secure information access using Extensible Markup Language (XML).

Xauth
Xauth is a UNIX tool for facilitating secure access to X Window System–based hosts.

Xhost
Xhost is a UNIX tool for specifying which hosts can make connections to a host running X Window System.

XKMS: XML Key Management Specification
XML Key Management Specification (XKMS) is a standard for encrypting information based on Extensible Markup Language (XML). XKMS comprises two services: the XML Key Information Service (X-KISS) and the XML Key Registration Service Specification (X-KRSS).

XMAS scan
XMAS scan refers to a port-scanning technique based on some obscure aspects of the Transmission Control Protocol (TCP) standard.

XMLDSIG: XML Digital Signatures
XML Digital Signatures (XMLDSIG) is a standard for creating and managing digital signatures using Extensible Markup Language (XML).

XMLENC: XML Encryption
XML Encryption(XMLENG) is a technology for encrypting information based on Extensible Markup Language (XML).

XOR: EXclusive OR
EXclusive OR (eXclusive-OR) encryption requires that both encryptor and decryptor have access to the encryption key, but the encryption algorithm, while extremely simple, is nearly unbreakable. Exclusive-OR encrytion works by using the boolean algebra function exclusive-OR (XOR). XOR is a binary operator (meaning that it takes two arguments - similar to the additional sign, for example). By its name, exclusive OR, it is easy to infer (correctly, no less) that it will return true if one, and only one, of the two operators is true.

Xscan
Xscan is a tool used to scan for hosts running X Window System.

XTACACS: Extended TACACS
XTACACS (Extended TACACS) is a later version of TACACS (Terminal Access Controller Access Control System), an authentication protocol common to Unix networks that allows a remote access server to forward a user's logon password to an authentication server to determine whether access can be allowed to a given system.

Xterm
In Unix computing, Xterm is the standard terminal emulator for the X Window System. Xterm was originally written as a stand-alone terminal emulator for the VAXStation 100 (VS100) by Mark Vandevoorde, prior to the X Window System. A user can have many different invocations of xterm running at once on the same display, each of which provides independent input/output for the process running in it (normally the process is a Unix shell).

Yersinia
Yersinia is a tool for performing layer 2 attacks, helping the pen-tester in his daily work checking the robustness of layer 2 protocols configuration. Yersinia can manipulate layer 2 network protocol and allow an attacker foil switches by injecting spurious Spanning Tree Protocol, DHCP, VLAN Trunking protocol and other messages into a network.

Ypgrab
Ypgrab is a tool for extracting password tables from Network Information System (NIS) hosts.

Zap
Zap is a tool for cleaning log files on UNIX systems.

Zero Day
A zero-day means the first day when a security vulnerability is discovered.

Zero-Day Exploit
A zero-day exploit is one that takes advantage of a security vulnerability on the same day that the vulnerability becomes generally known. Ordinarily, after someone detects that a software program contains a potential exposure to exploitation by a hacker, that person or company can notify the software company and sometimes the world at large so that actions can be taken to repair the exposure or defend against its exploitation. Given time, the software company can repair and distribute a fix to users.

Zeroization
Zeroization is the process of removing or eliminating the key from a cryptographic program or device.

Zombie
A zombie is an insecure computer on which malicious people have placed code that, when triggered at the same time as other zombie servers, will launch an overwhelming number of requests toward an attacked site, which will soon be unable to service legitimate requests from its users. A pulsing zombie is one that launches requests intermittently rather than all at once.

Zombie Army
Zombie army, also called botnet, is a number of Internet computers that have been set up to forward transmissions (including spam or viruses) to other computers on the Internet, without knowledge of their owner. Any such computer is referred to as a zombie.

Zombie Zapper
Zombie Zapper is a free and open source tool for stopping distributed denial of service (DDoS) attacks by telling a zombie system flooding packets to stop flooding.

Zone Transfer
A zone transfer is when a DNS server performs a complete dump of the database for a domain and sends the information from the primary DNS server to the secondary DNS servers

Zoo
In computer security, a zoo is a collection of viruses and worms that exist only in virus and anti-virus labs. Zoos are used to proactively study how malware programs are written and distributed.