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A zero-day (or zero-hour or day zero) attack or threat is an attack that exploits a previously unknown vulnerability in a computer application, one that developers have not had time to address and patch. It is called a "zero-day" because the programmer has had zero days to fix the flaw (in other words, a patch is not available). Once a patch is available, it is no longer a "zero-day exploit". It is common for individuals or companies who discover zero-day attacks to sell them to government agencies for use in cyberwarfare.
Malware writers are able to exploit zero-day vulnerabilities through several different attack vectors. Web browsers are a particular target because of their widespread distribution and usage. Attackers can also send e-mail attachments, which exploit vulnerabilities in the application opening the attachment. Exploits that take advantage of common file types are listed in databases like US-CERT. Malware can be engineered to take advantage of these file type exploits to compromise attacked systems or steal confidential data such as banking passwords and personal identity information.
Zero-day attacks occur during the vulnerability window that exists in the time between when vulnerability is first exploited and when software developers start to develop and publish a counter to that threat.
Conceptually, there is one more event in the zero-day attack time line, which is the users applying the fix, effectively closing the vulnerability window, but that can vary, as some users may simply stop using the affected software as soon as the problem surfaces. Meanwhile, others may never know of it at all, thus never fixing it and thereby keeping the vulnerability window open. Thus, the vulnerability window's length is usually just measured until the developer releases the fix. In any case, once the fix has been made public, the exploit is, by definition, no longer called a zero-day exploit.
Measuring the length of the vulnerability window can be difficult, as attackers do not announce when the vulnerability was first discovered. Developers may not want to distribute such information for commercial or security reasons. Developers also may not know if the vulnerability is being exploited when they fix it, and so may not record the vulnerability as a zero-day attack. By one estimate, "hackers exploit security vulnerabilities in software for 10 months on average before details of the holes surface in public," i.e., the average vulnerability window of a zero-day exploit is about 10 months. However, it can be easily shown that this window can be several years long. For example, in 2008, Microsoft confirmed a vulnerability in Internet Explorer, which affected some versions that were released in 2001. The date the vulnerability was first found by an attacker is not known; however, the vulnerability window in this case could have been up to 7 years. Some windows may never be closed, for example if they are hardwired in a device, requiring its replacement or the installation of additional hardware to protect the device from exploitation.
Sometimes exploitation events are seen shortly after the release of a security patch. By analyzing the patch, exploitation developers can more easily figure out how to exploit the underlying vulnerability, and attack the systems that have not been patched. Therefore the term "Exploit Wednesday" (after Patch Tuesday) was coined.
Microsoft is warning users that, after it would discontinue support for Windows XP starting on April 8, 2014, users running Windows XP will take risk of 'zero-day forever' because of reverse engineered security patches for newer Windows versions.
A special type of vulnerability management process focuses on finding and eliminating zero-day weaknesses. This unknown vulnerability management lifecycle is a security and quality assurance process that aims to ensure the security and robustness of both in-house and third party software products by finding and fixing unknown (zero-day) vulnerabilities. The unknown vulnerabilities management process consists of four phases: analyze, test, report and mitigate.
Zero-day protection is the ability to provide protection against zero-day exploits. Zero-day attacks can also remain undetected after they are launched.
Many techniques exist to limit the effectiveness of zero-day memory corruption vulnerabilities, such as buffer overflows. These protection mechanisms exist in contemporary operating systems such as Microsoft Windows 8, Windows 7, Windows Vista, Apple's Mac OS X, recent Oracle Solaris, Linux and possibly other Unix and Unix-like environments; Microsoft Windows XP Service Pack 2 includes limited protection against generic memory corruption vulnerabilities. Desktop and server protection software also exists to mitigate zero-day buffer overflow vulnerabilities.
"Multiple layers" provides service-agnostic protection and is the first line of defense should an exploit in any one layer be discovered. An example of this for a particular service is implementing access control lists in the service itself, restricting network access to it via local server firewalling (i.e., IP tables), and then protecting the entire network with a hardware firewall. All three layers provide redundant protection in case a compromise in any one of them occurs.
The use of port knocking or single packet authorization daemons may provide effective protection against zero-day exploits in network services. However these techniques are not suitable for environments with a large number of users.
Engineers and vendors such as Gama-Sec in Israel and DataClone Labs in Reno, Nevada are attempting to provide support with the Zeroday Project, which purports to provide information on upcoming attacks and provide support to vulnerable systems.
Keeping the computer’s software up-to-date is very important as well and it does help.
Users need to be careful when clicking on links or opening email attachments with images or PDF files, even if the sender is someone they know. This is how many cyber criminals deceive users, by pretending they are something they are not and gaining the user’s trust, as well as having a virus or other malware email copies of itself to the address lists of infected victims.
Utilize sites with Secure Socket Layer (SSL), which secures the information being passed between the user and the visited site.
Differing views surround the collection and use of zero-day vulnerability information. Many computer security vendors perform research on zero-day vulnerabilities in order to better understand the nature of vulnerabilities and their exploitation by individuals, computer worms and viruses. Alternatively, some vendors purchase vulnerabilities to augment their research capacity. An example of such a program is TippingPoint's Zero Day Initiative. While selling and buying these vulnerabilities is not technically illegal in most parts of the world, there is much controversy over the method of disclosure. A recent German decision to include Article 6 of the Convention on Cybercrime in the EU Framework Decision on Attacks against Information Systems may make selling or even manufacturing vulnerabilities illegal.
Most formal efforts follow some form of RFPolicy disclosure guidelines or the more recent OIS Guidelines for Security Vulnerability Reporting and Response. In general, these rules forbid the public disclosure of vulnerabilities without notification to the developer and adequate time to produce a patch.