Читать книгу Security Engineering - Ross Anderson - Страница 224

Popular operating systems such as Android, Linux and Windows are very large and complex, with their features tested daily by billions of users under very diverse circumstances. Many bugs are found, some of which give rise to vulnerabilities, which have a typical lifecycle. After discovery, a bug is reported to a CERT or to the vendor; a patch is shipped; the patch is reverse-engineered, and an exploit may be produced; and people who did not apply the patch in time may find that their machines have been compromised. In a minority of cases, the vulnerability is exploited at once rather than reported – called a zero-day exploit as attacks happen from day zero of the vulnerability's known existence. The economics, and the ecology, of the vulnerability lifecycle are the subject of study by security economists; I'll discuss them in Part 3.

The traditional goal of an attacker was to get a normal account on the system and then become the system administrator, so they could take over the system completely. The first step might have involved guessing, or social-engineering, a password, and then using an operating-system bug to escalate from user to root [1131].

The user/root distinction became less important in the twenty-first century for two reasons. First, Windows PCs were the most common online devices (until 2017 when Android overtook them) so they were the most common attack targets; and as they ran many applications as administrator, an application that could be compromised typically gave administrator access. Second, attackers come in two basic types: targeted attackers, who want to spy on a specific individual and whose goal is typically to acquire access to that person's accounts; and scale attackers, whose goal is typically to compromise large numbers of PCs, which they can organise into a botnet. This, too, doesn't require administrator access. Even if your mail client does not run as administrator, it can still be used by a spammer who takes control.

However, botnet herders do prefer to install rootkits which, as their name suggests, run as root; they are also known as remote access trojans or RATs. The user/root distinction does still matter in business environments, where you do not want such a kit installed as an advanced persistent threat by a hostile intelligence agency, or by a corporate espionage firm, or by a crime gang doing reconnaissance to set you up for a large fraud.

A separate distinction is whether an exploit is wormable – whether it can be used to spread malware quickly online from one machine to another without human intervention. The Morris worm was the first large-scale case of this, and there have been many since. I mentioned Wannacry and NotPetya in chapter 2; these used a vulnerability developed by the NSA and then leaked to other state actors. Operating system vendors react quickly to wormable exploits, typically releasing out-of-sequence patches, because of the scale of the damage they can do. The most troublesome wormable exploits at the time of writing are variants of Mirai, a worm used to take over IoT devices that use known root passwords. This appeared in October 2016 to exploit CCTV cameras, and hundreds of versions have been produced since, adapted to take over different vulnerable devices and recruit them into botnets. Wormable exploits often use root access but don't have to; it is sufficient that the exploit be capable of automatic onward transmission⁹. I will discuss the different types of malware in more detail in section 21.3.

However, the basic types of technical attack have not changed hugely in a generation and I'll now consider them briefly.