This information should be read by ANYONE using the Internet!
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Mac users often ask whether they should install "anti-virus" software. 
The answer usually given on ASC is "no." The answer is right, but it may 
give the wrong impression that there is no threat from what are loosely 
called "viruses." There  is a threat, and you need to educate yourself 
about it.

1. This is a comment on what you should—and should not—do to protect 
yourself from malicious software ("malware") that circulates on the 
Internet and gets onto a computer as an unintended consequence of the 
user's actions. It does not apply to software, such as keystroke 
loggers, that may be installed deliberately by an intruder who has 
hands-on access to the computer, or who has been able to log in to it 
remotely. That threat is in a different category, and there's no easy 
way to defend against it.

The comment is long because the issue is complex. The key points are in 
sections 5, 6, and 10.

OS X now implements three layers of built-in protection specifically 
against malware, not counting runtime protections such as execute 
disable, sandboxing, system library randomization, and address space 
layout randomization that may also guard against other kinds of exploits.

2. All versions of OS X since 10.6.7 have been able to detect known Mac 
malware in downloaded files, and to block insecure web plugins. This 
feature is transparent to the user. Internally Apple calls it "XProtect."

The malware recognition database used by XProtect is automatically 
updated; however, you shouldn't rely on it, because the attackers are 
always at least a day ahead of the defenders.

The following caveats apply to XProtect:

☞ It can be bypassed by some third-party networking software, such as 
BitTorrent clients and Java applets.

☞ It only applies to software downloaded from the network. Software 
installed from a CD or other media is not checked.

As new versions of OS X are released, it's not clear whether Apple will 
indefinitely continue to maintain the XProtect database of older 
versions such as 10.6. The security of obsolete system versions may 
eventually be degraded. Security updates to the code of obsolete systems 
will stop being released at some point, and that may leave them open to 
other kinds of attack besides malware.

3. Starting with OS X 10.7.5, there has been a second layer of built-in 
malware protection, designated "Gatekeeper" by Apple. By default, 
applications and Installer packages downloaded from the network will 
only run if they're digitally signed by a developer with a certificate 
issued by Apple. Software certified in this way hasn't necessarily been 
tested by Apple, but you can be reasonably sure that it hasn't been 
modified by anyone other than the developer. His identity is known to 
Apple, so he could be held legally responsible if he distributed 
malware. That may not mean much if the developer lives in a country with 
a weak legal system (see below.)

Gatekeeper doesn't depend on a database of known malware. It has, 
however, the same limitations as XProtect, and in addition the following:

☞ It can easily be disabled or overridden by the user.

☞ A malware attacker could get control of a code-signing certificate 
under false pretenses, or could simply ignore the consequences of 
distributing codesigned malware.

☞ An App Store developer could find a way to bypass Apple's oversight, 
or the oversight could fail due to human error.

Apple has so far failed to revoke the codesigning certificates of some 
known abusers, thereby diluting the value of Gatekeeper and the 
Developer ID program. These failures don't involve App Store products, 
however.

For the reasons given, App Store products, and—to a lesser extent—other 
applications recognized by Gatekeeper as signed, are safer than others, 
but they can't be considered absolutely safe. "Sandboxed" applications 
may prompt for access to private data, such as your contacts, or for 
access to the network. Think before granting that access. Sandbox 
security is based on user input. Never click through any request for 
authorization without thinking.

4. Starting with OS X 10.8.3, a third layer of protection has been 
added: a "Malware Removal Tool" (MRT). MRT runs automatically in the 
background when you update the OS. It checks for, and removes, malware 
that may have evaded the other protections via a Java exploit (see 
below.) MRT also runs when you install or update the Apple-supplied Java 
runtime (but not the Oracle runtime.) Like XProtect, MRT is effective 
against known threats, but not against unknown ones. It notifies you if 
it finds malware, but otherwise there's no user interface to MRT.

5. The built-in security features of OS X reduce the risk of malware 
attack, but they are not, and never will be, complete protection. 
Malware is foremost a problem of human behavior, and no technological 
fix alone is going to solve it. Trusting software to protect you will 
only make you more vulnerable.

The best defense is always going to be your own intelligence. With the 
possible exception of Java exploits, all known malware circulating on 
the Internet that affects a fully-updated installation of OS X 10.6 or 
later takes the form of so-called "Trojan horses," which can only have 
an effect if the victim is duped into running them. The threat therefore 
amounts to a battle of wits between you and Internet criminals. If 
you're better informed than they think you are, you'll win. That means, 
in practice, that you always stay within a safe harbor of computing 
practices. How do you know when you're leaving the safe harbor? Below 
are some warning signs of danger.

Software from an untrustworthy source

☞ Software of any kind is distributed via BitTorrent, or Usenet, or on a 
website that also distributes pirated music or movies.

☞ Software with a corporate brand, such as Adobe Flash Player, doesn't 
come directly from the developer’s website. Do not trust an alert from 
any website to update Flash, or your browser, or any other software.

☞ Rogue websites such as Softonic, Soft32, and CNET Download distribute 
free applications that have been packaged in a superfluous "installer."

☞ The software is advertised by means of spam or intrusive web ads. Any 
ad, on any site, that includes a direct link to a download should be 
ignored.

Software that is plainly illegal or does something illegal

☞ High-priced commercial software such as Photoshop is "cracked" or "free."

☞ An application helps you to infringe copyright, for instance by 
circumventing the copy protection on commercial software, or saving 
streamed media for reuse without permission. All "YouTube downloaders" 
are in this category, though not all are necessarily malicious.
Conditional or unsolicited offers from strangers

☞ A telephone caller or a web page tells you that you have a “virus” and 
offers to help you remove it. (Some reputable websites did legitimately 
warn visitors who were infected with the "DNSChanger" malware. That 
exception to this rule no longer applies.)

☞ A web site offers free content such as video or music, but to use it 
you must install a “codec,” “plug-in,” "player," "downloader," 
"extractor," or “certificate” that comes from that same site, or an 
unknown one.

☞ You win a prize in a contest you never entered.

☞ Someone on a message board such as this one is eager to help you, but 
only if you download an application of his choosing.

☞ A "FREE WI-FI !!!" network advertises itself in a public place such as 
an airport, but is not provided by the management.

☞ Anything online that you would expect to pay for is "free."

Unexpected events

☞ A file is downloaded automatically when you visit a web page, with no 
other action on your part. Delete any such file without opening it.

☞ You open what you think is a document and get an alert that it's "an 
application downloaded from the Internet." Click Cancel and delete the 
file. Even if you don't get the alert, you should still delete any file 
that isn't what you expected it to be.

☞ An application does something you don't expect, such as asking for 
permission to access your contacts, your location, or the Internet for 
no obvious reason.

☞ Software is attached to email that you didn't request, even if it 
comes (or seems to come) from someone you trust.

I don't say that leaving the safe harbor just once will necessarily 
result in disaster, but making a habit of it will weaken your defenses 
against malware attack. Any of the above scenarios should, at the very 
least, make you uncomfortable.

6. Java on the Web (not to be confused with JavaScript, to which it's 
not related, despite the similarity of the names) is a weak point in the 
security of any system. Java is, among other things, a platform for 
running complex applications in a web page, on the client. That was 
always a bad idea, and Java's developers have proven themselves 
incapable of implementing it without also creating a portal for malware 
to enter. Past Java exploits are the closest thing there has ever been 
to a Windows-style virus affecting OS X. Merely loading a page with 
malicious Java content could be harmful.

Fortunately, client-side Java on the Web is obsolete and mostly extinct. 
Only a few outmoded sites still use it. Try to hasten the process of 
extinction by avoiding those sites, if you have a choice. Forget about 
playing games or other non-essential uses of Java.
Java is not included in OS X 10.7 and later. Discrete Java installers 
are distributed by Apple and by Oracle (the developer of Java.) Don't 
use either one unless you need it. Most people don't. If Java is 
installed, disable it—not JavaScript—in your browsers.

Regardless of version, experience has shown that Java on the Web can't 
be trusted. If you must use a Java applet for a task on a specific site, 
enable Java only for that site in Safari. Never enable Java for a public 
website that carries third-party advertising. Use it only on well-known, 
login-protected, secure websites without ads. In Safari 6 or later, 
you'll see a lock icon in the left side of the address bar when visiting 
a secure site.

Stay within the safe harbor, and you’ll be as safe from malware as you 
can practically be. The rest of this comment concerns what you should 
not do to protect yourself.

7. Never install any commercial "anti-virus" (AV) or "Internet security" 
products for the Mac, as they are all worse than useless. If you need to 
be able to detect Windows malware in your files, use one of the free 
security apps in the Mac App Store—nothing else.

Why shouldn't you use commercial AV products?

☞ To recognize malware, the software depends on a database of known 
threats, which is always at least a day out of date. This technique is a 
proven failure, as a major AV software vendor has admitted. Most attacks 
are "zero-day"—that is, previously unknown. Recognition-based AV does 
not defend against such attacks, and the enterprise IT industry is 
coming to the realization that traditional AV software is worthless.

☞ Its design is predicated on the nonexistent threat that malware may be 
injected at any time, anywhere in the file system. Malware is downloaded 
from the network; it doesn't materialize from nowhere. In order to meet 
that nonexistent threat, commercial AV software modifies or duplicates 
low-level functions of the operating system, which is a waste of 
resources and a common cause of instability, bugs, and poor performance.

☞ By modifying the operating system, the software may also create 
weaknesses that could be exploited by malware attackers.

☞ Most importantly, a false sense of security is dangerous.

8. An AV product from the App Store, such as "ClamXav," has the same 
drawback as the commercial suites of being always out of date, but it 
does not inject low-level code into the operating system. That doesn't 
mean it's entirely harmless. It may report email messages that have 
"phishing" links in the body, or Windows malware in attachments, as 
infected files, and offer to delete or move them. Doing so will corrupt 
the Mail database. The messages should be deleted from within the Mail 
application.

An AV app is not needed, and cannot be relied upon, for protection 
against OS X malware. It's useful, if at all, only for detecting Windows 
malware, and even for that use it's not really effective, because new 
Windows malware is emerging much faster than OS X malware.
Windows malware can't harm you directly (unless, of course, you use 
Windows.) Just don't pass it on to anyone else. A malicious attachment 
in email is usually easy to recognize by the name alone. An actual example:

London Terror Moovie.avi [124 spaces] Checked By Norton Antivirus.exe

You don't need software to tell you that's a Windows trojan. Software 
may be able to tell you which trojan it is, but who cares? In practice, 
there's no reason to use recognition software unless an organizational 
policy requires it. Windows malware is so widespread that you should 
assume it's in every email attachment until proven otherwise.

Nevertheless, ClamXav or a similar product from the App Store may serve 
a purpose if it satisfies an ill-informed network administrator who says 
you must run some kind of AV application. It's free and it won't 
handicap the system.

The ClamXav developer won't try to "upsell" you to a paid version of the 
product. Other developers may do that. Don't be upsold. For one thing, 
you should not pay to protect Windows users from the consequences of 
their choice of computing platform. For another, a paid upgrade from a 
free app will probably have all the disadvantages mentioned in section 7.

9. It seems to be a common belief that the built-in Application Firewall 
acts as a barrier to infection, or prevents malware from functioning. It 
does neither. It blocks inbound connections to certain network services 
you're running, such as file sharing. It's disabled by default and you 
should leave it that way if you're behind a router on a private home or 
office network. Activate it only when you're on an untrusted network, 
for instance a public Wi-Fi hotspot, where you don't want to provide 
services. Disable any services you don't use in the Sharing preference 
pane. All are disabled by default.

10. As a Mac user, you don't have to live in fear that your computer may 
be infected every time you install software, read email, or visit a web 
page. But neither can you assume that you will always be safe from 
exploitation, no matter what you do. Navigating the Internet is like 
walking the streets of a big city. It's as safe or as dangerous as you 
choose to make it. The greatest harm done by security software is 
precisely its selling point: it makes people feel safe. They may then 
feel safe enough to take risks from which the software doesn't protect 
them. Nothing can lessen the need for safe computing practices.

Linc Davis
Oct 28, 2014

-- 
The only people who make a difference are the people who believe they can.
--- NewsGate v1.0 gamma 2