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October 29, 2014, 10:51 am
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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
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,
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
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
☞ 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."
☞ 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.
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
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
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.
Oct 28, 2014
The only people who make a difference are the people who believe they can.
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