virus scans

1. This comment applies to malicious software ("malware") that's installed unwittingly by the victim of a network attack. 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 victim's computer. That threat is in a different category, and there's no easy way to defend against it. If you have reason to suspect that you're the target of such an attack, you need expert help.

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, but internally Apple calls it "XProtect." The malware recognition database is automatically updated once a day; however, you shouldn't rely on it, because the attackers are always at least a day ahead of the defenders.

• It can be bypassed by some third-party networking software, such as BitTorrent clients and Java applets (see below.)
• It only applies to software downloaded from the network. Software installed from a CD or other media is not checked.

3. Starting with OS X 10.7.5, there has been another 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 actually been tested by Apple (unless it comes from the Mac App Store), 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. For most practical purposes, applications recognized by Gatekeeper as signed can be considered safe.

• 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 find some other way to evade Apple's controls.

For more information about Gatekeeper, see this Apple Support article.

• Any website that prompts you to install a “codec,” “plug-in,” "player," "archive extractor," or “certificate” that comes from that same site, or an unknown one, is untrustworthy.
• A web operator who tells you that you have a “virus,” or that anything else is wrong with your computer, or that you have won a prize in a contest you never entered, is trying to commit a crime with you as the victim. (Some reputable websites did legitimately warn users who were infected with the "DNSChanger" malware. That exception to this rule no longer applies.)
• Pirated copies or "cracks" of commercial software, no matter where they come from, are unsafe.
• Software of any kind downloaded from a BitTorrent or from a Usenet binary newsgroup is unsafe.
• Software with a corporate brand, such as Adobe Flash Player, must be downloaded directly from the developer’s website. If it comes from any other source, it's unsafe.

5. Java on the Internet ( 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. Its developers have had a lot of trouble getting it to do this 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" in OS X. Merely loading a page with malicious Java content could be harmful.

Java is not included in OS X 10.7 and later. A separate Java installer is distributed by Apple, and another one by Oracle (the developer of Java.) Don't use either one unless you need it. At the moment, the Oracle version is blocked by Apple because of known security flaws that make it unsafe to use on the Internet. If Java is installed, disable it — not JavaScript — in your browsers. In Safari, this is done by unchecking the box marked Enable Java in the Security tab of the preferences dialog. Few websites have Java content nowadays, so you probably won’t be missing much.

Regardless of version, experience has shown that Java on the Web can't be trusted. If you must use a Java applet for a specific task, enable Java only when needed for the task and disable it immediately when done. Close all other browser windows and tabs, and don't visit any other sites while Java is active. Eliminate Java from your online workflow whenever possible. If a web page prompts you to use Java to do something that can be done without it, such as streaming video or downloading files, don't.

Follow these guidelines, and you’ll be as safe from malware as you can reasonably be.

6. Never install any commercial "anti-virus" or "Internet security" products for the Mac, as they all do more harm than good, if they do any good at all. If you need to be able to detect Windows malware in your files, use the free software ClamXav — nothing else.

Why shouldn't you use commercial "anti-virus" products?

• Their 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, the 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 itself may create weaknesses that could be exploited by malware attackers.

7. ClamXav doesn't have these drawbacks. That doesn't mean it's entirely safe. 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.

A Windows malware attachment in email is usually easy to recognize. The file name will often be targeted at people who aren't very bright; for example:

♥♥♥♥♥♥♥♥♥♥♥♥♥♥!!!!!!!H0TBABEZ4U!!!!!!!.AVI♥♥♥♥♥♥♥♥♥♥♥♥♥♥.exe

ClamXav may be able to tell you which particular virus or trojan it is, but do you care? In practice, there's seldom a reason to use ClamXav unless a network administrator requires you to run an anti-virus application.

8. The greatest harm done by anti-virus software, in my opinion, is in its effect on human behavior. It does little or nothing to protect people from emerging threats, but they get a false sense of security from it, and then they may behave in ways that expose them to higher risk. Nothing can lessen the need for safe computing practices.

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.