You may have installed a variant of the "VSearch" ad-injection malware. Follow Apple Support's instructions to remove it.
If you have trouble following those instructions, see below.
Malware is always changing to get around the defenses against it. This procedure works as of now, as far as I know. It may not work in the future. Anyone finding this comment a few days or more after it was posted should look for a more recent discussion, or start a new one.
The VSearch malware tries to hide itself by varying the names of the files it installs. To remove it, you must first identify the naming pattern.
Triple-click the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:
/Library/LaunchDaemons
In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.
A folder named "LaunchDaemons" may open. Look inside it for two files with names of the form
com.something.daemon.plist
and
com.something.helper.plist
Here something is a variable string of characters, which can be different in each case. So far it has always been a string of letters without punctuation, such as "cloud," "dot," "highway," "submarine," or "trusteddownloads." Sometimes it's a meaningless string such as "e8dec5ae7fc75c28" rather than a word. Sometimes the string is "apple," and then you must be especially careful not to delete the wrong files, because many built-in OS X files have similar names.
If you find these files, leave the LaunchDaemons folder open, and open the following folder in the same way:
/Library/LaunchAgents
In this folder, there may be a file named
com.something.agent.plist
where the string something is the same as before.
If you feel confident that you've identified the above files, back up all data, then drag just those three files—nothing else—to the Trash. You may be prompted for your administrator login password. Close the Finder windows and restart the computer.
Don't delete the "LaunchAgents" or "LaunchDaemons" folder or anything else inside either one.
The malware is now permanently inactivated, as long as you never reinstall it. You can stop here if you like, or you can remove two remaining components for the sake of completeness.
Open this folder:
/Library/Application Support
If it has a subfolder named just
something
where something is the same string you saw before, drag that subfolder to the Trash and close the window.
Don't delete the "Application Support" folder or anything else inside it.
Finally, in this folder:
/System/Library/Frameworks
there may an item named exactly
v.framework
It's actually a folder, though it has a different icon than usual. This item always has the above name; it doesn't vary. Drag it to the Trash and close the window.
Don't delete the "Frameworks" folder or anything else inside it.
If you didn't find the files or you're not sure about the identification, post what you found.
If in doubt, or if you have no backups, change nothing at all.
The trouble may have started when you downloaded and ran an application called "MPlayerX." That's the name of a legitimate free movie player, but the name is also used fraudulently to distribute VSearch. If there is an item with that name in the Applications folder, delete it, and if you wish, replace it with the genuine article from mplayerx.org.
This trojan is often found on illegal websites that traffic in pirated content such as movies. If you, or anyone else who uses the computer, visit such sites and follow prompts to install software, you can expect more of the same, and worse, to follow. Never install any software that you downloaded from a bittorrent, or that was downloaded by someone else from an unknown source.
In the Security & Privacy pane of System Preferences, select the General tab. The radio button marked Anywhere should not be selected. If it is, click the lock icon to unlock the settings, then select one of the other buttons. After that, don't ignore a warning that you are about to run or install an application from an unknown developer.
Then, still in System Preferences, open the App Store or Software Update pane and check the box marked
Install system data files and security updates (OS X 10.10 or later)
or
Download updates automatically (OS X 10.9 or earlier)
if it's not already checked.
