Keystroke logging
Keystroke logging, often referred to as keylogging or keyboard capturing, is the action of recording (logging) the keys struck on a [keyboard][2], typically covertly, so that the person using the keyboard is unaware that their actions are being monitored. Data can then be retrieved by the person operating the logging program. A keylogger can be either software or hardware.
While the programs themselves are legal, with many of them being designed to allow employers to oversee the use of their computers, keyloggers are most often used for the purpose of stealing passwords and other confidential information.
Software-based Keyloggers
- Hypervisor-based: The keylogger can theoretically reside in a malware hypervisor running underneath the operating system, which thus remains untouched. It effectively becomes a virtual machine. Blue Pill is a conceptual example.
- Kernel-based: A program on the machine obtains root access to hide itself in the OS and intercepts keystrokes that pass through the kernel. This method is difficult both to write and to combat. Such keyloggers reside at the kernel level, which makes them difficult to detect, especially for user-mode applications that don't have root access.
- API-based: These keyloggers hook keyboard APIs inside a running application. The keylogger registers keystroke events, as if it was a normal piece of the application instead of malware. The keylogger receives an event each time the user presses or releases a key. The keylogger simply records it.
- Form grabbing based: Form grabbing-based keyloggers log [web form][33] submissions by recording the web browsing on submit events. This happens when the user completes a form and submits it, usually by clicking a button or hitting enter. This type of keylogger records form data before it is passed over the Internet.
- Javascript-based: A malicious script tag is injected into a targeted web page, and listens for key events such as
onKeyUp(). Scripts can be injected via a variety of methods, including cross-site scripting, man-in-the-browser, man-in-the-middle, or a compromise of the remote web site. - Memory injection based: Memory Injection (MitB)-based keyloggers perform their logging function by altering the memory tables associated with the browser and other system functions. By patching the memory tables or injecting directly into memory, this technique can be used by malware authors to bypass Windows UAC (User Account Control). The Zeus and SpyEye trojans use this method exclusively.
Related features
Software keyloggers may be augmented with features that capture user information without relying on keyboard key presses as the sole input. Some of these features include:
- Clipboard logging. Anything that has been copied to the clipboard can be captured by the program.
- Screen logging. Screenshots are taken to capture graphics-based information. Applications with screen logging abilities may take screenshots of the whole screen, of just one application, or even just around the mouse cursor. They may take these screenshots periodically or in response to user behaviours (for example, when a user clicks the mouse). A practical application that is used by some keyloggers with this screen logging ability, is to take small screenshots around where a mouse has just clicked; thus defeating web-based keyboards (for example, the web-based screen keyboards that are often used by banks), and any web-based on-screen keyboard without screenshot protection.
Hardware-based Keyloggers
- Firmware-based: BIOS-level firmware that handles keyboard events can be modified to record these events as they are processed. Physical and/or root-level access is required to the machine, and the software loaded into the BIOS needs to be created for the specific hardware that it will be running on.
- Keyboard hardware: Hardware keyloggers are used for keystroke logging by means of a hardware circuit that is attached somewhere in between the computer keyboard and the computer, typically inline with the keyboard's cable connector. There are also USB connectors based Hardware keyloggers as well as ones for Laptop computers (the Mini-PCI card plugs into the expansion slot of a laptop). More stealthy implementations can be installed or built into standard keyboards, so that no device is visible on the external cable. Both types log all keyboard activity to their internal memory, which can be subsequently accessed, for example, by typing in a secret key sequence. A hardware keylogger has an advantage over a software solution: it is not dependent on being installed on the target computer's operating system and therefore will not interfere with any program running on the target machine or hardly be detected by any software. However its physical presence may be detected if, for example, it is installed outside the case as an inline device between the computer and the keyboard. Some of these implementations have the ability to be controlled and monitored remotely by means of a wireless communication standard.
- Wireless keyboard and mouse sniffers: These passive sniffers collect packets of data being transferred from a wireless keyboard and its receiver. As encryption may be used to secure the wireless communications between the two devices, this may need to be cracked beforehand if the transmissions are to be read. In some cases this enables an attacker to type arbitrary commands into a victim's computer.
- Keyboard overlays: Criminals have been known to use keyboard overlays on ATMs to capture people's PINs. Each keypress is registered by the keyboard of the ATM as well as the criminal's keypad that is placed over it. The device is designed to look like an integrated part of the machine so that bank customers are unaware of its presence.
- Acoustic keyloggers: Acoustic cryptanalysis can be used to monitor the sound created by someone typing on a computer. Each key on the keyboard makes a subtly different acoustic signature when struck. It is then possible to identify which keystroke signature relates to which keyboard character via statistical methods such as frequency analysis. The repetition frequency of similar acoustic keystroke signatures, the timings between different keyboard strokes and other context information such as the probable language in which the user is writing are used in this analysis to map sounds to letters. A fairly long recording (1000 or more keystrokes) is required so that a big enough sample is collected.
- Electromagnetic emissions: It is possible to capture the electromagnetic emissions of a wired keyboard from up to 20 metres (66 ft) away, without being physically wired to it. In 2009, Swiss researchers tested 11 different USB, PS/2 and laptop keyboards in a semi-anechoic chamber and found them all vulnerable, primarily because of the prohibitive cost of adding shielding during manufacture. The researchers used a wide-band receiver to tune into the specific frequency of the emissions radiated from the keyboards.
- Optical surveillance: Optical surveillance, while not a keylogger in the classical sense, is nonetheless an approach that can be used to capture passwords or PINs. A strategically placed camera, such as a hidden surveillance camera at an ATM, can allow a criminal to watch a PIN or password being entered.
- Physical evidence: For a keypad that is used only to enter a security code, the keys which are in actual use will have evidence of use from many fingerprints. A passcode of four digits, if the four digits in question are known, is reduced from 10,000 possibilities to just 24 possibilities (104 versus 4! (factorial of 4)). These could then be used on separate occasions for a manual "brute force attack".
- Smartphone sensors: Researchers have demonstrated that it is possible to capture the keystrokes of nearby computer keyboards using only the commodity accelerometer found in smartphones. The attack is made possible by placing a smartphone near a keyboard on the same desk. The smartphone's accelerometer can then detect the vibrations created by typing on the keyboard, and then translate this raw accelerometer signal into readable sentences with as much as 80 percent accuracy. The technique involves working through probability by detecting pairs of keystrokes, rather than individual keys. Similar techniques have also been shown to be effective at capturing keystrokes on touchscreen keyboards while in some cases, in combination with gyroscope or with the ambient-light sensor.
Countermeasures
Denial of physical access to sensitive computers, e.g. by locking the server room, is the most effective means of preventing hardware keylogger installation. Visual inspection is the easiest way of detecting hardware keyloggers. But there are also some techniques that can be used for most hardware keyloggers on the market, to detect them via software.
The main risk associated with keylogger use is that physical access is needed twice: initially to install the keylogger, and secondly to retrieve it. Thus, if the victim discovers the keylogger, they can then set up a sting operation to catch the person in the act of retrieving it. This could include camera surveillance or the review of access card swipe records to determine who gained physical access to the area during the time period that the keylogger was removed.
See also Keystroke logging countermeasures.