Security arrangements for extended USB protocol stack of a USB host system
Security arrangements for a universal serial bus (USB) protocol stack of a USB host system are provided. The security arrangements prevent an unauthorized or suspicious USB device from communicating with the host system, detect suspicious activity originating from a device which is already communicating with the host system and may provide notification to a user.
BACKGROUND
1. Technical Field
The present disclosure relates generally to universal serial bus (USB) host systems and, more specifically, to security arrangements for an extended USB protocol stack of a USB host system.
2. Background Information
Over the past 15 years, USB has emerged as the preferred, if not dominant, is technology for interfacing personal computers (PCs) or other host systems with a wide variety of devices including mass storage, smartphones, digital cameras, media players, keyboards and the like. Users enjoy the convenience and simplicity of USB, while manufacturers and software developers enjoy the benefits of having a single standard interface to support, although there are several different USB specifications that include different speeds.
While the USB protocol has proved sufficiently robust for typical user data transfers over short distances, the protocol does not include any security features to prevent or limit the effects of malicious activity originating from a USB device. For example, the USB protocol for wired communication does not require authentication of a USB device before permitting communication between the device and a host system. Thus, if a wired USB device identifies itself to a host system and the host has the appropriate driver available for that device, communication is established and the USB device is simply presumed to be non-malicious. Consequently, an unauthorized or malicious user could gain access to, disable or otherwise interfere with a host system by programming a USB device to launch an attack, deliver a virus, download data from the host system and the like.
SUMMARY OF THE INVENTION
In brief summary, the present invention provides security arrangements which prevent a suspicious or unauthorized USB device from communicating with a host system, or prevents continued communications between a USB device and the host system where suspicious activity is detected originating from a USB device. User involvement in the security arrangements may be permitted or not as desired. For example, a user may be given a visual warning of the presence of a suspicious USB device along with the option to permit or deny communication between that device and the host system. Alternatively, a suspicious USB device may be denied communication with the host system without user involvement.
In accordance with a first embodiment of the invention, security arrangements are provided through an extension of the USB protocol stack, e.g., through the addition of a security control module (SCM) to the stack. In accordance with a second embodiment of the invention, security arrangements are provided through a host controller. The first and second embodiments may be combined such that the USB protocol stack and host controller cooperate to provide the desired security arrangements. In accordance with yet another embodiment of the invention, at least part of the security arrangements are provided in connection with the host system's BIOS functions. To meet the requirements of a particular application or environment, various embodiments of the invention may be implemented in either hardware or software or a combination of both.
In accordance with various embodiments of the invention, a variety of security techniques may be applied either individually or in combination. First, a security control application interface (SCAI), which interfaces with the SCM, may be used to examine a new USB device when it is initially connected to the host system and prior to allowing communication between the device and the host's operating system. In determining whether to allow such communication, the SCAI may examine a variety of information from the USB device, including its device, configuration, interface and string descriptors, looking for suspicious patterns. Similarly, the SCAI may look for suspicious combinations of functions associated with the USB device. The SCAI may also include an application programming interface through which anti-virus applications, firewall applications or other applications may examine or test the USB device or provide further information to be used in the determining whether to allow the device to communicate with the host's operating system. The SCAI may also provide a user configuration interface through which a user may direct various actions including selecting or changing a desired security level, identifying in advance pre-approved or disapproved USB devices, and the like.
The SCAI may also interface with a device identification database or other data is structure which may be used to maintain a whitelist of approved USB devices, a blacklist of prohibited USB devices, or a combination of both. When a new USB device is initially connected to the host system, the SCAI may retrieve specific information about the device (e.g., its manufacturer ID, product ID or other information) and compare such information to the whitelist and/or blacklist to determine if the USB device is clear to communicate with the host system. The contents of the device identification database may be updated in the field by any of a variety of techniques including host-initiated, automatic communication with a designated website, remote access to the host system by an authorized service provider, and user-initiated action.
In addition, a run-time device identification module (DIM) may be provided and interfaced with the SCM, device identification database and additional modules which perform static analysis or heuristic analysis on activity originating from USB devices. Based on the results of static or heuristic analysis (or both), the DIM may output a dynamic indication of whether a particular USB device is operating as expected or in a suspicious manner. Such indication is provided to an admission control module (ACM) which may determine when suspicious activity rises to the level that action is warranted. The ACM communicates with a threat notification module which may provide a visual, message-based or other notification to a user of the perceived threat and request the user to take appropriate action which could include manually approving or disapproving a particular USB device. In addition, the ACM may, without user involvement, block a USB device from communicating with or further communicating with the host system based on an observed violation of security policy.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which:
FIG. 1 is a block diagram of a USB host system;
FIG. 2 is a block diagram of a conventional USB protocol stack; and
FIG. 3 is a block diagram of an extended USB protocol stack, which includes security arrangements in accordance with embodiments of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
FIG. 1 shows a block diagram of a conventional USB host system 100 which may, for example, represent a personal computer (PC). The USB host system100 illustratively includes a processor 110, memory 120, a Basic Input/Output System (BIOS) 130 and a USB host controller 150 interconnected by a system bus 140. The BIOS 130 provides a firmware interface that performs various testing and initialization functions (such as, e.g., power-on self test, identification and initialization of system devices) prior to executing boot loader software needed to bootstrap or "boot" the system 100. The USB host controller150 contains the mechanical, electrical and signaling circuitry needed to connect the host system 100 to USB devices 236 coupled to a USB bus 155. To that end, the USB host controller 150 provides a hardware interface for the USB devices 236, which may include a keyboard, mouse, mass storage, digital camera as well as many other types of devices.
The memory 120 may include a plurality of locations that are addressable by processor 110, BIOS 130 and/or USB host controller 150 for storing software programs and data structures associated with the embodiments described herein. The processor 110 may include logic adapted to execute the software programs and manipulate (i.e., control) the data structures, such as a device identification database described herein. An exemplary processor 110 may include single or multi-core Central Processing Unit (CPU), Graphics Processing Unit (GPU), Field Programmable Gate Array (FPGA) and/or Application Specific Integrated Circuit (ASIC).
An operating system 122, portions of which are typically resident in memory 120and executed by the processor 110, functionally organizes the host system 100by, inter alia, invoking operations in support of software processes and/or applications executing on the host system. The operating system 122 is illustratively organized as a series of software layers including protocol stacks organized as file system, network and/or direct input stacks, as described herein. A suitable operating system 122 may include the Windows® series of operating systems from Microsoft Corp. of Redmond, Wash., the Linux® operating system, the MAC OS® series of operating systems from Apple Inc. of Cupertino, Calif., and the UNIX® series of operating system, among others. The software processes/applications may include various processes/applications of an extended USB protocol stack security arrangement 302 and, as such, contain computer executable instructions executed by processor 110 of host system 100 to perform functions described herein.
It will be apparent to those skilled in the art that other types of processors and memory, including various tangible non-transitory computer-readable media, may be used to store and execute program instructions pertaining to the system described herein. Also, while the embodiments herein are described in terms of software processes/applications stored in memory, alternative embodiments also include those processes/applications being embodied as modules consisting of hardware, software, firmware, or combinations thereof.
FIG. 2 is a block diagram of a USB host system 200 with a conventional USB protocol stack 232. File system applications 202 may include directory and file browsers provided by the operating system and other suppliers (e.g., "Explorer" from Microsoft Corp. and "Finder" from Apple Inc.). Anti-virus applications 204may include software available from Symantec, McAfee and other suppliers. Network applications 206 may include web browsers available from Microsoft Corp. and other suppliers. Firewall applications 208 may include software available from Kaspersky, Online Armor and other suppliers. Direct input applications 210 may include word processing and spreadsheet applications available from Microsoft and other suppliers.
A file system stack 212, examples of which are NTFS and FAT32, may form part of the operating system 122 of host system 200 and enables communication between file is system applications 202 and a mass storage function driver 218. Similarly, a network stack 214 may form part of the operating system and supports various network protocols such as TCP/IP, HTTP and others, thus enabling communication between network applications 206 and a network function driver220. A direct input stack 216 which may also form part of the operating system supports direct user input (e.g., keyboard strokes or mouse clicks) and enables communication between direct input applications 210 and human interface device (HID) class function drivers 222.
Drivers 218, 220 and 222 communicate with a USB composite driver 224. In turn, USB composite driver 224communicates with a USB stack 232 which enables communication between composite driver 224 and USB host controller (hardware) 234. USB stack 232 includes a USB hub driver 226, USBD interface driver 228 and USB host controller driver 230.
FIG. 3 shows a block diagram of a USB host system 300 which includes an extended USB protocol stack 302 and which embodies USB security arrangements in accordance with at least one embodiment of the present invention. For clarity and brevity, functional blocks shown in FIG. 3 which were previously described are identified by their prior reference numbers.
The extended USB stack 302 includes a security control module (SCM) 304. SCM 304 is preferably disposed between USBD interface driver 228 and USB host controller driver 230 and thus enables (or disables) communication between those drivers. SCM 304 is interfaced with a run-time device identification module (DIM) 306 and a security control application interface (SCAI) module 318. DIM 306 is interfaced with static analysis functions 310, heuristic analysis functions 312 and an admission control module (ACM) 308. A threat notification module 314 is interfaced with ACM 308and a user notification interface module 322. A device identification database (DID) 316 is interfaced with both DIM 306 and SCAI 318. A user configuration interface module 320 is also interfaced with SCAI 318.
In an illustrative embodiment, the software processes and/or modules, including is the drivers and functions, described herein may be implemented as separately-scheduled processes within the USB host system 300; however, in an alternate embodiment, the modules may be implemented as pieces of code within a single process. Communication between modules of the host system 300 may thus illustratively be effected through the use of message passing, shared memory or remote procedure calls (RPCs) between the modules. A suitable message-passing, shared memory and/or RPC mechanism provided by the operating system 122 to transfer information between modules (processes) is the Inter Process Communication (IPC) mechanism.
The functions of the SCM, SCAI, DIM and ACM may be distributed among various combinations of the host system, the host system's BIOS and the USB protocol stack. For example in USB host system illustrated in FIG. 2, when a new USB device is connected to the host controller 234, the host system 200 detects the new device (usually by monitoring a USB data line) and sends a packet (or packets) of information to the new USB device and receives packets from the device. This initial exchange of packets does not represent the data transfers that will occur in the typical operation of a USB device. Rather, this initial exchange of packets allows the host system to select the correct driver for the new USB device. At some time during this initialization, the criteria used by the SCM and SCAI to detect unauthorized USB devices may be applied such that the host system may prevent further communications when a suspicious or unauthorized USB device is detected. In practice, the functions and criteria necessary to implement a desired security arrangement may be distributed in various ways among the host system, its I/O system and the SCM/SCAI modules shown in FIG. 3.
A similar distribution of functions may be implemented among the host system, its BIOS, and the SCM, DIM and ACM modules for run time suspicious activity as described below with respect to the DIM and ACM module functions.
The operation of the security arrangements (e.g., modules and/or functions) shown in FIG. 3 will now be described. At a high level, SCM 304, in response to input received from either SCAI 318 or DIM 306 (or both) functions to either enable or disable communication between a given USB device 236 and the remainder of USB host system 300. More specifically, if a given USB device 236 is identified as blacklisted or suspicious, as described in detail below, SCM 304 cooperates with the SCAI 318 and/or DIM 306 to block communication between USB host controller driver 230 and USBD interface driver228 with respect to that USB device. Conversely, if a given USB device 236 is identified as whitelisted (cleared) or not suspicious, SCM 304 permits communication between USB host controller driver 230 and USBD interface driver 228 with respect to that USB device.
When a new USB device 236 is first connected to USB host controller hardware 234 and prior to allowing any communication between that device and the remainder of host system 300, SCM 304 reports the presence of the new USB device to SCAI 318 and run-time DIM 306. In response, SCAI 318 may request information regarding the new USB device, including its device, configuration (interface) and string descriptors, and possibly other information such as the device's functions. Within such information, SCAI 318 looks for suspicious patterns such as mismatched descriptors, unconventional combinations of functions and the like.
Certain aspects of descriptors which are germane will now be described. In general, each descriptor starts with a single byte defining the descriptor's length. A device descriptor may include the device class and USB specification version number. A configuration descriptor might include the interface descriptor and the total number of bytes in the configuration descriptors. The interface descriptor, if not in the configuration descriptors, includes alternative settings and classes. The string descriptor may include manufacturer, e.g., vendor or manufacturer identification (ID), product, e.g., product ID and other information strings, e.g., serial number. The descriptors provide sufficient information to at least allow a host system to select the proper driver for the USB device.
In addition, or alternatively, SCAI 318 may compare information about the new USB device with information stored in DID316 or, if host system 300 has Internet connectivity, SCAI 318 may query a web resource (e.g., database) to determine if the new USB device has been identified as either safe or suspicious with a default condition of suspicious. DID 316 is preferably used to maintain a whitelist of approved USB devices, a blacklist of prohibited USB devices, or a combination of both. SCAI 318 may retrieve specific information about the USB device (e.g., manufacturer ID, product ID and/or other information) and compare such information to the whitelist and/or blacklist to determine if the device should be permitted to communicate with host system 300.
The above functions of the SCM 304 together with the SCAI 318 may be performed partly in the host 300 and the host's BIOS 130. The comparing of the new USB device's information to the listed information that precludes the USB device from communicating with the host could be part of the host's operating system 122 as would be understood by those skilled in the art.
In order to provide SCAI 318 with further information to consider in determining whether the new USB device should be permitted to communicate with host system 300, SCAI 318 may also include an application programming interface (API)324 through which anti-virus applications, firewall applications or other applications may examine or test the new USB device or provide further information to be used in determining whether to allow the device to communicate (or to continue to communicate) with the host system. Thus, for example, if an anti-virus application detected a suspected virus on the new USB device, that information could cause SCAI 318 and SCM 304 to block communication until the virus was neutralized.
In addition, SCAI 318 may also receive information from a user through user configuration interface 320. Through interface320, a user may direct various actions including selecting or changing a desired security level (e.g., low, medium or high) depending upon the user's environment or perceived risk level, identifying in advance preapproved or disapproved USB devices for addition to the whitelist or blacklist maintained in DID 316, saving a user's security settings and the like. A USB thumb drive (not shown) or other storage device could be used to both store a user's security settings, including a copy of DID 316, and provide a convenient mechanism for porting such information to an embedded system. By storing such information in a password protected, encrypted file, loss or theft of the thumb drive would not compromise security. In addition to serving as a storage and transport mechanism, the thumb drive could also is function as a dongle that must be physically attached to a given system in order for the system to operate. A user's security settings could also be stored in a secure website or network cloud, which would allow for easy retrieval for mobile users.
As discussed above, the contents of DID 316 may be updated in the field by any of a variety of techniques including host-initiated, automatic communication with a designated website, remote access to the host system by an authorized service provider, and user-initiated action.
Once a particular USB device 236 is cleared to communicate with host system 300, additional security arrangements may be applied. Specifically, once a particular USB device 236 begins communication with host system 300, run-time device identification module (DIM) 306 may monitor such communications for suspicious or unauthorized activity. Run-time DIM306 may rely upon either static analysis functions 310 or heuristic analysis functions 312 (or both), as well as information from DID 316, in looking for suspicious activities. An example of a static analysis function is deep packet inspection in which the data, and possibly the commands, contained in packets originating from the USB device are examined for suspicious patterns. Similarly, packets originating from host system 300 and destined for one of USB devices 236 may be examined to determine if the data contained in the packets is appropriate for the USB device.
Another example of a static analysis function is applicable to a USB mass storage device. Run-time DIM 306 may store a hash of certain predetermined storage locations on the USB mass storage device using hash techniques known to those skilled in the art. During operation the USB mass storage device, run-time DIM 306 may monitor the device for suspicious changes in the hash and output an appropriate indication to ACM 308.
Another example of a static analysis function is applicable to suspicious activity identified with USB HID devices, such as a HID keyboard, connected to the USB host controller. For example, run-time DIM 306 may detect and reject pressing of the Windows® key of the keyboard, thereby defeating a variety of Windows®-R (nm) is attempts (attacks) that would run known programs to gain administrative access. Similarly, with respect to a HID mouse identified as suspicious, run-time DIM 306 may detect and reject left- or right-clicks which attempt to gain administrative access, including access to menus that run programs or allow unprotected access to host system 300.
An example of a heuristic analysis function is to monitor the rate of keystrokes originating from a USB device that has identified itself as a keyboard. If the keystroke rate exceeds a predetermined limit representing the maximum typing speed of a human, then that USB device would be flagged as suspicious by DIM 306 and an appropriate indication output to ACM 308. ACM 308 would then determine what action was warranted. In this example, an excessively high rate of keystrokes might indicate malicious activity (e.g., a denial of service type attack), but might also indicate that something has fallen on the keyboard or a key has stuck and the user has not noticed the problem.
Depending on the seriousness of the observed threat, ACM 308 could automatically block further communication from the suspicious USB device without user involvement, allow further communication but issue a threat notification via module314, allow further communication but make a record of it, or take other action. Threat notification module 314, in turn, may present a human perceivable warning of the detected threat through user notification interface 322. In the case in which host system 300 is a PC or other system that includes an attached video display or television, the human perceivable warning may be in the form of a warning displayed as video. In the case in which host system 300 is an embedded system that does not include an attached video display, the human perceivable warning may be in the form of an audible warning, a flashing light or other display on a front panel of the system that is normally visible to a user, or the like. In cases where host system 300 includes Internet connectivity or PSTN connectivity, a warning in the form of email, text message or recorded announcement phone call could also be provided.
In accordance with yet another embodiment of the invention, at least part of the security arrangements described above may be provided in connection with a host system's BIOS functions. To guard against malicious activity originating from a USB device during the time period before the host system's operating system becomes active, the host system's BIOS 130may be extended such that only a limited number of specifically defined, trusted USB devices will be recognized initially and allowed to operate. For example, the host system's 130 BIOS may be extended such that it initially recognizes only one keyboard and one mouse each of which is must be identified by a minimum number of descriptors. Once the operating system becomes active, a user could input information through user configuration interface 320 to identify a different keyboard or mouse which the BIOS should subsequently recognize.
Desired ones of the previously described security arrangements could also be provided through enhancements to the host controller. For example, by modifying the hardware or software (or both) of the host controller to perform desired security arrangements, improved performance in terms of speed may be achieved along with an increased resistance to tampering or malicious attempts to circumvent the arrangements.
The foregoing description has been directed to particular embodiments of this invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. Specifically, it should be noted that embodiments of the present invention may be implemented in connection with various USB host systems and are not limited to host systems that rely on a particular operating system. Also, the procedures, processes and/or modules described herein may be implemented in hardware, software, embodied as a computer-readable medium having program instructions, firmware, or a combination thereof.