Abstract:
When a file is created on a computer, an archive copy of that file is also created and separately stored. Upon a subsequent access to the active copy of that file, a comparison between the active copy and the full archived copy is made to detect any changes. If there are not any changes, then the active copy of the file is assumed to be clean from malicious alteration and the access request is permitted. If an alteration has been made and is detected, then further countermeasures are triggered, such as full virus scanning of that file or blocking of its use. This archiving and comparison technique may be selectively applied to a subset of file types, such as executable files and dynamic link libraries, which are known to be infrequently modified during normal user operations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of data processing systems. More particularly, this invention relates to the detection of the malicious alteration of stored computer files, such as, for example, by computer viruses infecting stored computer files. 
     2. Description of the Prior Art 
     It is known to provide anti-virus computer systems that seek to detect if stored computer programs have been subject to computer virus infection whereby the computer file is altered by the computer virus. These anti-virus computer systems typically operate by having a stored library of virus characteristics and then searching the computer files to detect those characteristics. As the number of known computer viruses increases, then the amount of processing needed to search a computer file for any characteristics of all those known computer viruses also increases. Furthermore, the typical number of computer files held within a computer system is also rapidly increasing. This combination of factors produces a problem that the processing load and processing time required to perform comprehensive anti-virus scanning is becoming disadvantageously large. As an example, a fall on-demand scan of every file held on a network storage device may take so long to run that insufficient time is available during the normal out-of-hours periods available to complete such an on-demand scan. 
     U.S. Pat. No. 5,619,095, U.S. Pat. No. 5,502,815 and U.S. Pat. No. 5,473,815 describe systems that seek to detect alterations in computer files by generating data characteristics of the computer file when first created and then comparing this with similar data generated upon an access attempt to that file to see if that file has been altered. 
     SUMMARY OF THE INVENTION 
     Viewed from one aspect the present invention provides a computer program product comprising a computer program operable to control a computer to detect a malicious alteration to a stored computer file, said computer program comprising: 
     file comparing logic operable to compare said stored computer file with an archive copy of said computer file stored when said stored computer file was created; and 
     comparison response logic operable if said file comparing logic detects that said stored computer file and said archive computer file do not match to trigger further countermeasures against a potential malicious alteration. 
     The invention recognises that with the rapid increase in capacity and lowering cost of data storage capacity within computer systems, it is practical to store a relatively comprehensive archive set of the computer files that are created when the active stored computer files were created. Furthermore, the invention recognises that in many cases there is no normal reason why these computer files would be altered after they had been created. As an example, if an archive set of executable files and dynamic link library files is kept, then the average computer user would not normally make any alterations to this type of file since their active data is stored in documents, spreadsheets, databases and other file types. Accordingly, this archive of essentially static files may be used to compare the active stored files with these archived files to see if there has been any changes in the stored active files. This direct comparison is in fact quicker than seeking to detect many tens of thousands of different computer viruses that may be present within that file. Thus, providing the file was uninfected when it was first stored, then, if it is unaltered, it will still be uninfected and can be rapidly passed as clean during normal use. If a stored computer file has been altered from the archive copy of that computer file, then further countermeasures may be triggered. 
     As one example of such counter measures, altered computer files may then be referred on to an anti-virus scanning system which then scans in detail within those altered files to detect the characteristics of any known computer viruses. 
     The archive copy of the computer files may be stored in a variety of ways, such as in a simple unencrypted form. However, preferred embodiments seek to increase security by storing the archive copy of the computer files in an encrypted form or on a PGP disk or similar encrypted system. 
     The archive copy of the computer files may for security be stored on a separate physical device. Alternatively, in some cases simplicity and speed may be improved when the archive copies are simply stored in a different part of the same physical device, such as on a separate network volume defined upon that physical device. Both the original and the archive copies could be stored on network shares. 
     As previously mentioned, the technique is particularly well suited to the detection of malicious alteration to file types that are normally static and remain unaltered. Examples of such file types are executable file types (such as EXE and COM files) and dynamic link library file types. 
     The system can be made substantially automatic as far as the user is concerned if an automated mechanism is provided to create the archive file copies when the original active stored computer files are created or copied onto the device. This automatic archive copy generator may also be selective in the file types which it targets. 
     Complementary aspects of the invention also provide a method of operating a computer and a computer apparatus in accordance with the above described techniques. 
     The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a portion of a computer system showing the relationship of the anti-virus systems to normal file access operations; 
         FIGS. 2 and 3  schematically illustrate possible storage locations for archive copies of computer files; 
         FIG. 4  is a flow diagram illustrating processing in accordance with a first embodiment; 
         FIG. 5  is a flow diagram illustrating processing in accordance with a second embodiment; and 
         FIG. 6  is a diagram schematically illustrating a general purpose computer of the type that may be used to implement the present techniques. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates the relationship between an operating system  2 , an anti-virus system  4  and a data storage device  6 . In normal operation file access requests from application programs are passed to the operating system  2 , which then controls the servicing of those file access requests by the data storage device  6 . When an anti-virus system  4  is present, then this serves to intercept the normal file access requests and pass their details together with the file concerned to the anti-virus system  4 . The anti-virus system  4  can then conduct anti-virus countermeasures, such as scanning for viruses, worms, Trojans, malware and the like. If the anti-virus system  4  detects that the file being accessed is clean, then this is indicated back to the operating system  2  and the operating system  2  then services the file access request for the application program in the normal way. Conversely, if the anti-virus system  4  detects a computer virus or other malicious content (such as a Trojan or a worm), then countermeasures are triggered, such as quarantining, cleaning or deletion. 
       FIG. 2  schematically illustrates a computer  8  containing a first data storage device  10  and a second data storage device  12 . High capacity, high speed data storage devices are becoming less expensive and accordingly the provision of a comparatively large storage capacity within a computer  8  is quite practical. In operation, the active copies of computer files are stored upon the first data storage device  10 . Archive copies of all executable and DLL files are stored to the second data storage device  12  as they created for the first time upon the first data storage device  10 . These archive copies may then be compared with the main active copies upon access to those active copies at a later time to detect if there has been any alteration in those active copies. If there has been an alteration, then further countermeasures may be triggered, such as thorough anti-virus scanning. 
       FIG. 2  illustrates the second data storage device  12  as being incorporated within the same computer  8 . This may be convenient for high speed access. However, it will be appreciated that the second data storage device  12  could be physically located within a different computer, such as on a different computer on the same computer network, providing the computer  8  does have access to that second data storage device  12  to retrieve the archived filed copies when needed. 
       FIG. 3  illustrates another embodiment. In this embodiment the computer  14  includes a single data storage device  14 . In this case the active copies of the computer files and the archived copies of the computer files are stored on the same data storage device, but in different portions of that device, such as in different logical volumes defined on the device. 
     The archived copies of the computer files could be stored in an unencrypted plain form directly corresponding to the active copies of the files. However, in order to improve security, the archive copies may be encrypted for storage and require decryption to their original state prior to comparison with the active copies. The archive copies could alternatively be stored upon a PGP or other secure data storage drive or device. Other known encryption techniques may be employed. 
       FIG. 4  illustrates a first embodiment. At step  18 , when a file access request has been made, a check is performed to determine if a file is being created for the first time. If a file is being created for the first time, then that file is scanned for viruses at step  20 . Step  22  determines whether or not the results of the virus scan indicated that the file being created was free of computer viruses (or other malicious content or unwanted content). If the file being created did contain any computer viruses, then processing proceeds to step  24  at which anti-virus (or other) countermeasures, such as user or administrator alerts, quarantining, deletion, cleaning etc. are triggered. If the file being created is free of computer viruses, then step  24  determines whether or not the file type of the file being created is one for which archive copies are kept. In a preferred embodiment, archive copies are kept for executable and DLL file types which are unlikely to be altered by a user during normal operation. If archive copies are not being kept, then processing proceeds to step  26  at which the access requested, in this case file creation, is permitted. If archive copies are being made for this file type, then these are created at step  28  before processing proceeds to step  26 . 
     If the test at step  18  indicated that the access request was not one for file creation, then processing proceeds to step  30  at which a check is made to see if there is a stored archive copy of the file to which the access request is being made. If there is no stored copy, then processing proceeds to step  32 , at which standard scanning for computer viruses in accordance with the normal library of virus definition data takes place. If this virus scanning indicates that the file is free from viruses at step  34 , then processing proceeds to step  26  to permit the access. If the scanning indicates the presence of a virus, then anti-virus measures at step  36  are triggered. 
     If the test at step  30  indicates that an archived copy of the file to which the access request is being made is stored, then step  38  performs a byte-by-byte or other form of comparison of full copies of the currently active computer file and the archived computer file to check that they fully match. If the two copies do fully match, then no alterations have been made to that computer file since it was created and accordingly since the computer file was scanned for viruses when it was created, then the computer file can be treated as clean. If the comparison at step  38  does not reveal a match, then processing proceeds to step  32  where a normal scan for viruses is triggered. 
     It will be appreciated that periodically full on-demand virus scans of all the computer files stored, irrespective of whether there are any archive copies may be beneficial in order to provide protection against computer viruses that may have been infecting those files at the time when they were first created on the system, but were not yet known to the anti-virus systems, and accordingly were first categorised as clean and archived even though they were in fact infected. Nevertheless, for normal day-to-day operation the test conducted at steps  38  to compare the active copy of the file with the archive copy of the file and treat the file as clean if these match, provides a significant reduction in the amount of processing required and accordingly is advantageous. 
     It will be appreciated that step  28  could apply user defined rules to determine whether or not an archive copy is made. For example, a user could be prompted to confirm that they wish to make an archive copy. Archive copies could always be made. Archive copies could be made when the origin of the files matched a predetermined list of file types or other combinations of factors. 
     Step  38  in  FIG. 4  is illustrated as passing a non-matching current copy through to step  32  for scanning for viruses. As an alternative, files which do not match could simply be blocked from use, or processing passed to the anti-virus actions at step  36  without requiring the scanning of step  32 . 
     The processing illustrated in  FIG. 4  is performed when a file is accessed. It may be that when embodied within an on-access scanner, this processing is carried out upon the first access to that file since activation of the scanner. Such scanners can keep a record of previously accessed and passed-as-clean files such that they avoid re-scanning them or checking them in other ways upon subsequent accesses when they know that they have not in the intervening period been modified. This type of mechanism to reduce the processing load may be combined with the techniques described herein. 
     The match comparison conducted at step  38  could take a variety of forms. A byte-by-byte comparison or binary comparison could be performed in some embodiments. Alternatively, each full copy of the file could be subject to processing, such as generation of an MD5 checksum or similar, and then these results compared to verify a match between the files concerned. 
       FIG. 5  illustrates processing in accordance with a second embodiment. The generation of archive copies in the first place proceeds in the same manner as for  FIG. 4 . The difference between the processing of  FIG. 5  and that of  FIG. 4  starts at the comparison step between the archive copy and the currently active copy that is performed at step  40 . In this embodiment if the two copies do not match, then processing proceeds to step  42  at which the user is notified of the occurrence of the non-match. The user may define a set of rules for how processing proceeds further from this point. One possibility would be for the user to waive their right to notification and automatically restore the altered file from the archived copy at step  44 . Another option may be to prompt the user for confirmation of the restore operation or to selectively restore based upon the origin of the file, the file types or some other rule. 
     If processing proceeds to step  44  and the user confirms the restore operation, then the currently active non-matching copy is replaced by the archived copy at step  46  and then processing proceeds to permit access at step  48 . This provides file repair. 
     This repair technique synergistically combines with the pure alteration detection technique of  FIG. 4 . 
       FIG. 6  illustrates a general purpose computer  200  of the type that may be used to perform the above described techniques. The general purpose computer  200  includes a central processing unit  202 , a read only memory  204 , a random access memory  206 , a hard disk drive  208 , a display driver  210  with attached display  211 , a user input/output circuit  212  with attached keyboard  213  and mouse  215 , a network card  214  connected to a network connection and a PC computer on a card  218  all connected to a common system bus  216 . In operation, the central processing unit  202  executes a computer program that may be stored within the read only memory  204 , the random access memory  206 , the hard disk drive  208  or downloaded over the network card  214 . Results of this processing may be displayed on the display  211  via the display driver  210 . User inputs for triggering and controlling the processing are received via the user input/output circuit  212  from the keyboard  213  and mouse  215 . The central processing unit  202  may use the random access  206  as its working memory. A computer program may be loaded into the computer  200  via a recording medium such as a floppy disk drive or compact disk. Alternatively, the computer program may be loaded in via the network card  214  from a remote storage drive. The PC on a card  218  may comprise its own essentially independent computer with its own working memory, CPU and other control circuitry that can co-operate with the other elements in  FIG. 4  via the system bus  216 . The system bus  216  is a comparatively high bandwidth connection allowing rapid and efficient communication. 
     Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.