Method for protecting executable software programs against infection by software viruses

A method is disclosed for protecting executable computer programs against infection by a computer virus program. The invented method prevents writing operations that attempt to modify portions of the program, such as the program's entry point or first instructions. A writing operation that attempts to write data to the program is intercepted and analyzed before the operation is allowed to be processed. The method selects significant data and stores the data, in order to retain information indicative of the program prior to any modification thereof. The invented method then determines if the writing operation is attempting to modify the significant data, and if it is determined that the writing operation is attempting to modify the data, an alarm is generated and operation is denied. If it is determined that the writing operation is not attempting to modify the data, the writing operation as allowed to continue. Additionally, the program can be restored to its initial state using the stored information and data. The method of the present invention uses the stored data indicative of the significant data of the program to restore the program to its initial state and undo all the modifications that the virus may have made to the program.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to computer software, and more 
particularly, to a method for protecting executable software programs 
against infection by computer software virus programs. 
2. Description of Related Art 
Computer software virus programs are malicious programs adapted to corrupt 
other executable computer software programs, such as by replicating 
themselves over an existing program or by adding code to the beginning or 
end of the program. Since their appearance, the number, performance, and 
intelligence of virus programs has greatly increased. The persons who 
write computer viruses continue to find means for making their viruses 
increasingly transparent and difficult to detect by a computer user. 
However, the methods used by various virus programs for infecting computer 
programs are substantially similar. Most computer viruses replicate 
themselves by infecting executable programs. Executable programs typically 
comprise a series of instructions that are executed by a central 
processing unit (CPU) of a computer containing the program, when the 
program is invoked. The program contains a series of instructions referred 
to as "first instructions" . These instructions are located within the 
program, in a location that is either fixed by a disk operating system 
(DOS) that the executable program is designed for, or in a location 
referred to as the "program entry point". The program entry point is 
indicated by an "entry point indicator" inside the program and may be 
different for each program. 
The objective of computer viruses is to obtain control of a desired 
executable program, before normal processing of the program begins. 
Therefore, the virus program must have its instructions executed by the 
CPU, before the CPU begins processing the instructions of the executable 
program. For the virus to be executed before the executable program, the 
virus must either modify the first instructions or entry point of the 
program, so that the virus instructions will be processed at execution 
time, before the computer program is processed. Infection of the program 
typically comprises the virus adding its executable instructions or code 
to the program. The virus then causes the program to be invoked with the 
first instructions of the virus, instead of the program's first 
instructions, corrupting and possibly destroying the program. 
Most viruses cannot be detected by computer users without using programs 
specifically designed for virus detection referred to as "anti-virus" 
programs. The anti-virus programs are written for detection, and possibly 
destruction, of viruses. The most important goal of anti-virus programs is 
to detect the presence of a virus in a computer system as early as 
possible. Once a virus is detected, the anti-virus program typically 
signals the user, for informing the user that a virus was detected. Since 
the virus often writes its code into the program at several different 
locations, restoring the program is a time consuming process. Further, as 
the virus code is in the program at different locations, it is 
substantially difficult to be absolutely sure that the virus code is 
completely removed from the program when the program is restored, without 
damaging the program itself. 
There are systems in the prior art for protecting executable programs from 
infections by virus programs. One such system denies performing any 
writing operation on any executable program, without taking account of 
modified bytes of the program. This system is used by some anti-virus 
programs. A disadvantage to this system is that writing to an executable 
program is a completely legal operation, and protection systems that deny 
writing to an executable program typically produce false positives 
(wrongly identifying a legal operation or program as a virus program), 
rather than stopping real virus operations and programs. However, 
modifying an executable program's entry point or first instructions is 
rarely performed by normal executable programs and is often performed by 
virus programs. 
Avoiding false positives is one of the more important functions of an 
anti-virus program, in order to provide a reliable program and achieve the 
users trust. A preferred method for detecting virus programs without 
making false positives, is to provide a method that generates an alarm 
signal only when it is confirmed that an operation is executed by a virus. 
However, when an operation executed by a virus is detected, the virus 
could have made changes and modifications to the program. 
One method for recovery of a computer program infected by a computer virus 
is disclosed in U.S. Pat. No. 5,408,642, to Mann. The disclosed method 
takes a unique fingerprint of a program to be recovered, along with data 
relating to the beginning portion of the program, and stores the 
fingerprint and data at a separate location. A fingerprinted program 
thought to be infected by a virus is processed for generating a 
fingerprint of a string of data of the program. The generated fingerprint 
is compared to the stored fingerprint, to determine if they match. If the 
fingerprints do not match, a value utilized to select the string can be 
incremented and the comparison process repeated. 
U.S. Pat. No. 5,349,655, to Mann, discloses a method for recovery of a 
computer program infected by a computer virus, similar to the method 
disclosed in U.S. Pat. No. 5,408,642, to Mann. The disclosed method 
includes generating strings of data prior to infection by a virus and 
storing the strings. Second strings of data are then generated and 
compared to the prior strings of data, to determine if the data has been 
corrupted by a virus and for recovering the data. 
U.S. Pat. No. 5,359,659, to Rosenthal, is directed to a method for securing 
software against corruption by computer virus programs. The disclosed 
method includes coupling security routines capable of detecting the 
presence of any virus infection, or other corruption, to a program. The 
loading information of an executable program is modified so that upon any 
attempt to execute the program, the security routines will execute first 
and scan for viruses or other corruption. If any viruses or corruption are 
detected, execution of the program is aborted and a warning is displayed. 
If no viruses or corruption are detected, the security routines are 
removed from the computer's memory and the program continues normally. 
U.S. Pat. No. 5,319,776, to Hile et al., discloses a method for in transit 
detection of computer virus with a safeguard. The disclosed method tests 
data in transit between a source medium and a destination medium. Each 
character of an incoming data stream is tested against multiple search 
strings representing the signatures of multiple known computer viruses. 
When a virus is detected, the incoming data is prevented from remaining on 
the destination storage medium. 
U.S. Pat. No. 5,121,345, to Lentz, discloses a system and method for 
protecting the integrity of computer data and software from a computer 
virus. U.S. Pat. No. 5,367,682, to Chang, discloses data processing virus 
protection circuitry including a permanent memory for storing a redundant 
partition table. 
Although the methods and systems disclosed in the above enumerated prior 
art references have improved features, they fail to disclose all of the 
advantageous features achieved by the method of the present invention. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
method for protecting an executable computer program against infection by 
a computer virus program; 
It is another object of the present invention to provide a method for 
protecting executable programs against infection by a computer virus that 
has a low probability of generating false positive recognition of a 
computer virus; 
It is a further object of the present invention to provide a method for 
protecting executable programs against infection by viruses that detects 
virus infection actions made on executable programs; 
It is another object of the present invention to provide a method for 
protecting an executable program against infection by a virus that does 
not require any modification of the executable program; 
It is a still further object of the present invention to provide a method 
for protecting executable programs against infection by viruses that does 
not require a database of known computer virus programs; 
It is yet another object of the present invention to provide a method for 
protecting executable programs against infection by viruses that 
automatically detects executable program infections, without the need for 
checking each executable program contained in a computer system; 
It is a further object of the present invention to provide a method for 
protecting executable programs against infection by viruses that signals a 
user when a virus attempts to start spreading itself through the computer 
system; and 
It is still another object of the present invention to provide a method for 
protecting executable programs against infection by viruses that is 
capable of restoring executable programs upon detection and denial of a 
suspected virus operation. 
SUMMARY OF THE INVENTION 
These and other objects and advantages of the present invention are 
achieved by providing an improved method for protecting an executable 
computer program against infection by a computer virus program. 
Executable programs comprise a series of instructions that are executed by 
a central processing unit (CPU) of a computer system containing the 
program, when the program is invoked. The program contains a series of 
first instructions that are located within the program, at a location that 
is either fixed by a disk operating system (DOS) or in the program entry 
point. The program entry point is indicated by an entry point indicator 
inside the program. 
The method of the present invention comprises denying writing operations 
that attempt to modify either the executable program's entry point or 
first instructions. Any writing operation that intends to write data to a 
program is first intercepted, then analyzed before the operation is 
allowed to be processed. 
Before an executable program is initially modified, the method of the 
present invention selects significant portions of the program's 
information and data, and stores the information and data in a desired 
memory location, such as on a magnetic hard disk for example, in order to 
retain information indicative of the program, prior to any modification 
thereof. The invented method then determines if a writing operation is 
attempting to modify the significant portions, such as the entry point 
indicator or the first instructions, of the program. If it is determined 
that the writing operation is attempting to modify these portions, the 
method generates an alarm and the writing operation is denied. If it is 
determined that the writing operation is not attempting to modify these 
portions, the writing operation is allowed to continue. 
Additionally, the program is restored to its initial state using the stored 
information and data. Storing portions of the significant data of the 
program, such as the program size and entry point or first instructions, 
before any modification to the program is made is an important feature of 
the invented method. When a virus is detected, while trying to modify the 
first instructions or entry point of a program, it is possible that by the 
time that the virus is detected, the virus may have overwritten a portion 
of the program. The virus may modify the program's header or made other 
modifications to the program. The method of the present invention uses the 
data indicative of the stored portions of the program to restore the 
program to its initial state and undo the modifications that the virus may 
have been made to the program.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following description is provided to enable any person skilled in the 
art to make and use the invention and sets forth the best modes 
contemplated by the inventor of carrying out the invention. Various 
modifications, however, will remain readily apparent to those skilled in 
the art, since the generic principles of the present invention have been 
defined herein. Further, while the following discussion is directed to a 
single occurrence of implementing the method of the present invention on a 
single executable computer program, it is to be understood that the method 
may be used with several different executable programs on a storage 
medium. 
Referring now to FIG. 1 of the drawings, there is shown a schematic 
representation of a computer system 10 for implementation of the method of 
the present invention. The computer system 10 typically comprises a 
central processing unit (CPU) 12 for processing data, an input means such 
as a keyboard 14 for data processing, and an output means such as a video 
monitor 16 so that a user can view data being processed by the CPU 12. The 
computer system 10 is further provided with a known magnetic storage 
medium 18, such as a hard disk drive. 
For data processing, the user typically calls an executable computer 
program (to be discussed thoroughly hereafter) stored on the hard disk 18 
into a memory location 20, such as a well known random access memory 
(RAM). The user then utilizes the program to process data. After 
processing, the program is closed and stored on the hard disk 18. Any data 
that is processed using the program may also be stored on the hard disk 
18, or the data may be stored on a floppy diskette (not shown). 
Referring now to FIGS. 2A-2C of the drawings, there are shown a plurality 
of executable programs 30A, 30B, 30C, as is well known in the art. 
Executable programs 30 typically comprise a series of instructions or code 
32 that are executed by the computer's CPU 12, when the program 30 is 
called into the memory 20. FIG. 2A shows a first type of program 30A 
wherein the initial code 32 comprises a header portion 34. The header 34 
contains specific information regarding the program 30A, such as program 
loading information and an entry point indicator 36. The entry point 
indicator 36 points out the location in the program 30A of the program's 
first instructions 38. The program code 32 is generally substantially 
larger than the header portion 34 of the program 30A. This type of program 
30A is commonly found in programs designed to function in an MS-DOS 
(Microsoft-Disk Operating System, registered trademark of Microsoft 
Corporation) operating system. Such a program usually is provided with an 
"EXE" suffix, to indicate to a user that the program 30A can be executed 
by the user. 
FIGS. 2B and 2C show a second type of executable program 30B that is not 
provided with a header 34 or an entry point indicator 36. The first 
instructions 38 of the program 30B, shown in FIG. 2B are located at 
position 0 or the first line of code 32 of the program 30B. The second 
type of program 30B is commonly found in programs that are designated with 
a "COM" suffix and are designed to function in the MS-DOS operating 
system. If the first instruction 38 of the program 30B is a "JUMP" or 
"CALL" instruction, that instructs the CPU 12 to process code 32 located 
elsewhere in the program 30B, then the location pointed to by the JUMP or 
CALL instruction is considered the program entry point 36. In this 
instance, the instructions pointed to by this JUMP or CALL instruction are 
considered to be the program's first instructions 38, though they are not 
actually the program's first instructions. 
Referring now to FIGS. 3A-3B, the objective of computer virus programs is 
to obtain control of the executable program 30, before normal processing 
of the program 30 begins. Infection of the program 30 typically comprises 
a virus program 40A adding its executable instructions or code 42 to the 
program 30. The virus 40A then causes the program 30 to be invoked with 
the virus instructions 42, instead of the programs first instructions 38, 
corrupting and possibly destroying the program 30. Therefore, the virus 
program 40A must have its instructions 42 executed by the CPU 12, before 
the CPU 12 begins processing the program's code 32. For the virus 40A to 
be executed before the program 30, the virus 40A must either modify the 
first instructions 38, or entry point 36, of the program 30, so that the 
code 42 of the virus 40A will be processed prior to the program 30 being 
processed. 
The virus program 40A is attempting to infect the program 30 by attaching 
its code 42 to an end 44 of the program 30. The virus 40A also attempts to 
modify the entry point indicator 36 of the program 30 by overwriting the 
beginning of the program 30, and thus the entry point indicator 36, with 
its own entry point indicator 46. Therefore, the virus code 42 becomes the 
first instructions of the program 30, instead of the program's first 
instructions 38. 
FIGS. 4A-4B show a second type of virus program 40B. The second type of 
virus 40B attempts to infect the program 30 by attaching its code 42 to 
the end 44 of the program 30. However, the virus 40B does not attempt to 
modify the entry point indicator 36 of the program 30, it attempts to 
modify the program's first instructions 38. The virus 40B typically places 
a JUMP instruction 48 overwriting the first instructions 38 of the program 
30 that instructs the CPU 12 to process the virus code 42. 
FIGS. 5A-5B show a third type of virus program 40C. The third type of virus 
40C attempts to infect the program 30 by overwriting the first portion of 
the program 30, such as the header 34 (shown in FIG. 2A) and additional 
code 32, with its own code 42 and first instructions 46. Since the entry 
point indicator 36 is located at the beginning of the program 30, the 
entry point indicator 36 or any first instructions 38 are overwritten. The 
virus 40C further displaces the rest of the program code 32 forward and 
writes back the portion of the program code 32 it overwrote after the 
virus code 42 and before the displaced program code 32, so that the 
program 30 remains intact and can function. 
Referring now to FIG. 6 of the drawings, there is shown a flow diagram 50 
of the method of the preferred embodiment of the present invention. It is 
to be understood, that the invented method may comprise a computer 
software program or other suitable means for implementing the present 
invention in the computer system 10, or other appropriate device. 
An attempt to write to an executable program 30 is detected by the invented 
method as shown in block 52. In decision block 54 it is determined if an 
attempt to write new data to the program 30 is the first such writing 
attempt made. If it is not the first attempt to write new data to the 
program 30, then the process continues to decision block 56. At decision 
block 56 it is determined whether the write attempt will write new data 
over the entry point indicator 36 or first instructions 38 of the program 
30, thus modifying either the indicator 36 or instructions 38, or possibly 
both. If it is determined that the indicator 36 and instructions 38 will 
not be modified, then at process block 58 the write data attempt is 
allowed to write the new data to the program 30, after which the method of 
the present invention terminates at block 60. 
Returning to decision block 54, if it is determined that the attempt to 
write new data to the program 30 is the first such attempt, then the 
preferred method proceeds to process block 62. At process block 62 
predetermined data that contains important information regarding the 
program 30, such as the program entry point 36, first instructions 38, or 
size of the program 30, as well as any other desired data, or 
predetermined combinations thereof, is read from the program 30 and 
written to a different desired storage location. The storage location may 
be a different location on the hard disk 18 other than the program 
location, or a completely separate storage means, such as a floppy 
diskette for example (not shown). 
After the predetermined data is properly stored, the method continues to 
decision block 56. If it is determined at decision block 56 that the write 
attempt will write new data over the entry point indicator 36 or first 
instructions 38, or possibly both, of the program 30, an alarm signal is 
generated at process block 64. The alarm signal is generated to inform the 
user of the program 30 that a virus program 40 may be attempting to 
corrupt the program 30. Preferably, the method allows the user to 
determine whether to prevent the write data attempt from writing the new 
data to the program 30, or to allow the write data attempt to write the 
new data to the program 30. 
Once the alarm is generated, the method continues to process block 66 where 
the program 30 is restored. The program 30 is restored by reading the 
predetermined data of the program 30 on which the write data attempt was 
made from the second storage medium and writing the data to the program 
30. The program 30 is further truncated to the representative size of the 
program 30, using information contained in the predetermined data. The 
method continues to process block 68 where the write data attempt is 
denied for preventing the writing new data to the program. Additionally, 
the present invention can prevent all further attempts at writing new data 
to the program 30 at process block 68. The method then continues to block 
60 where the method of the present invention terminates. 
Thus, there has been described an improved method for protecting an 
executable computer program against infection by a computer virus program. 
While the preferred embodiment of the present invention has been disclosed 
herein, certain changes and modifications will readily occur to those 
skilled in the art. For example, although only one entry point indicator 
per program is discussed, several entry point indicators can be 
considered, starting with a number such as 1, and incrementing this value 
as long as the last entry point indicator found points on a JUMP or CALL 
instruction. Furthermore, although the preferred embodiment generates an 
alarm if there is an attempt to modify the entry point or first 
instructions of the executable program, the alarm may be generated only if 
the number of the modified bytes of data doesn't exceed a predetermined 
value. Additionally, although the entry point and first instructions 
modifications are denied, the method of the present invention is capable 
of restoring the program after the entry point and first instructions are 
modified. Also, the present invention has been described in conjunction 
with a program for a microprocessor of the computer system, it is obvious 
that dedicated hardware circuitry could be provided within the computer to 
perform the invented method. 
Those skilled in the art will appreciate that various adaptations and 
modifications of the just-described preferred embodiments can be 
configured without departing from the scope and spirit of the invention. 
Therefore, it is to be understood that, within the scope of the appended 
claims, the invention may be practiced other than as specifically 
described herein.