第四步：过滤动态输出内容中的特殊字符

　　在实际应用中，判断哪些字符或者字符组合可能导致攻击是不明确的。因此，直接选择安全的字符集要比排除不信任的字符集更方便。例如，如果用户需要在一个表单项中填写他的年龄，开发者就可以直接地限定这个表单项的取值为数字0到9的组合，而不需要再接受其他字符。这样处理后，将大大地降低未知攻击的可能性。

　　过滤处理可以作为数据输入的一部分、数据输出的一部分或者两者兼而有之。当作为数据输出的一部分时，建议在数据呈现给用户前对之进行过滤处理。如果处理正确，就可以确保所有的动态内容被过滤成为纯净的东西。

　　下面分别列出用C++、JavaScript以及Perl语言编写的过滤代码，你可以根据实际情况选择其中一种：

C++
 
BYTE IsBadChar[] = {
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0xFF,0xFF,0x00,0x00,0xFF,0xFF,0xFF,0xFF,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0xFF,0xFF,0x00,0xFF,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00
};
 
DWORD FilterBuffer(BYTE  * pString,DWORD cChLen){
BYTE * PBad  = pString;
BYTE * pGood = pString;
DWORD i=0;
if (!pString) return 0;
for (i=0;pBad[i];i++){
if (!IsBadChar[pBad[i]]) *pGood++ = pBad[i];
};
return pGood-pString;
}