bash CVE-2014-6271 CVE-2014-7169
昨天CVE-2014-6271曝出, 网上给出的测试代码env X='() { :; }; echo vuln' bash -c "echo ..."很好理解, 原因也很好找, http://www.zhihu.com/question/25539470/answer/31055176?utm_source=weibo&utm_medium=weibo_share&utm_content=share_answer&utm_campaign=share_button.
但是对第二个绕过测试代码很不解, env X='() { (a)=>\' bash -c "echo echo vuln";cat echo
这段测试代码目的是造成bash错误, 即(a)= 这段语句会语法出错,
这时, 一个全局变量eol_ungetc_lookahead为'>'
然后yyerror会被调用, 输出语法错误提示, 并且调用reset_parser, 之后回到variables.c中的initilize_shell_variables函数, 调用report_error, 由于exit_immediately_on_error仍然为0, 所以程序会继续运行.
当调用shell_getc时, 因为eol_ungetc_lookahead不为空, 那么返回它的值, 也即为之前出错的时候的字符'>'
然后读取后面将执行的命令, "echo echo vuln" 合并即为">echo echo vuln" 就会创建一个新文件
同样, 将'>'字符改成'<', 会读取系统一些文件, 如env -i X='() { (a)=>\' bash -c "/etc/passwd cat".
补充: 为什么要在最后加上\, 因为shell_getc在碰到\的时候, 会restart_read, 然后直接返回EOF, 同时会触发两次shell_ungetc, 让eol_ungetc_lookahead变量得到'>'. 没有\的时候, shell_ungetc的执行路径会不同,
通过输入精心制造的特殊数据, 控制程序的执行路径向自己希望的方向走, nice
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