现在普遍的Nginx + PHP cgi的做法是在配置文件中,　通过正则匹配(Nginx(PHP/fastcgi)的PATH_INFO问题)设置SCRIPT_FILENAME, 今天小顿发现了一个这种方式的安全漏洞.
比如, 有http://www.laruence.com/fake.jpg, 那么通过构造如下的URL, 就可以看到fake.jpg的二进制内容:

1. http://www.laruence.com/fake.jpg/foo.php

为什么会这样呢?
比如, 如下的nginx conf:

1. location ~ \.php($|/) {
2.      fastcgi_pass 127.0.0.1:9000;
3.      fastcgi_index index.php;
4.      set $script $uri;
5.      set $path_info "";
6.      if ($uri ~ "^(.+\.php)(/.*)") {
7.           set $script $1;
8.           set $path_info $2;
9.      include fastcgi_params;
10.      fastcgi_param SCRIPT_FILENAME $document_root$script;
11.      fastcgi_param SCRIPT_NAME $script;
12.      fastcgi_param PATH_INFO $path_info;

通过正则匹配以后, SCRIPT_NAME会被设置为"fake.jpg/foo.php", 继而构造成SCRIPT_FILENAME传递个PHP CGI, 但是PHP又为什么会接受这样的参数, 并且把a.jpg解析呢?
这就要说到PHP的cgi SAPI中的参数, fix_pathinfo了:

1. ; cgi.fix_pathinfo provides *real* PATH_INFO/PATH_TRANSLATED support for CGI. PHP's
2. ; previous behaviour was to set PATH_TRANSLATED to SCRIPT_FILENAME, and to not grok
3. ; what PATH_INFO is. For more information on PATH_INFO, see the cgi specs. Setting
4. ; this to 1 will cause PHP CGI to fix it's paths to conform to the spec. A setting
5. ; of zero causes PHP to behave as before. Default is 1. You should fix your scripts
6. ; to use SCRIPT_FILENAME rather than PATH_TRANSLATED.
7. cgi.fix_pathinfo=1

如果开启了这个选项, 那么就会触发在PHP中的如下逻辑:

1. * if the file doesn't exist, try to extract PATH_INFO out
2. * of it by stat'ing back through the '/'
3. * this fixes url's like /info.php/test
4. if (script_path_translated &&
5.      (script_path_translated_len = strlen(script_path_translated)) > 0 &&
6.      (script_path_translated[script_path_translated_len-1] == '/' ||
7. ....//以下省略.

到这里, PHP会认为SCRIPT_FILENAME是fake.jpg, 而foo.php是PATH_INFO, 然后PHP就把fake.jpg当作一个PHP文件来解释执行... So...
这个隐患的危害用小顿的话来说, 是巨大的.
对于一些论坛来说, 如果上传一个图片(实际上是恶意的PHP脚本), 继而构造这样的访问请求...
所以, 大家如果有用这种服务器搭配的, 请排查, 如果有隐患, 请关闭fix_pathinfo(默认是开启的).
详细漏洞信息, 请移步小顿的BLOG: 80Sec
另: 我认为这个和Nginx没啥关系, 不属于Nginx的漏洞. 是配置的问题, 现在到处都在说是Nginx的Bug, 不妥不妥.