2017年6月8日木曜日

CVE-2017-5638(Struts2) PoC with SELinux

We did "Famous" Struts2 vulnerability(CVE-2017-5638) PoC to check how SELinux can mitigate that vulnerability. During the PoC, we found current policy problem, then reported it on bugzilla. So, I would write the information for that PoC and SELinux policy problem.
(Written by Kazuki Omo:ka-omo@sios.com).

Prepare for PoC

Here is a description how to reproduce it. I used CentOS 7.3(CentOS-7-x86_64-DVD-1611.iso)image for the PoC. I used VMWare Guest(CPU: 1, Memory: 2GB) for the PoC.
  1. Install tomcat and related packages for working Struts2.
  2. Download and install vulnerable version of Struts2. I used both of struts-2.5.10. Copy struts2-showcase.war under /var/lib/tomcat/webapps
    root@cent7enc:~# ls /var/ls /var/lib/tomcat/webapps/*war
    /var/lib/tomcat/webapps/struts2-showcase.war
    
  3. Download and copy the PoC code on remote. There are many sample site for the PoC, then I'm not explaining it in here.
  4. To avoid normal Unix permission check, I changed /etc/shadow permission to 755.
    root@cent7enc:~# ls -lZ /etc/shadow
    -rw-r--r--. root root system_u:object_r:shadow_t:s0        /etc/shadow
    

PoC with no SELinux(SELinux Permissive)

  1. Confirm SELinux is Permissive mode;
    root@cent7enc:~# getenforce
    Permissive
    
  2. Run PoC from remote host(jssosug@vmhost);
     jsossug@vmhost:~$ python attack.py http://172.16.148.130:8080/struts2-showcase/showcase.action "cat /etc/shadow"
     CVE: 2017-5638 - Apache Struts2 S2-045
     cmd: cat /etc/shadow
     root:XXXXXX.::0:99999:7:::
     bin:*:17110:0:99999:7:::
     daemon:*:17110:0:99999:7:::
     --snip--
     sshd:!!:17247::::::
     jssosug:XXXXXXXXXXXX::0:99999:7:::
    
     jsossug@vmhost:~$
    

PoC with SELinux Enabled(SELinux Enforcing)

  1. Reboot and set SELinux as Enforcing.
    root@cent7enc:~# getenforce
    Permissive
    
  2. Run PoC code again. Even if SELinux is Enforcing, still you can see /etc/shadow(so bad...);
    jsossug@vmhost:~$ python attack.py http://172.16.148.130:8080/struts2-showcase/showcase.action "cat /etc/shadow"
    CVE: 2017-5638 - Apache Struts2 S2-045
    cmd: cat /etc/shadow
    
    root:XXXXXX.::0:99999:7:::
    bin:*:17110:0:99999:7:::
    daemon:*:17110:0:99999:7:::
    --snip--
    sshd:!!:17247::::::
    jssosug:XXXXXXXXXXXX::0:99999:7:::
    
    jsossug@vmhost:~$
    

Check SELinux Policy

Now we understand that SELinux can't mitigate CVE-2017-6074. Why it is happened?
  1. For understanding it, attack from remote(vmhost) by using below command;
    jsossug@vmhost:~$ python attack.py http://172.16.148.130:8080/struts2-showcase/showcase.action "vi /tmp/abcd" 
    CVE: 2017-5638 - Apache Struts2 S2-045
    cmd: vi /tmp/abcd
    
  2. On Struts PC, check domain who is running "vi /tmp/abcd";
    root@cent7enc:~# ps axZ|grep abcd
    system_u:system_r:tomcat_t:s0         3251 ?                S          0:00 vi /tmp/abcd
    
  3. Check "tomcat_t" inheriented domain with seinfo;
    root@cent7enc:~# seinfo -ttomcat_t -x
    tomcat_t
      can_change_object_identity
      can_load_kernmodule
      can_setbool
      can_setenforce
      corenet_unconfined_type
      corenet_unlabeled_type
      devices_unconfined_type
      domain
      files_unconfined_type
      filesystem_unconfined_type
      kern_unconfined
      kernel_system_state_reader
      process_uncond_exempt
      selinux_unconfined_type
      storage_unconfined_type
      unconfined_domain_type
      dbusd_unconfined
      daemon
      syslog_client_type
      sepgsql_unconfined_type
      tomcat_domain
      userdom_filetrans_type
      x_domain
      xserver_unconfined_type
    
So we found tomcat_t is in several "unconfined" domain. I reported it on bugzilla(https://bugzilla.redhat.com/show_bug.cgi?id=1432083), then fixed version of policy(selinux-policy-3.13.1-145.el7.noarch.rpm)

Check SELinux Updated Policy

  1. Fixed version of policy is in RHEL-7.4Beta image. Just for confirmation, I installed those policy on CentOS7(PoC).
    root@cent7enc:~# rpm -Fvh selinux-policy-3.13.1-145.el7.noarch.rpm selinux-policy-targeted-3.13.1-145.el7.noarch.rpm
    
    root@cent7enc:~# getenforce
    Enforcing
    
  2. Run PoC from remote same as before;
    jsossug@vmhost:~$ python attack.py http://172.16.148.130:8080/struts2-showcase/showcase.action "cat /etc/shadow" 
    CVE: 2017-5638 - Apache Struts2 S2-045
    cmd: cat /etc/shadow
    
    cat: /etc/shadow: Permission denied
    
  3. Check AVC log on Struts PC;
    type=AVC msg=audit(1496882036.860:219): avc:  denied  { read } for  pid=4413 comm="cat" name="shadow" dev="dm-1" ino=34456196 scontext=system_u:system_r:tomcat_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file
    
  4. Check "tomcat_t" inheriented domain with seinfo;
    root@cent7enc:~# seinfo -ttomcat_t -x
    tomcat_t
      nsswitch_domain
      corenet_unlabeled_type
      domain
      kernel_system_state_reader
      netlabel_peer_type
      daemon
      syslog_client_type
      pcmcia_typeattr_7
      pcmcia_typeattr_6
      pcmcia_typeattr_5
      pcmcia_typeattr_4
      pcmcia_typeattr_3
      pcmcia_typeattr_2
      pcmcia_typeattr_1
      tomcat_domain
    
    So now we can see no "Unconfined" domain in there.

Conclusion

From this PoC we can say
  1. SELinux can mitigate Struts2 vulnerability if "Policy is good.";
  2. Last SELinux Policy is treating "tomcat_t" as "unconfined domain".
  3. Latest version of SELinux Policy will solve the problem.

0 件のコメント:

コメントを投稿