12 Restricting agent checks

Overview

You can control which item keys Zabbix agent or agent 2 is allowed or denied to use when executing item checks, remote commands, or scripts.

To do that, use these agent configuration parameters to define allow/deny rules:

• AllowKey=<pattern>
• DenyKey=<pattern>

The <pattern> must contain a single item key and supports wildcards (*). The wildcard matches any number of any characters in its position, and can be used to match item keys or parameters (e.g., vfs.file.*[*]).

In contrast to other agent configuration parameters, you can specify an unlimited number of AllowKey or DenyKey parameters.

Important notes

• All system.run items are disabled by default (even when DenyKey is empty), as if DenyKey=system.run[*] was set as the last rule. Because of this, you can allow specific system.run items without explicitly denying other system.run items.

• An item specified in AllowKey must also be specified in DenyKey (except for system.run items); otherwise, Zabbix agent will not start.

• If possible, use AllowKey to allow only required items and deny everything else. Some keys can be abused to read unintended files via path traversal (e.g., vfs.file.contents["../../../../etc/passwd"]), and new Zabbix agent versions may introduce keys not covered by your DenyKey rules.

• AllowKey and DenyKey configuration does not affect HostnameItem, HostMetadataItem, or HostInterfaceItem agent parameters.

• Denied items become unsupported without any hints or error messages; for example:

  • Zabbix agent --print (-p) command-line parameter will not show denied item keys.
  • Zabbix agent --test (-t) command-line parameter will return "Unsupported item key." for denied item keys.
  • Zabbix agent log file will not log denied remote commands if LogRemoteCommands=1.

Allow/deny rule order

You can specify an unlimited number of AllowKey or DenyKey rules, though their order matters.

• Rules are evaluated one by one, from top to bottom.
• When an item key matches a rule, it is either allowed or denied, and rule evaluation stops.

For example, when evaluating vfs.file.contents[/etc/passwd], the rules are processed as follows:

AllowKey=vfs.file.contents[/tmp/app.log]    # Item key pattern does not match, agent proceeds to the next rule.
       AllowKey=vfs.file.contents[/etc/passwd]     # Item key pattern matches; agent allows the item check and stops rule evaluation.
       DenyKey=vfs.file.*[*]                       # Agent ignores the rule, as the evaluation has stopped.

The following rule order will deny the item check:

DenyKey=vfs.file.*[*]                       # Item key pattern matches; agent denies the item check and stops rule evaluation.
       AllowKey=vfs.file.contents[/etc/passwd]     # Agent ignores the rule, as the evaluation has stopped.
       AllowKey=vfs.file.contents[/tmp/app.log]    # Agent ignores the rule, as the evaluation has stopped.

Examples

The following examples show common configuration patterns for AllowKey and DenyKey.

Allowing specific checks and commands

Allow only two vfs.file item checks and two system.run commands:

AllowKey=vfs.file.contents[/tmp/app.log]
       AllowKey=vfs.file.size[/tmp/app.log]
       AllowKey=system.run[/usr/bin/uptime]
       AllowKey=system.run[/usr/bin/df -h /]
       DenyKey=vfs.file.*[*]

Allowing scripts

Allow Zabbix agent to execute scripts on hosts via all available methods:

• Global scripts that can be executed in the frontend or via API (this method always uses the system.run[myscript.sh] key)
• Remote commands from action operations (this method always uses the system.run[myscript.sh,nowait] key)
• system.run Zabbix agent items with the script, for example:
  • system.run[myscript.sh]
  • system.run[myscript.sh,wait]
  • system.run[myscript.sh,nowait]

AllowKey=system.run[myscript.sh,*]

To control the wait/nowait parameter, you must set a different rule. For example, you can allow only system.run[myscript.sh,wait] items, thus excluding other methods:

AllowKey=system.run[myscript.sh,wait]

Securing allow/deny rules

This example shows how to secure overly permissive AllowKey or DenyKey rules.

Consider the following rules:

AllowKey=system.run["C:\Program^ Files\Zabbix^ Agent^ 2\scripts\test.bat*"]
       DenyKey=vfs.file.*
       DenyKey=system.cpu.load[*]

These rules contain a wildcard (*), which can be misused:

• The test.bat script can be executed with any arguments, including unintended ones.
• The vfs.file.* pattern matches only item keys without parameters; however, all vfs.file items require parameters.
• The system.cpu.load[*] pattern matches only item keys with parameters; however system.cpu.load items do not require parameters.

To secure these rules, explicitly allow executing test.bat only with specific arguments, and deny correct item key patterns; for example:

AllowKey=system.run["C:\Program^ Files\Zabbix^ Agent^ 2\scripts\test.bat status"]
       AllowKey=system.run["C:\Program^ Files\Zabbix^ Agent^ 2\scripts\test.bat version"]
       DenyKey=vfs.file.*[*]
       DenyKey=system.cpu.load
       DenyKey=system.cpu.load[*]

You can test the rules by running the following commands, which will return ZBX_NOTSUPPORTED.

cd "C:\Program Files\Zabbix Agent 2"
       zabbix_agent2.exe -t system.run["C:\Program^ Files\Zabbix^ Agent^ 2\scripts\test.bat debug"]
       zabbix_agent2.exe -t vfs.file.size["C:\ProgramData\MyApp\config.ini"]
       zabbix_agent2.exe -t vfs.file.contents["C:\Windows\System32\drivers\etc\hosts"]
       zabbix_agent2.exe -t system.cpu.load
       zabbix_agent2.exe -t system.cpu.load[all,avg1]

Pattern examples

The following table shows how item key patterns are matched:

• A key matches the pattern only if it meets all conditions in the Matches column.
• Parameters must be fully enclosed in square brackets (e.g., vfs.file.contents[* and vfs.file.contents*utf8] are invalid patterns).