9.8 KiB
There are two YARA related tables in osquery, which serve very different purposes. The first table, called yara_events, uses osquery's Events framework to monitor for filesystem changes and will execute YARA when a file change event fires. The second table, called yara, is an on-demand YARA scanning table.
Both of these tables are considered a beta feature right now.
YARA Configuration
The configuration for osquery is simple. Here is an example config:
{
// Description of the YARA feature.
"yara": {
"signatures": {
// Each key is an arbitrary group name to give the signatures listed
"sig_group_1": [ "/Users/wxs/sigs/foo.sig", "/Users/wxs/sigs/bar.sig" ],
"sig_group_2": [ "/Users/wxs/sigs/baz.sig" ]
},
"file_paths": {
// Each key is a key from file_paths
// The value is a list of signature groups to run when an event fires
// These will be watched for and scanned when the event framework
// fire off an event to yara_events table
"system_binaries": [ "sig_group_1" ],
"tmp": [ "sig_group_1", "sig_group_2" ]
}
},
// Paths to watch for filesystem events
"file_paths": {
"system_binaries": [ "/usr/bin/%", "/usr/sbin/%" ],
"tmp": [ "/Users/%/tmp/%%", "/tmp/%" ]
}
}
The first thing to notice is the file_paths section, which is used to describe which paths to monitor for changes. Each key is an arbitrary category name and the value is a list of paths. The syntax used is documented on the osquery wildcarding rules described on the FIM page. The paths, when expanded out by osquery, are monitored for changes and processed by the file_events table.
The second thing to notice is the yara section, which contains the configuration to use for YARA within osquery. The yara section contains two keys: signatures and file_paths. The signatures key contains a set of arbitrary key names, called "signature groups". The value for each of these groups are the paths to the signature files that will be compiled and stored within osquery. The paths to the signature files can be absolute or relative to /etc/osquery/yara/. The file_paths key maps the category name for an event described in the global file_paths section to a signature grouping to use when scanning.
For example, when a file in /usr/bin/ and /usr/sbin/ is changed it will be scanned with sig_group_1, which consists of foo.sig and bar.sig. When a file in /Users/%/tmp/ (recursively) is changed it will be scanned with sig_group_1 and sig_group_2, which consists of all three signature files.
yara_events table
Using the configuration above you can see it in action. While osquery was running I executed touch /Users/wxs/tmp/foo in another terminal. Here is the relevant queries to show what happened:
osquery> select * from file_events;
+--------------------+----------+------------+---------+----------------+----------------------------------+------------------------------------------+------------------------------------------------------------------+
| target_path | category | time | action | transaction_id | md5 | sha1 | sha256 |
+--------------------+----------+------------+---------+----------------+----------------------------------+------------------------------------------+------------------------------------------------------------------+
| /Users/wxs/tmp/foo | tmp | 1430078285 | CREATED | 33859499 | d41d8cd98f00b204e9800998ecf8427e | da39a3ee5e6b4b0d3255bfef95601890afd80709 | e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 |
+--------------------+----------+------------+---------+----------------+----------------------------------+------------------------------------------+------------------------------------------------------------------+
osquery> select * from yara_events;
+--------------------+----------+------------+---------+----------------+-------------+-------+
| target_path | category | time | action | transaction_id | matches | count |
+--------------------+----------+------------+---------+----------------+-------------+-------+
| /Users/wxs/tmp/foo | tmp | 1430078285 | CREATED | 33859499 | always_true | 1 |
+--------------------+----------+------------+---------+----------------+-------------+-------+
osquery>
The file_events table recorded that a file named /Users/wxs/tmp/foo was created with the corresponding hashes and a timestamp.
The yara_events table recorded that 1 matching rule (always_true) was found when the file was created. In this example every file will always have at least one match because I am using a rule which always evaluates to true. In the next example I'll issue the same command to create a file in a monitored directory but have removed the always_true rule from my signature files.
osquery> select * from yara_events;
+--------------------+----------+------------+---------+----------------+-------------+-------+
| target_path | category | time | action | transaction_id | matches | count |
+--------------------+----------+------------+---------+----------------+-------------+-------+
| /Users/wxs/tmp/foo | tmp | 1430078285 | CREATED | 33859499 | always_true | 1 |
| /Users/wxs/tmp/foo | tmp | 1430078524 | CREATED | 33860795 | | 0 |
+--------------------+----------+------------+---------+----------------+-------------+-------+
osquery>
As you can see, even though no matches were found a row is still created and stored.
On-demand YARA scanning
The yara table is used for on-demand scanning. With this table you can arbitrarily YARA scan any available file on the filesystem with any available signature files or signature group from the configuration. In order to scan, the table must be given a constraint which says where to scan and what to scan with.
In order to determine where to scan, the table accepts either a path or a pattern constraint. The path constraint must be a full path to a single file. There is no expansion or recursion with this constraint. The pattern constraint follows the same wildcard rules mentioned before.
Once the "where" is out of the way, you must specify the "what" part. This is done through either the sigfile or sig_group constraints. The sigfile constraint can be either an absolute path to a signature file on disk or a path relative to /var/osquery/. The signature file will be compiled only for the execution of this one query and removed afterwards. The sig_group constraint must consist of a named signature grouping from your configuration file.
Here are some examples of the yara table in action:
osquery> select * from yara where path="/bin/ls" and sig_group="sig_group_1";
+---------+-------------+-------+-------------+---------+---------+
| path | matches | count | sig_group | sigfile | pattern |
+---------+-------------+-------+-------------+---------+---------+
| /bin/ls | always_true | 1 | sig_group_1 | | |
+---------+-------------+-------+-------------+---------+---------+
osquery> select * from yara where path="/bin/ls" and sig_group="sig_group_2";
+---------+---------+-------+-------------+---------+---------+
| path | matches | count | sig_group | sigfile | pattern |
+---------+---------+-------+-------------+---------+---------+
| /bin/ls | | 0 | sig_group_2 | | |
+---------+---------+-------+-------------+---------+---------+
osquery>
As you can see in these examples, I'm scanning the same file with two different signature groups and getting different results.
osquery> select * from yara where pattern="/bin/%sh" and sig_group="sig_group_1";
+-----------+-------------+-------+-------------+---------+----------+
| path | matches | count | sig_group | sigfile | pattern |
+-----------+-------------+-------+-------------+---------+----------+
| /bin/bash | always_true | 1 | sig_group_1 | | /bin/%sh |
| /bin/csh | always_true | 1 | sig_group_1 | | /bin/%sh |
| /bin/ksh | always_true | 1 | sig_group_1 | | /bin/%sh |
| /bin/sh | always_true | 1 | sig_group_1 | | /bin/%sh |
| /bin/tcsh | always_true | 1 | sig_group_1 | | /bin/%sh |
| /bin/zsh | always_true | 1 | sig_group_1 | | /bin/%sh |
+-----------+-------------+-------+-------------+---------+----------+
osquery>
The above illustrates using the pattern constraint to scan /bin/%sh with a signature group.
osquery> select * from yara where pattern="/bin/%sh" and sigfile="/Users/wxs/sigs/baz.sig";
+-----------+---------+-------+-----------+-------------------------+----------+
| path | matches | count | sig_group | sigfile | pattern |
+-----------+---------+-------+-----------+-------------------------+----------+
| /bin/bash | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
| /bin/csh | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
| /bin/ksh | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
| /bin/sh | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
| /bin/tcsh | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
| /bin/zsh | | 0 | | /Users/wxs/sigs/baz.sig | /bin/%sh |
+-----------+---------+-------+-----------+-------------------------+----------+
osquery>
The above is an example of using an absolute path for sigfile combined with pattern.