Alert on SSH Login from new IP with OSSEC

Published: May 27, 2019

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A useful security alert condition is a login from a new IP address. In this post we’ll explore how to set this up with OSSEC.

The “First time user logged in” alert

By default OSSEC sends an alert the first time a user logs in. We can use ossec-logtest to observe this:

$ cat ossec-lest-log
May 22 02:13:22 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ
$ cat ossec-test-log | /var/ossec/bin/ossec-logtest 
2019/05/28 00:24:33 ossec-testrule: INFO: Reading local decoder file.
2019/05/28 00:24:33 ossec-testrule: INFO: Started (pid: 5253).
2019/05/28 00:24:33 ossec-testrule: INFO: Started (pid: 5253).
ossec-testrule: Type one log per line.



**Phase 1: Completed pre-decoding.
       full event: 'May 22 02:13:22 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'
       hostname: 'localhost'
       program_name: 'sshd'
       log: 'Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'

**Phase 2: Completed decoding.
       decoder: 'sshd'
       dstuser: 'vagrant'
       srcip: '10.0.2.2'

**Phase 3: Completed filtering (rules).
       Rule id: '10100'
       Level: '4'
       Description: 'First time user logged in.'
**Alert to be generated.

When we have two logins from separate IPs the “First time user logged in” alert only first on the first login (as expected)…

$ cat ossec-test-log-2
May 22 02:13:22 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ
May 22 02:13:33 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.3 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ
$ cat ossec-test-log-2 | /var/ossec/bin/ossec-logtest 
2019/05/28 00:26:25 ossec-testrule: INFO: Reading local decoder file.
2019/05/28 00:26:25 ossec-testrule: INFO: Started (pid: 5255).
ossec-testrule: Type one log per line.



**Phase 1: Completed pre-decoding.
       full event: 'May 22 02:13:22 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'
       hostname: 'localhost'
       program_name: 'sshd'
       log: 'Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'

**Phase 2: Completed decoding.
       decoder: 'sshd'
       dstuser: 'vagrant'
       srcip: '10.0.2.2'

**Phase 3: Completed filtering (rules).
       Rule id: '10100'
       Level: '4'
       Description: 'First time user logged in.'
**Alert to be generated.




**Phase 1: Completed pre-decoding.
       full event: 'May 22 02:13:33 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.3 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'
       hostname: 'localhost'
       program_name: 'sshd'
       log: 'Accepted publickey for vagrant from 10.0.2.3 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'

**Phase 2: Completed decoding.
       decoder: 'sshd'
       dstuser: 'vagrant'
       srcip: '10.0.2.3'

**Phase 3: Completed filtering (rules).
       Rule id: '5715'
       Level: '3'
       Description: 'SSHD authentication success.'
**Alert to be generated.

<if_fts> and <fts>

We can see the configuration of the rule in /var/ossec/rules/syslog_rules.xml

<group name="syslog,fts,">
  <rule id="10100" level="4">
    <if_group>authentication_success</if_group>
    <options>alert_by_email</options>
    <if_fts></if_fts>
    <group>authentication_success</group>
    <description>First time user logged in.</description>
  </rule>
</group>

There’s not much official documentation on this, but the <if_fts> tag is the key to the operation of the alert.

The Decoder

fts (first time seen) is set by the decoder. If you look in /var/ossec/etc/decoder.xml you’ll see the following:

<decoder name="sshd-success">
  <parent>sshd</parent>
  <prematch>^Accepted</prematch>
  <regex offset="after_prematch">^ \S+ for (\S+) from (\S+) port </regex>
  <order>user, srcip</order>
  <fts>name, user, location</fts>
</decoder>

In the <fts> node you’ll see it uses name, user and location. If we add srcip to the fts node we can track new logins by IP.

NOTE: It seems less than ideal to modify decoders.xml directly to make this change, however, per this blog post there is no way to override decoders in OSSEC.

<decoder name="sshd-success">
  <parent>sshd</parent>
  <prematch>^Accepted</prematch>
  <regex offset="after_prematch">^ \S+ for (\S+) from (\S+) port </regex>
  <order>user, srcip</order>
  <fts>name, user, location, srcip</fts>
</decoder>

Re-Testing

Now you’ll see that if you test with ossec-logtest again it alerts “First time user logged in” for each new IP.

# cat ossec-test-log | /var/ossec/bin/ossec-logtest 
2019/05/28 00:33:24 ossec-testrule: INFO: Reading local decoder file.
2019/05/28 00:33:24 ossec-testrule: INFO: Started (pid: 5275).
ossec-testrule: Type one log per line.



**Phase 1: Completed pre-decoding.
       full event: 'May 22 02:13:22 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'
       hostname: 'localhost'
       program_name: 'sshd'
       log: 'Accepted publickey for vagrant from 10.0.2.2 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'

**Phase 2: Completed decoding.
       decoder: 'sshd'
       dstuser: 'vagrant'
       srcip: '10.0.2.2'

**Phase 3: Completed filtering (rules).
       Rule id: '10100'
       Level: '4'
       Description: 'First time user logged in.'
**Alert to be generated.




**Phase 1: Completed pre-decoding.
       full event: 'May 22 02:13:33 localhost sshd[13949]: Accepted publickey for vagrant from 10.0.2.3 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'
       hostname: 'localhost'
       program_name: 'sshd'
       log: 'Accepted publickey for vagrant from 10.0.2.3 port 64565 ssh2: RSA SHA256:WeegtaAAFxNXdrRFSJfQ7Yc1sJQLOqYZTzr4uRjByyQ'

**Phase 2: Completed decoding.
       decoder: 'sshd'
       dstuser: 'vagrant'
       srcip: '10.0.2.3'

**Phase 3: Completed filtering (rules).
       Rule id: '10100'
       Level: '4'
       Description: 'First time user logged in.'
**Alert to be generated.

Max Chadwick Hi, I'm Max!

I'm a software developer who mainly works in PHP, but also dabbles in Ruby and Go. Technical topics that interest me are monitoring, security and performance.

During the day I solve challenging technical problems at Something Digital where I mainly work with the Magento platform. I also blog about tech, work on open source and hunt for bugs.

If you'd like to get in touch with me the best way is on Twitter.