Detection: Linux Auditd Possible Access Or Modification Of Sshd Config File

Description

The following analytic detects access, deletion or modification of the ssh_config file on Linux systems. It leverages data from Linux Auditd, focusing on events of type PATH with a nametype of ("NORMAL", "CREATE", "DELETE"). This activity could be significant because unauthorized changes to ssh_config can allow threat actors to redirect port connections or use unauthorized keys, potentially compromising the system. Correlate this with related EXECVE or PROCTITLE events to identify the process or user responsible for the access or modification. If confirmed malicious, this could lead to unauthorized access, privilege escalation, or persistent backdoor access, posing a severe security risk.

Search

 1`linux_auditd`
 2(
 3  (type=PATH nametype IN ("NORMAL", "CREATE", "DELETE"))
 4  OR
 5  type=CWD
 6)
 7
 8| rex "msg=audit\([^)]*:(?<audit_id>\d+)\)"
 9
10
11| stats
12  values(type) as types
13  values(name) as names
14  values(nametype) as nametype
15  values(cwd) as cwd_list
16  values(_time) as event_times
17  by audit_id, host
18
19
20| eval current_working_directory = coalesce(mvindex(cwd_list, 0), "N/A")
21
22| eval candidate_paths = mvmap(names, if(match(names, "^/"), names, current_working_directory + "/" + names))
23
24| eval matched_paths = mvfilter(match(candidate_paths, "/etc/ssh/ssh_config.*"))
25
26| eval match_count = mvcount(matched_paths)
27
28| eval reconstructed_path = mvindex(matched_paths, 0)
29
30| eval e_time = mvindex(event_times, 0)
31
32| where match_count > 0
33
34| rename host as dest
35
36
37| stats count min(e_time) as firstTime max(e_time) as lastTime
38  values(nametype) as nametype
39  by current_working_directory
40     reconstructed_path
41     match_count
42     dest
43     audit_id
44
45
46| `security_content_ctime(firstTime)`
47
48| `security_content_ctime(lastTime)`
49
50|  `linux_auditd_possible_access_or_modification_of_sshd_config_file_filter`

Data Source

Name	Platform	Sourcetype	Source
Linux Auditd Cwd	Linux	'auditd'	'auditd'
Linux Auditd Path	Linux	'auditd'	'auditd'

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

To implement this detection, the process begins by ingesting auditd data, that consist SYSCALL, TYPE, EXECVE and PROCTITLE events, which captures command-line executions and process details on Unix/Linux systems. These logs should be ingested and processed using Splunk Add-on for Unix and Linux (https://splunkbase.splunk.com/app/833), which is essential for correctly parsing and categorizing the data. The next step involves normalizing the field names to match the field names set by the Splunk Common Information Model (CIM) to ensure consistency across different data sources and enhance the efficiency of data modeling and make sure the type=CWD record type is activate in your auditd configuration. This approach enables effective monitoring and detection of linux endpoints where auditd is deployed.

Known False Positives

Administrator or network operator can use this commandline for automation purposes. Please update the filter macros to remove false positives.

Associated Analytic Story

• Compromised Linux Host

• Linux Living Off The Land

• Linux Persistence Techniques

• Linux Privilege Escalation

Risk Based Analytics (RBA)

Risk Message:

$reconstructed_path$ has been accessed with type $nametype$ on host - [$dest$]

References

• https://www.hackingarticles.in/ssh-penetration-testing-port-22/

• https://attack.mitre.org/techniques/T1098/004/

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 8