Expression

Let's whitelist a specific user-agent (of course, it's just an example, don't do this in production !).

Since we are using data that is present from the parsing stage we can do this within Parsing Whitelist level. Please see introduction for your OS specific paths.

name: crowdsecurity/whitelists
description: "Whitelist events from private ipv4 addresses"
whitelist:
  reason: "private ipv4 ranges"
  expression:
   - evt.Parsed.http_user_agent == 'MySecretUserAgent'

Reload CrowdSec

sudo systemctl reload crowdsec

For the record, I edited nikto's configuration to use 'MySecretUserAgent' as user-agent, and thus :

nikto -host myfqdn.com

tail -f /var/log/crowdsec.log

CrowdSec will inform you some lines have been discarded because they are whitelisted by the expression.

How can I find which data is present from parsing stage?

You can use cscli explain to generate output from a given log line or log file.

For example:

sudo cscli explain --log '5.5.8.5 - - [04/Jan/2020:07:25:02 +0000] "GET /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo HTTP/1.1" 404 522 "-" "MySecretUserAgent"' --type nginx -v

Output:

line: 5.5.8.5 - - [04/Jan/2020:07:25:02 +0000] "GET /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo HTTP/1.1" 404 522 "-" "MySecretUserAgent"
	├ s00-raw
	|	├ 🟢 crowdsecurity/non-syslog (+5 ~8)
	|		├ update evt.ExpectMode : %!s(int=0) -> 1
	|		├ update evt.Stage :  -> s01-parse
	|		├ update evt.Line.Raw :  -> 5.5.8.5 - - [04/Jan/2020:07:25:02 +0000] "GET /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo HTTP/1.1" 404 522 "-" "MySecretUserAgent"
	|		├ update evt.Line.Src :  -> /tmp/cscli_explain156736029/cscli_test_tmp.log
	|		├ update evt.Line.Time : 0001-01-01 00:00:00 +0000 UTC -> 2023-07-21 14:05:09.67803335 +0000 UTC
	|		├ create evt.Line.Labels.type : nginx
	|		├ update evt.Line.Process : %!s(bool=false) -> true
	|		├ update evt.Line.Module :  -> file
	|		├ create evt.Parsed.message : 5.5.8.5 - - [04/Jan/2020:07:25:02 +0000] "GET /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo HTTP/1.1" 404 522 "-" "MySecretUserAgent"
	|		├ create evt.Parsed.program : nginx
	|		├ update evt.Time : 0001-01-01 00:00:00 +0000 UTC -> 2023-07-21 14:05:09.678072613 +0000 UTC
	|		├ create evt.Meta.datasource_path : /tmp/cscli_explain156736029/cscli_test_tmp.log
	|		├ create evt.Meta.datasource_type : file
	├ s01-parse
	|	├ 🟢 crowdsecurity/nginx-logs (+22 ~2)
	|		├ update evt.Stage : s01-parse -> s02-enrich
	|		├ create evt.Parsed.remote_addr : 5.5.8.5
	|		├ create evt.Parsed.request_length : 
	|		├ create evt.Parsed.verb : GET
	|		├ create evt.Parsed.http_user_agent : MySecretUserAgent
	|		├ create evt.Parsed.request : /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo
	|		├ create evt.Parsed.body_bytes_sent : 522
	|		├ create evt.Parsed.remote_user : -
	|		├ create evt.Parsed.time_local : 04/Jan/2020:07:25:02 +0000
	|		├ create evt.Parsed.http_referer : -
	|		├ create evt.Parsed.request_time : 
	|		├ create evt.Parsed.proxy_alternative_upstream_name : 
	|		├ create evt.Parsed.proxy_upstream_name : 
	|		├ create evt.Parsed.status : 404
	|		├ create evt.Parsed.target_fqdn : 
	|		├ create evt.Parsed.http_version : 1.1
	|		├ update evt.StrTime :  -> 04/Jan/2020:07:25:02 +0000
	|		├ create evt.Meta.http_status : 404
	|		├ create evt.Meta.http_user_agent : MySecretUserAgent
	|		├ create evt.Meta.log_type : http_access-log
	|		├ create evt.Meta.service : http
	|		├ create evt.Meta.http_path : /.well-known/acme-challenge/FMuukC2JOJ5HKmLBujjE_BkDo
	|		├ create evt.Meta.http_verb : GET
	|		├ create evt.Meta.source_ip : 5.5.8.5
	├ s02-enrich
	|	├ 🟢 crowdsecurity/dateparse-enrich (+2 ~2)
	|		├ create evt.Enriched.MarshaledTime : 2020-01-04T07:25:02Z
	|		├ update evt.Time : 2023-07-21 14:05:09.678072613 +0000 UTC -> 2020-01-04 07:25:02 +0000 UTC
	|		├ update evt.MarshaledTime :  -> 2020-01-04T07:25:02Z
	|		├ create evt.Meta.timestamp : 2020-01-04T07:25:02Z
	|	├ 🟢 crowdsecurity/geoip-enrich (+13)
	|		├ create evt.Enriched.ASNumber : 6805
	|		├ create evt.Enriched.Latitude : 51.299300
	|		├ create evt.Enriched.SourceRange : 5.4.0.0/14
	|		├ create evt.Enriched.ASNOrg : Telefonica Germany
	|		├ create evt.Enriched.IsInEU : true
	|		├ create evt.Enriched.IsoCode : DE
	|		├ create evt.Enriched.Longitude : 9.491000
	|		├ create evt.Enriched.ASNNumber : 6805
	|		├ create evt.Meta.ASNOrg : Telefonica Germany
	|		├ create evt.Meta.IsInEU : true
	|		├ create evt.Meta.IsoCode : DE
	|		├ create evt.Meta.ASNNumber : 6805
	|		├ create evt.Meta.SourceRange : 5.4.0.0/14
	|	├ 🟢 crowdsecurity/http-logs (+7)
	|		├ create evt.Parsed.impact_completion : false
	|		├ create evt.Parsed.file_ext : 
	|		├ create evt.Parsed.file_frag : FMuukC2JOJ5HKmLBujjE_BkDo
	|		├ create evt.Parsed.file_name : FMuukC2JOJ5HKmLBujjE_BkDo
	|		├ create evt.Parsed.static_ressource : false
	|		├ create evt.Parsed.file_dir : /.well-known/acme-challenge/
	|		├ create evt.Meta.http_args_len : 0
	|	└ 🟢 crowdsecurity/whitelists (unchanged)
	├-------- parser success 🟢
	├ Scenarios
		├ 🟢 crowdsecurity/http-crawl-non_statics
		└ 🟢 crowdsecurity/http-probing

You can see what data can be used from s01-parse stage.