Whitelists in parsing

When a whitelist is present in parsing /etc/crowdsec/parsers/..., it will be checked/discarded before being poured to any bucket. These whitelists intentionally generate no logs and are useful to discard noisy false positive sources.

Whitelist by IP address

Let's assume we have a setup with a crowdsecurity/nginx collection enabled and no whitelists.

Thus, if I "attack" myself :

nikto -host myfqdn.com

my own IP address will be flagged as being an attacker :

$ tail -f /var/log/crowdsec.log 
ime="07-07-2020 16:13:16" level=warning msg="80.x.x.x triggered a 4h0m0s ip ban remediation for [crowdsecurity/http-bad-user-agent]" bucket_id=cool-smoke event_time="2020-07-07 16:13:16.579581642 +0200 CEST m=+358819.413561109" scenario=crowdsecurity/http-bad-user-agent source_ip=80.x.x.x
time="07-07-2020 16:13:16" level=warning msg="80.x.x.x triggered a 4h0m0s ip ban remediation for [crowdsecurity/http-probing]" bucket_id=green-silence event_time="2020-07-07 16:13:16.737579458 +0200 CEST m=+358819.571558901" scenario=crowdsecurity/http-probing source_ip=80.x.x.x
time="07-07-2020 16:13:17" level=warning msg="80.x.x.x triggered a 4h0m0s ip ban remediation for [crowdsecurity/http-crawl-non_statics]" bucket_id=purple-snowflake event_time="2020-07-07 16:13:17.353641625 +0200 CEST m=+358820.187621068" scenario=crowdsecurity/http-crawl-non_statics source_ip=80.x.x.x
time="07-07-2020 16:13:18" level=warning msg="80.x.x.x triggered a 4h0m0s ip ban remediation for [crowdsecurity/http-sensitive-files]" bucket_id=small-hill event_time="2020-07-07 16:13:18.005919055 +0200 CEST m=+358820.839898498" scenario=crowdsecurity/http-sensitive-files source_ip=80.x.x.x
^C
sudo cscli decisions list
4 local decisions:
+--------+---------------+-----------------------------------+------+--------+---------+---------------------------+--------+------------+
| SOURCE |      IP       |              REASON               | BANS | ACTION | COUNTRY |            AS             | EVENTS | EXPIRATION |
+--------+---------------+-----------------------------------+------+--------+---------+---------------------------+--------+------------+
| local  | 80.x.x.x   | crowdsecurity/http-bad-user-agent |    4 | ban    | FR      | 21502 SFR SA              |     60 | 3h59m3s    |
...

Create the whitelist by IP address

Let's create a /etc/crowdsec/parsers/s02-enrich/mywhitelists.yaml file with the following content :

name: my/whitelist
description: "Whitelist events from my ip addresses"
whitelist:
  reason: "my ip ranges"
  ip:
    - "80.x.x.x"

Reload CrowdSec

sudo systemctl reload crowdsec

Test the whitelist

Thus, if we restart our attack :

nikto -host myfqdn.com

And we don't get bans :

$ tail -f /var/log/crowdsec.log  
...
^C
sudo cscli decisions list
No local decisions.
And 21 records from API, 15 distinct AS, 12 distinct countries

Here, we don't get any logs, as the event have been discarded at parsing time.

Create whitelist by expression

Now, let's make something more tricky : let's whitelist a specific user-agent (of course, it's just an example, don't do this at home !). The hub's taxonomy will helps us to find which data is present in which field.

Let's change our whitelist to :

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

Again, let's restart 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  
...
time="07-05-2020 09:39:09" level=info msg="Event is whitelisted by Expr !" filter= name=solitary-leaf stage=s02-enrich
...

Whitelist in PostOverflows

Whitelists in PostOverflows are applied after the bucket overflow happens.

warning

In PostOverflows, the evt.Parsed object will be empty at this stage.

It means that you must work with the evt.Overflow object for your expression.

It has the advantage of being triggered only once we are about to take decision about an IP or Range, and thus happens a lot less often.

A good example is the crowdsecurity/whitelist-good-actors collection.

But let's craft ours based on our previous example ! First of all, install the crowdsecurity/rdns postoverflow : it will be in charge of enriching overflows with reverse dns information of the offending IP address.

Let's put the following file in /etc/crowdsec/postoverflows/s01-whitelist/mywhitelists.yaml :

name: me/my_cool_whitelist
description: lets whitelist our own reverse dns
whitelist:
  reason: dont ban my ISP
  expression:
  #this is the reverse of my ip, you can get it by performing a "host" command on your public IP for example
    - evt.Enriched.reverse_dns endsWith '.asnieres.rev.numericable.fr.'

After reloading CrowdSec, and launching (again!) nikto :

nikto -host myfqdn.com

$ tail -f /var/log/crowdsec.log
ime="07-07-2020 17:11:09" level=info msg="Ban for 80.x.x.x whitelisted, reason [dont ban my ISP]" id=cold-sunset name=me/my_cool_whitelist stage=s01
time="07-07-2020 17:11:09" level=info msg="node warning : no remediation" bucket_id=blue-cloud event_time="2020-07-07 17:11:09.175068053 +0200 CEST m=+2308.040825320" scenario=crowdsecurity/http-probing source_ip=80.x.x.x
time="07-07-2020 17:11:09" level=info msg="Processing Overflow with no decisions 80.x.x.x performed 'crowdsecurity/http-probing' (11 events over 313.983994ms) at 2020-07-07 17:11:09.175068053 +0200 CEST m=+2308.040825320" bucket_id=blue-cloud event_time="2020-07-07 17:11:09.175068053 +0200 CEST m=+2308.040825320" scenario=crowdsecurity/http-probing source_ip=80.x.x.x
...

This time, we can see that logs are being produced when the event is discarded.