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Version: v1.3.4

Format

Scenario configuration example​

A way to detect a http scanner might be to track the number of distinct non-existing pages it's requesting. The scenario might look like this:

type: leaky
name: crowdsecurity/http-scan-uniques_404
description: "Detect multiple unique 404 from a single ip"
filter: "evt.Meta.service == 'http' && evt.Meta.http_status in ['404', '403', '400']"
groupby: "evt.Meta.source_ip"
distinct: "evt.Meta.http_path"
capacity: 5
leakspeed: "10s"
blackhole: 5m
labels:
 service: http
 type: scan
 remediation: true

Scenario directives​

type​

type: leaky|trigger|counter

Defines the type of the bucket. Currently three types are supported :

  • leaky : a leaky bucket that must be configured with a capacity and a leakspeed
  • trigger : a bucket that overflows as soon as an event is poured (it is like a leaky bucket is a capacity of 0)
  • counter : a bucket that only overflows every duration. It is especially useful to count things.

name​

name: github_account_name/my_scenario_name

or

name: my_author_name/my_scenario_name

The name is mandatory.

It must be unique. This will define the scenario's name in the hub.

description​

description: A scenario that detect XXXX behavior

The description is mandatory.

It is a short description, probably one sentence, describing what it detects.

filter​

filter: expression

filter must be a valid expr expression that will be evaluated against the event.

If filter evaluation returns true or is absent, event will be pour in the bucket.

If filter returns false or a non-boolean, the event will be skipped for this bucket.

Here is the expr documentation.

Examples :

  • evt.Meta.log_type == 'telnet_new_session'
  • evt.Meta.log_type in ['http_access-log', 'http_error-log'] && evt.Parsed.static_ressource == 'false'
  • evt.Meta.log_type == 'ssh_failed-auth'

duration​

duration: 45s
duration: 10m

Only applies to counter buckets.

A duration after which the bucket will overflow. The format must be compatible with golang ParseDuration format

Examples :

type: counter
name: crowdsecurity/ban-reports-ssh_bf_report
description: "Count unique ips performing ssh bruteforce"
filter: "evt.Overflow.Scenario == 'ssh_bruteforce'"
distinct: "evt.Overflow.Source_ip"
capacity: -1
duration: 10m
labels:
  service: ssh

groupby​

groupby: evt.Meta.source_ip

An expression that must return a string. This string will be used as a partition for the buckets.

Examples​

Here, each source_ip will get its own bucket.

type: leaky
...
groupby: evt.Meta.source_ip
...

Here, each unique combo of source_ip + target_username will get its own bucket.

type: leaky
...
groupby: evt.Meta.source_ip + '--' + evt.Parsed.target_username
...

distinct​

distinct: evt.Meta.http_path

An expression that must return a string. The event will be poured only if the string is not already present in the bucket.

Examples​

This will ensure that events that keep triggering the same .Meta.http_path will be poured only once.

type: leaky
...
distinct: "evt.Meta.http_path"
...

Assuming we received the 3 following events :

  • evt.Meta.http_path = /
  • evt.Meta.http_path = /test
  • evt.Meta.http_path = /

Only the first 2 events will be poured to the bucket.

The 3rd one will not be poured as the bucket already contains an event with evt.Meta.http_path == /

capacity​

capacity: 5

Only applies to leaky buckets.

A positive integer representing the bucket capacity. If there are more than capacity item in the bucket, it will overflow.

leakspeed​

leakspeed: "10s"

Only applies to leaky buckets.

A duration that represent how often an event will be leaking from the bucket.

Must be compatible with golang ParseDuration format.

Example​

Here the bucket will leak one item every 10 seconds, and can hold up to 5 items before overflowing.

type: leaky
...
leakspeed: "10s"
capacity: 5
...

labels​

labels:
 service: ssh
 type: bruteforce
 remediation: true

Labels is a list of label: values that provide context to an overflow. The labels are (currently) not stored in the database, nor they are sent to the API.

Special labels :​
  • The remediation label, if set to true indicate the the originating IP should be banned.

Example​

The IP address that triggered the overflow (.Meta.source_ip) will be banned.

type: leaky
...
labels:
 service: ssh
 type: bruteforce
 remediation: true

blackhole​

blackhole: 10m

A duration for which a bucket will be "silenced" after overflowing. This is intended to limit / avoid spam of buckets that might be very rapidly triggered.

The blackhole only applies to the individual bucket rather than the whole scenario.

Must be compatible with golang ParseDuration format.

Example​

The same source_ip won't be able to trigger this overflow more than once every 10 minutes. The potential overflows in the meanwhile will be discarded (but will still appear in logs as being blackholed).

type: trigger
...
blackhole: 10m
groupby: evt.Meta.source_ip

debug​

debug: true|false

default: false

If set to to true, enabled scenario level debugging. It is meant to help understanding scenario behavior by providing contextual logging :

debug of filters and expression results
DEBU[31-07-2020 16:34:58] eval(evt.Meta.log_type in ["http_access-log", "http_error-log"] && any(File("bad_user_agents.txt"), {evt.Parsed.http_user_agent contains #})) = TRUE  cfg=still-feather file=config/scenarios/http-bad-user-agent.yaml name=crowdsecurity/http-bad-user-agent
DEBU[31-07-2020 16:34:58] eval variables:                               cfg=still-feather file=config/scenarios/http-bad-user-agent.yaml name=crowdsecurity/http-bad-user-agent
DEBU[31-07-2020 16:34:58]        evt.Meta.log_type = 'http_access-log'  cfg=still-feather file=config/scenarios/http-bad-user-agent.yaml name=crowdsecurity/http-bad-user-agent
DEBU[31-07-2020 16:34:58]        evt.Parsed.http_user_agent = 'Mozilla/5.00 (Nikto/2.1.5) (Evasions:None) (Test:002810)'  cfg=still-feather file=config/scenarios/http-bad-user-agent.yaml name=crowdsecurity/http-bad-user-agent

reprocess​

reprocess: true|false

default: false

If set to true, the resulting overflow will be sent again in the scenario/parsing pipeline. It is useful when you want to have further scenarios that will rely on past-overflows to take decisions.

cache_size​

cache_size: 5

By default, a bucket holds capacity events "in memory". However, for a number of cases, you don't want this, as it might lead to excessive memory consumption.

By setting cache_size to a positive integer, we can control the maximum in-memory cache size of the bucket, without changing its capacity and such. It is useful when buckets are likely to stay alive for a long time or ingest a lot of events to avoid storing a lot of events in memory.

overflow_filter​

overflow_filter: any(queue.Queue, { .Enriched.IsInEU  == "true" })

overflow_filter is an expression that is run when the bucket overflows. If this expression is present and returns false, the overflow will be discarded.

cancel_on​

cancel_on: evt.Parsed.something == 'somevalue'

cancel_on is an expression that runs on each event poured to the bucket. If the cancel_on expression returns true, the bucket is immediately destroyed (and doesn't overflow).

data​

data:
  - source_url: https://URL/TO/FILE
    dest_file: LOCAL_FILENAME
    [type: (regexp|string)]

data allows to specify an external source of data.

This section is only relevant when cscli is used to install scenario from hub, as it will download the source_url and store it to dest_file.

When the scenario is not installed from the hub, CrowdSec won't download the URL, but the file must exist for the scenario to be loaded correctly.

The type is mandatory if you want to evaluate the data in the file, and should be regex for valid (re2) regular expression per line or string for string per line.

The regexps will be compiled, the strings will be loaded into a list and both will be kept in memory. Without specifying a type, the file will be downloaded and stored as file and not in memory.

You can refer to the content of the downloaded file(s) by using either the File() or RegexpInFile() function in an expression:

filter: 'evt.Meta.log_type in ["http_access-log", "http_error-log"] and any(File("backdoors.txt"), { evt.Parsed.request contains #})'

Example​

name: crowdsecurity/cdn-whitelist
...
data:
  - source_url: https://www.cloudflare.com/ips-v4
    dest_file: cloudflare_ips.txt
    type: string

format​

format: 2.0

CrowdSec has a notion of format support for parsers and scenarios for compatibility management. Running cscli version will show you such compatibility matrix :

sudo cscli version
2020/11/05 09:35:05 version: v0.3.6-183e34c966c475e0d2cdb3c60d0b7426499aa573
2020/11/05 09:35:05 Codename: beta
2020/11/05 09:35:05 BuildDate: 2020-11-04_17:56:46
2020/11/05 09:35:05 GoVersion: 1.13
2020/11/05 09:35:05 Constraint_parser: >= 1.0, < 2.0
2020/11/05 09:35:05 Constraint_scenario: >= 1.0, < 3.0
2020/11/05 09:35:05 Constraint_api: v1
2020/11/05 09:35:05 Constraint_acquis: >= 1.0, < 2.0

scope​

scope:
  type: Range
  expression: evt.Parsed.mySourceRange

While most scenarios might focus on IP addresses, CrowdSec and Bouncers can work with any scope. The scope directive allows you to override the default scope :

  • type is a string representing the scope name
  • expression is an expr expression that will be evaluated to fetch the value

let's imagine a scenario such as :

# ssh bruteforce
type: leaky
name: crowdsecurity/ssh-enforce-mfa
description: "Enforce mfa on users that have been bruteforced"
filter: "evt.Meta.log_type == 'ssh_failed-auth'"
leakspeed: "10s"
capacity: 5
groupby: evt.Meta.source_ip
blackhole: 1m
labels:
 service: ssh
 type: bruteforce
 remediation: true
scope:
 type: username
 expression: evt.Meta.target_user

and a profile such as :

name: enforce_mfa
filters:
 - 'Alert.Remediation == true && Alert.GetScope() == "username"'
decisions:
 - type: enforce_mfa
   scope: "username"
   duration: 1h
on_success: continue

the resulting overflow will be :

$ ./cscli -c dev.yaml decisions list
+----+----------+---------------+-------------------------------+-------------+---------+----+--------+------------------+
| ID |  SOURCE  |  SCOPE:VALUE  |            REASON             |   ACTION    | COUNTRY | AS | EVENTS |    EXPIRATION    |
+----+----------+---------------+-------------------------------+-------------+---------+----+--------+------------------+
|  2 | crowdsec | username:rura | crowdsecurity/ssh-enforce-mfa | enforce_mfa |         |    |      6 | 59m46.121840343s |
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