A Falco rules file is a YAML file containing three types of elements:
|Rules||Conditions under which an alert should be generated. A rule is accompanied by a descriptive output string that is sent with the alert.|
|Macros||Rule condition snippets that can be re-used inside rules and even other macros. Macros provide a way to name common patterns and factor out redundancies in rules.|
|Lists||Collections of items that can be included in rules, macros, or other lists. Unlike rules and macros, lists cannot be parsed as filtering expressions.|
There are also two optional elements related to versioning:
|Used to track compatibility between rules content and the falco engine version.|
|Used to track compatibility between rules content and plugin versions.|
From time to time, we make changes to the rules file format that are not backwards-compatible with older versions of Falco. Similarly, libsinsp and libscap may define new filtercheck fields, operators, etc. We want to denote that a given set of rules depends on the fields/operators from those libraries.
As of Falco version 0.14.0, the Falco rules support explicit versioning of both the Falco engine and the Falco rules file.
falco executable and the
falco_engine C++ object now support returning a version number. The initial version is 2 (implying that prior versions were 1). We will increment this version any time we make an incompatible change to the rules file format or add new filtercheck fields/operators to Falco.
The Falco rules files included with Falco include a new top-level object,
required_engine_version: N, that specifies the minimum engine version required to read this rules file. If not included, no version check is performed when reading the rules file. Here's an example:
# This rules file requires a falco with falco engine version 7. - required_engine_version: 7
If a rules file has an
engine_version greater than the Falco engine version, the rules file is loaded and an error is returned.
A Falco rule is a node containing the following keys:
|yes||A short, unique name for the rule.|
|yes||A filtering expression that is applied against events to check whether they match the rule.|
|yes||A longer description of what the rule detects.|
|yes||Specifies the message that should be output if a matching event occurs. See output.|
|yes||A case-insensitive representation of the severity of the event. Should be one of the following: |
|no||A set of exceptions that cause the rule to not generate an alert.|
|no||If set to |
|no||A list of tags applied to the rule (more on this below).|
|no||If set to |
|no||If set to |
|no||The event source for which this rule should be evaluated. Typical values are |
The key part of a rule is the condition field. A condition is a Boolean predicate expressed using the condition syntax. It is possible to express conditions on all supported events using their respective supported fields.
Here's an example of a condition that alerts whenever a bash shell is run inside a container:
container.id != host and proc.name = bash
The first clause checks that the event happened in a container (where
container.id is equal to
"host" if the event happened on a regular host). The second clause checks that the process name is
bash. Since this condition does not include a clause with a system call it will only check event metadata. Because of that, if a bash shell does start up in a container, Falco outputs events for every syscall that is performed by that shell.
If you want to be alerted only for each successful spawn of a shell in a container, add the appropriate event type and direction to the condition:
evt.type = execve and evt.dir=< and container.id != host and proc.name = bash
Therefore, a complete rule using the above condition might be:
- rule: shell_in_container desc: notice shell activity within a container condition: evt.type = execve and evt.dir=< and container.id != host and proc.name = bash output: shell in a container (user=%user.name container_id=%container.id container_name=%container.name shell=%proc.name parent=%proc.pname cmdline=%proc.cmdline) priority: WARNING
Conditions allow you to check for many aspects of each supported event. To learn more, see the condition language.
As noted above, macros provide a way to define common sub-portions of rules in a reusable way. By looking at the condition above it looks like both
evt.type = execve and evt.dir=< and
container.id != host would be used many by other rules, so to make our job easier we can easily define macros for both:
- macro: container condition: container.id != host
- macro: spawned_process condition: evt.type = execve and evt.dir=<
With this macro defined, we can then rewrite the above rule's condition as
spawned_process and container and proc.name = bash.
For many more examples of rules and macros, take a look the documentation on default macros and the
rules/falco_rules.yaml file. In fact, both the macros above are part of the default list!
Lists are named collections of items that you can include in rules, macros, or even other lists. Please note that lists cannot be parsed as filtering expressions. Each list node has the following keys:
|The unique name for the list (as a slug)|
|The list of values|
Here are some example lists as well as a macro that uses them:
- list: shell_binaries items: [bash, csh, ksh, sh, tcsh, zsh, dash] - list: userexec_binaries items: [sudo, su] - list: known_binaries items: [shell_binaries, userexec_binaries] - macro: safe_procs condition: proc.name in (known_binaries)
Referring to a list inserts the list items in the macro, rule, or list.
Lists can contain other lists.
If you use multiple Falco rules files, you might want to append new items to an existing list, rule, or macro. To do that, define an item with the same name as an existing item and add an
append: true attribute to the list. When appending lists, items are added to the end of the list. When appending rules/macros, the additional text is appended to the condition: field of the rule/macro.
Note that when appending to lists, rules or macros, the order of the rule configuration files matters! For example if you append to an existing default rule (e.g.
Terminal shell in container), you must ensure your custom configuration file (e.g.
/etc/falco/rules.d/custom-rules.yaml) is loaded after the default configuration file (
/etc/falco/falco_rules.yaml). This can be configured with multiple
-r parameters in the right order, directly inside the falco configuration file (
rules_file or if you use the official Helm chart, via the
In all of the examples below, it's assumed one is running Falco via
falco -r /etc/falco/falco_rules.yaml -r /etc/falco/falco_rules.local.yaml, or has the default entries for
rules_file in falco.yaml, which has
/etc/falco/falco.yaml first and
Here's an example of appending to lists:
- list: my_programs items: [ls, cat, pwd] - rule: my_programs_opened_file desc: track whenever a set of programs opens a file condition: proc.name in (my_programs) and (evt.type=open or evt.type=openat) output: a tracked program opened a file (user=%user.name command=%proc.cmdline file=%fd.name) priority: INFO
- list: my_programs append: true items: [cp]
my_programs_opened_file would trigger whenever any of
cp opened a file.
Here's an example of appending to macros:
- macro: access_file condition: evt.type=open - rule: program_accesses_file desc: track whenever a set of programs opens a file condition: proc.name in (cat, ls) and (access_file) output: a tracked program opened a file (user=%user.name command=%proc.cmdline file=%fd.name) priority: INFO
- macro: access_file append: true condition: or evt.type=openat
program_accesses_file would trigger when
cat either used
openat on a file.
Here's an example of appending to rules:
- rule: program_accesses_file desc: track whenever a set of programs opens a file condition: proc.name in (cat, ls) and evt.type=open output: a tracked program opened a file (user=%user.name command=%proc.cmdline file=%fd.name) priority: INFO
- rule: program_accesses_file append: true condition: and not user.name=root
program_accesses_file would trigger when
cat either used
open on a file, but not if the user was root.
Remember that when appending rules and macros, the text of the second rule/macro is simply added to the condition of the first rule/macro. This can result in unintended results if the original rule/macro has potentially ambiguous logical operators. Here's an example:
- rule: my_rule desc: ... condition: evt.type=open and proc.name=apache output: ... - rule: my_rule append: true condition: or proc.name=nginx
proc.name=nginx be interpreted as relative to the
and proc.name=apache, that is to allow either apache/nginx to open files, or relative to the
evt.type=open, that is to allow apache to open files or to allow nginx to do anything?
In cases like this, be sure to scope the logical operators of the original condition with parentheses when possible, or avoid appending conditions when not possible.
Even though Falco provides a quite powerful default ruleset, you sometimes need to disable some of these default rules since they do not work properly in your environment. Luckily Falco offers you multiple possibilities to do so.
Most of the default rules offer some kind of
consider_* macros which are already part of the rule conditions. These
consider_* macros are usually set to
(always_true) which basically enables or disabled the regarding rule. Now if you want to enable a by default disabled rule (e.g.
Unexpected outbound connection destination), you just have to override the rule's
consider_* macro (
consider_all_outbound_conns in this case) inside your custom Falco configuration.
Example for your custom Falco configuration (note the
- macro: consider_all_outbound_conns condition: (always_true)
Please note again that the order of the specified configuration file matters! The last defined macro with the same name wins.
Falco offers the following parameters to limit which default rules should be enabled/used and which not:
-D <substring> Disable any rules with names having the substring <substring>. Can be specified multiple times. -T <tag> Disable any rules with a tag=<tag>. Can be specified multiple times. Can not be specified with -t. -t <tag> Only run those rules with a tag=<tag>. Can be specified multiple times. Can not be specified with -T/-D.
These parameters can also be specified as Helm chart value (
extraArgs) if you are deploying Falco via the official Helm chart.
Last but not the least, you can just disable a rule that is enabled by default using the
enabled: false rule property.
This is especially useful for rules which do not provide a
consider_* macro in the default condition.
Ensure that the custom configuration file loads after the default configuration file. You can configure the right order using multiple
-r parameters, directly inside the falco configuration file
rules_file. If you are using the official Helm chart, then configure the order with the
For example to disable the
User mgmt binaries default rule in
/etc/falco/falco_rules.yaml define a custom rule in
- rule: User mgmt binaries enabled: false
At the same time, disabled rules can be re-enabled by using the
enabled: true rule property. For instance, the
Change thread namespace rule in
/etc/falco/falco_rules.yaml that is disabled by default, can be manually enabled with:
- rule: Change thread namespace enabled: true
A rule output is a string that can use the same fields that conditions can use prepended by
% to perform interpolation, akin to
printf. For example:
Disallowed SSH Connection (command=%proc.cmdline connection=%fd.name user=%user.name user_loginuid=%user.loginuid container_id=%container.id image=%container.image.repository)
Disallowed SSH Connection (command=sshd connection=127.0.0.1:34705->10.0.0.120:22 user=root user_loginuid=-1 container_id=host image=<NA>)
Note that it's not necessary that all fields are set in the specific event. As you can see in the example above if the connection happens outside a container the field
%container.image.repository would not be set and
<NA> is displayed instead.
Every Falco rule has a priority which indicates how serious a violation of the rule is. The priority is included in the message/JSON output/etc. Here are the available priorities:
The general guidelines used to assign priorities to rules are the following:
One exception is that the rule "Run shell untrusted", which is fairly FP-prone, has a priority of
As of 0.6.0, rules have an optional set of tags that are used to categorize the ruleset into groups of related rules. Here's an example:
- rule: File Open by Privileged Container desc: Any open by a privileged container. Exceptions are made for known trusted images. condition: (open_read or open_write) and container and container.privileged=true and not trusted_containers output: File opened for read/write by privileged container (user=%user.name command=%proc.cmdline %container.info file=%fd.name) priority: WARNING tags: [container, cis]
In this case, the rule "File Open by Privileged Container" has been given the tags "container" and "cis". If the tags key is not present for a given rule or the list is empty, a rule has no tags.
Here's how you can use tags:
-T <tag>argument to disable rules having a given tag.
-Tcan be specified multiple times. For example, to skip all rules with the "filesystem" and "cis" tags you would run falco with
falco -T filesystem -T cis ....
-Tcan not be specified with
-t <tag>argument to only run those rules having a given tag.
-tcan be specified multiple times. For example, to only run those rules with the "filesystem" and "cis" tags, you would run falco with
falco -t filesystem -t cis ....
-tcan not be specified with
-D <pattern>(disable rules by rule name regex).
We've also gone through the default ruleset and tagged all the rules with an initial set of tags. Here are the tags we've used:
|The rule relates to reading/writing files|
|The rule relates to any software/package management tool like rpm, dpkg, etc.|
|The rule relates to starting a new process or changing the state of a current process|
|The rule relates to databases|
|The rule only works outside of containers|
|The rule specifically relates to starting shells|
|The rule only works inside containers|
|The rule is related to the CIS Docker benchmark|
|The rule relates to management of users or changing the identity of a running process|
|The rule relates to network activity|
Rules can have multiple tags if they relate to multiple of the above. Every rule in the falco ruleset currently has at least one tag.
To allow for grouping rules by event type, which improves performance, Falco prefers rule conditions that have at least one
evt.type= operator, at the beginning of the condition, before any negative operators (i.e.
!=). If a condition does not have any
evt.type= operator, Falco logs a warning like:
Rule no_evttype: warning (no-evttype): proc.name=foo did not contain any evt.type restriction, meaning that it will run for all event types. This has a significant performance penalty. Consider adding an evt.type restriction if possible.
If a rule has an
evt.type operator in the latter portion of the condition, Falco logs a warning like this:
Rule evttype_not_equals: warning (trailing-evttype): evt.type!=execve does not have all evt.type restrictions at the beginning of the condition, or uses a negative match (i.e. "not"/"!=") for some evt.type restriction. This has a performance penalty, as the rule can not be limited to specific event types. Consider moving all evt.type restrictions to the beginning of the rule and/or replacing negative matches with positive matches if possible.
In some cases, rules may need to contain special characters like
(, spaces, etc. For example, you may need to look for a
(systemd), including the surrounding parentheses.
You can use
" to capture these special characters. Here's an example:
- rule: Any Open Activity by Systemd desc: Detects all open events by systemd. condition: evt.type=open and proc.name="(systemd)" or proc.name=systemd output: "File opened by systemd (user=%user.name command=%proc.cmdline file=%fd.name)" priority: WARNING
When including items in lists, ensure that the double quotes are not interpreted from your YAML file by surrounding the quoted string with single quotes. Here's an example:
- list: systemd_procs items: [systemd, '"(systemd)"'] - rule: Any Open Activity by Systemd desc: Detects all open events by systemd. condition: evt.type=open and proc.name in (systemd_procs) output: "File opened by systemd (user=%user.name command=%proc.cmdline file=%fd.name)" priority: WARNING
For performance reasons, some system calls are currently discarded before Falco processes them. You can see the complete list by running falco with
-i. If you'd like to run Falco against all events, including system calls in the above list, you can run Falco with the
-A flag. For more information, see supported events.
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