YAML
Yet Another Markup Language
Description
First of all, this great article explains all sorts of tricks and weirdness in YAML:
Insecure Deserialization
In YAML the !
character can mean a tag, which allows you to execute a function in the host language with a parameter that comes right after (because why not). Many parsers implement this as it is required by the spec, but if attackers have control over the YAML file, even partially, they can use these tags to run arbitrary functions with arbitrary arguments.
A common target for this is a function that executes shell commands, where you can gain Remote Code Execution. The following examples all execute the id
command and allow you to execute any arbitrary commands:
Ruby
Payload (>2.7, source)
Python
Payload
JavaScript - js-yaml
(<4.0)
js-yaml
(<4.0)This popular JavaScript library allows the creation of arbitrary functions like .toString()
which can be called accidentally, when using load()
instead of safeLoad()
in versions below 4:
Payloads
Java - SnakeYAML (<2.0)
Payload
/payload.jar
file:
Explanation (search "remote jar file")
Proof of Concept with
build.sh
script (changeexec()
)
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