Exploitation

The example below is a vulnerable YAML code that can be exploited.

Python 2

import yaml
​
# Input can be whater text or a file with this content
yaml.load(input)

Python 3

In Python 3, the default loader changed to a safe Loader, and to exploit this vulnerability should be enable UnsafeLoader explicitly.

import yaml
​
# Input can be whater text or a file with this content
yaml.load(input, Loader=yaml.UnsafeLoader)

Example payload to exploit this vulnerability in a file sample.yaml os a direct input if allowed.

!!python/object/apply:os.system ["cat /etc/passwd"]

Fix

Fixing this vulnerability is relatively easy.

Replace the usage of yaml.load() function with yaml.safe_load()

In Python 3, yaml.load() uses as default data Loader FullLoader which avoids code execution.