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YAML

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.
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Exploitation
Fix