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The vulnerability arises when applications deserialize untrusted pickle data without proper safeguards. Attackers can craft pickle payloads that instantiate standard library classes like logging.FileHandler to bypass common blocklists aimed at preventing remote code execution (RCE). The logging.FileHandler class, when deserialized, opens a file in append mode via its init method. If the specified file path does not exist, the handler creates a new zero-byte file at that location. This behavior allows unauthenticated attackers to create empty files arbitrarily on the filesystem. The exploit leverages the reduce method in a custom class to return a tuple specifying FileHandler and its arguments, such as (‘evil.log’,). When pickle.loads processes this payload, it reconstructs the FileHandler object, triggering file creation. This method sidesteps defenses that only filter for RCE keywords like os.system or subprocess.Popen. The file creation is limited to new files; existing files are opened without modification, preventing data overwrite. However, this primitive enables filesystem pollution—attackers can create lock files (e.g., .lock, maintenance.lock) that application logic may rely on, potentially causing denial-of-service (DoS) or disrupting startup sequences. The vulnerability requires the application to deserialize pickle from untrusted sources, such as network inputs or user-supplied data. Mitigation involves avoiding pickle for untrusted data or implementing strict allowlists for deserializable classes.
Platform: Python
Version: Not specified
Vulnerability: Unsafe Pickle Deserialization
Severity: High
Date: Not specified
Prediction: No patch ETA
What Undercode Say:
Analytics:
python3 -c "import pickle; payload = pickle.dumps(<strong>import</strong>('logging').FileHandler('evil.log')); open('exploit.pkl','wb').write(payload)"
import pickle
class Exploit:
def <strong>reduce</strong>(self):
from logging import FileHandler
return (FileHandler, ('evil.log',))
pickle.loads(pickle.dumps(Exploit()))
How Exploit:
Craft pickle payload with FileHandler. Deserialize payload on target. Creates specified empty file.
Protection from this CVE:
Avoid unpickling untrusted data. Use JSON or XML. Implement class allowlists.
Impact:
Arbitrary file creation. Filesystem pollution. Potential DoS.
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Sources:
Reported By: github.com
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