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 |...