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

Summary

# 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 ```python 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) ```python 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 ```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.