picklescan-marshal-functiontype-bypass-poc

PoC — picklescan blocklist evasion via marshal.loads + types.FunctionType

Proof-of-concept accompanying a huntr Model File Vulnerabilities (MFV) report against picklescan 1.0.5.

This is a security research artifact. The payload is deliberately harmless: it writes a local file named cyfra_marker containing EXEC_OK. It does not touch the network, delete anything, or write outside the working directory.

The issue

picklescan blocks types.CodeType ("can construct arbitrary code objects") but:

1. marshal is absent from the blocklist entirely — and marshal.loads(bytes) returns a code object, so the CodeType entry is bypassable by design.
2. types.FunctionType is not blocked, so that code object can be turned into a callable and invoked through a second REDUCE.

Result: a pickle that executes arbitrary code on a plain pickle.load() is reported as merely suspicious, with Infected files: 0 and Dangerous globals: 0.

Reproduce

pip install picklescan==1.0.5
python generate_poc.py

python -m picklescan.cli --path chain.pkl
#   Scanned files: 1
#   Infected files: 0
#   Suspicious globals: 2
#   Dangerous globals: 0      <-- not flagged as dangerous

python -c "import pickle; pickle.load(open('chain.pkl','rb'))"
cat cyfra_marker              # EXEC_OK  -> arbitrary code executed

No custom loader, no container trick, no attacker-controlled prerequisite: the standard library loader runs it. For comparison, fickling 0.1.12 flags the identical file as LIKELY_OVERTLY_MALICIOUS, showing detection is feasible.

Suggested fix

• Add marshal (loads/load) to _unsafe_globals as dangerous.
• Add types.FunctionType (and consider types.MethodType) alongside CodeType.

Attribution

Cyfra Tech Solutions (Roman Arce Bran), Costa Rica.