---
id: CVE-2026-16760
title: >-
  Fickling check_safety() bypass: likely_safe_imports grants module-origin trust
  to unvalidated standard library callables
summary: >-
  In Trail of Bits fickling through 0.1.12, the _process_import() method in
  fickle.py adds an imported name to the likely_safe_imports set whenever the
  source module is present in sys.stdlib_module_names and the name is absent
  from the UNS…
severity: high
cvss: 8.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-693
  - CWE-502
vendor: trailofbits
product: fickling
affected:
  - fickling <= 0.1.12
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-11-06T16:00:50.929044Z'
published: '2026-10-30'
updated: '2026-11-06'
sourceUpdated: '2026-11-06T16:01:12.427Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-16760'
references:
  - url: 'https://bombadil.systems/advisories.html'
    label: Bombadil Systems advisory listing
  - url: 'https://pypi.org/project/fickling/0.1.12/'
    label: fickling 0.1.12 on PyPI
  - url: 'https://github.com/trailofbits/fickling'
    label: fickling source repository
  - url: 'https://github.com/trailofbits/fickling/commit/d011709'
    label: >-
      Vendor remediation merge commit d011709 (adds readline, dataclasses, and
      mailcap to UNSAFE_IMPORTS on master; the module-origin trust root cause
      remains)
tags:
  - cve.org
epss: 0.00455
epssPercentile: 0.39693
ingestedAt: '2026-11-05T16:24:31.466Z'
---

## Overview

In Trail of Bits fickling through 0.1.12, the _process_import() method in fickle.py adds an imported name to the likely_safe_imports set whenever the source module is present in sys.stdlib_module_names and the name is absent from the UNSAFE_IMPORTS denylist, without inspecting what the name resolves to or how it behaves. The OvertlyBadEvals analysis pass in analysis.py then unconditionally skips any call whose function name appears in likely_safe_imports, including calls that would otherwise be flagged as overtly dangerous. As a result, any callable reachable through a non-blocked standard library module inherits module-level trust regardless of its own behavior, and check_safety() returns LIKELY_SAFE with zero findings for pickle payloads that reach code execution or file write through such callables. Demonstrated primitives: dataclasses._create_fn, which passes an attacker-controlled function body to exec() and yields arbitrary code execution on Python 3.7 through 3.12 (the internal _create_fn function was removed in Python 3.13); mailcap.findmatch, which internally calls os.system() on a test command taken from an attacker-controlled caps dictionary, yielding arbitrary code execution on Python versions through 3.12 (the mailcap module was removed in Python 3.13); and readline.add_history() followed by readline.write_history_file(), which writes attacker-controlled content to an arbitrary filesystem path (POSIX platforms, where the readline module is available). fickling.load() chains check_safety() into pickle.loads() as an explicit security gate, so a LIKELY_SAFE verdict causes the payload to be deserialized and executed with the privileges of the victim process. The root cause is the inference of callable safety from module origin; it is independent of, and separately fixable from, the dotted __qualname__ resolution weakness tracked as CVE-2026-118938, which this advisory does not address. As of 2026-10-30 the three demonstrated modules were added to UNSAFE_IMPORTS on the vendor master branch (commit d011709) pending release; released versions through 0.1.12 remain affected, and the module-origin trust root cause persists, as demonstrated by the related CVE-2026-118938 remaining exploitable on that same build.

## Affected

- `fickling <= 0.1.12`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.