Fickling check_safety() bypass: likely_safe_imports grants module-origin trust to unvalidated standard library callables

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-16760
StatePUBLISHED
AssignerBombadilSystems
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-30 22:16:54 UTC
Updated2026-10-30 22:16:54 UTC
DescriptionIn 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.

Risk And Classification

Primary CVSS: v3.1 8.8 HIGH from aa17e1a1-c329-4d6e-a1ed-8d0188aea082

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Problem Types: CWE-502 | CWE-693 | CWE-693 CWE-693 Protection Mechanism Failure | CWE-502 CWE-502 Deserialization of Untrusted Data

VersionSourceTypeScoreSeverityVector
3.1aa17e1a1-c329-4d6e-a1ed-8d0188aea082Secondary8.8HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
3.1CNACVSS8.8HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Trailofbits Fickling affected 0.1.12 custom Not specified

References

ReferenceSourceLinkTags
github.com/trailofbits/fickling aa17e1a1-c329-4d6e-a1ed-8d0188aea082 github.com
github.com/trailofbits/fickling/commit/d011709 aa17e1a1-c329-4d6e-a1ed-8d0188aea082 github.com
bombadil.systems/advisories.html aa17e1a1-c329-4d6e-a1ed-8d0188aea082 bombadil.systems
pypi.org/project/fickling/0.1.12 aa17e1a1-c329-4d6e-a1ed-8d0188aea082 pypi.org
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Christopher Aziz (Bombadil Systems LLC) (en)

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