{"id":"CVE-2026-16760","state":"PUBLISHED","title":"Fickling check_safety() bypass: likely_safe_imports grants module-origin trust to unvalidated standard library callables","description":"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.","publishedAt":"2026-10-30T21:16:31.353Z","updatedAt":"2026-10-30T21:16:31.353Z","severity":{"score":8.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H","source":"BombadilSystems"},"assigner":"BombadilSystems","products":["trailofbits fickling"],"affected":[{"vendor":"trailofbits","product":"fickling","packageName":"fickling","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThanOrEqual":"0.1.12","versionType":"custom"}],"collectionURL":"https://pypi.org/project/fickling/"}],"references":["https://bombadil.systems/advisories.html","https://pypi.org/project/fickling/0.1.12/","https://github.com/trailofbits/fickling","https://github.com/trailofbits/fickling/commit/d011709"],"weaknesses":["CWE-693","CWE-502"],"ingestedAt":"2026-10-30T21:20:06.288Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/16xxx/CVE-2026-16760.json","commit":"8b3b8e28cf7db225d80596c8efe0ae24249fe3c9","baselineCommit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-16760","state":"PUBLISHED","datePublished":"2026-10-30T21:16:31.353Z","dateUpdated":"2026-10-30T21:16:31.353Z","assignerShortName":"BombadilSystems","assignerOrgId":"aa17e1a1-c329-4d6e-a1ed-8d0188aea082","dateReserved":"2026-07-23T14:00:36.878Z"},"containers":{"cna":{"title":"Fickling check_safety() bypass: likely_safe_imports grants module-origin trust to unvalidated standard library callables","descriptions":[{"lang":"en","value":"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":[{"vendor":"trailofbits","product":"fickling","packageName":"fickling","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThanOrEqual":"0.1.12","versionType":"custom"}],"collectionURL":"https://pypi.org/project/fickling/"}],"references":[{"url":"https://bombadil.systems/advisories.html","name":"Bombadil Systems advisory listing"},{"url":"https://pypi.org/project/fickling/0.1.12/","name":"fickling 0.1.12 on PyPI"},{"url":"https://github.com/trailofbits/fickling","name":"fickling source repository"},{"url":"https://github.com/trailofbits/fickling/commit/d011709","name":"Vendor remediation merge commit d011709 (adds readline, dataclasses, and mailcap to UNSAFE_IMPORTS on master; the module-origin trust root cause remains)"}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV3_1":{"version":"3.1","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"REQUIRED","scope":"UNCHANGED","confidentialityImpact":"HIGH","integrityImpact":"HIGH","availabilityImpact":"HIGH","baseSeverity":"HIGH","baseScore":8.8,"vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-693","lang":"en","description":"CWE-693 Protection Mechanism Failure","type":"CWE"}]},{"descriptions":[{"cweId":"CWE-502","lang":"en","description":"CWE-502 Deserialization of Untrusted Data","type":"CWE"}]}],"providerMetadata":{"shortName":"BombadilSystems","orgId":"aa17e1a1-c329-4d6e-a1ed-8d0188aea082","dateUpdated":"2026-10-30T21:16:31.353Z"},"impacts":[{"descriptions":[{"lang":"en","value":"An attacker can craft a malicious pickle that invokes a dangerous callable imported from a standard library module not present in UNSAFE_IMPORTS. The name is added to likely_safe_imports by _process_import(), causing OvertlyBadEvals to skip the call, so check_safety() returns LIKELY_SAFE and fickling.load() proceeds to deserialize the payload. This yields arbitrary code execution via dataclasses._create_fn or mailcap.findmatch on Python versions through 3.12, and arbitrary file write with attacker-controlled content via readline.write_history_file on POSIX platforms across supported Python versions. Because the flaw is in the module-origin trust model rather than in any specific module list, adding the demonstrated modules to UNSAFE_IMPORTS does not address the root cause; additional dangerous callables remain reachable through other non-blocked standard library modules."}]}],"credits":[{"lang":"en","value":"Christopher Aziz (Bombadil Systems LLC)","type":"finder"}],"source":{"discovery":"UNKNOWN"},"x_generator":{"engine":"Vulnogram 1.0.5"}}}}}