CVE-2026-16760 in fickling
Summary
by MITRE • 10/31/2026
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.
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Analysis
by VulDB Data Team • 10/31/2026
The vulnerability in Trail of Bits fickling version 0.1.12 stems from a fundamental flaw in its security assessment logic within the _process_import() method located in fickle.py. This function is responsible for determining which imports are considered safe during static analysis by adding them to a likely_safe_imports set if their source module exists in sys.stdlib_module_names and the specific name is not present in the UNSAFE_IMPORTS denylist. The critical technical flaw lies in the fact that this process does inspect what the imported name actually resolves to or analyze its behavioral characteristics. Instead, it relies entirely on a heuristic assumption that any callable originating from Python standard library modules is inherently safe. This approach ignores the reality that many standard library functions are designed for general-purpose use and can be weaponized if their inputs are attacker-controlled. Consequently, the analysis pass known as OvertlyBadEvals in analysis.py unconditionally skips flagging calls whose function names appear in likely_safe_imports, even when those calls would otherwise trigger security alerts due to dangerous operations such as code execution or file system modifications.
This flawed trust model allows malicious pickle payloads to bypass safety checks and achieve arbitrary code execution or unauthorized file writes through specific standard library modules that were not previously blacklisted. One demonstrated primitive involves dataclasses._create_fn, a function present in Python versions 3.7 through 3.12 which was removed in version 3.13. This function accepts an attacker-controlled function body and passes it directly to exec(), resulting in immediate arbitrary code execution within the context of the deserializing process. Another significant vector is mailcap.findmatch, available up to Python 3.12 before its removal in 3.13. This module internally invokes os.system() using a test command derived from an attacker-controlled caps dictionary, thereby enabling shell injection and arbitrary code execution. Additionally, on POSIX platforms where the readline module remains available, attackers can exploit readline.add_history() combined with readline.write_history_file(). By manipulating these functions, it is possible to write attacker-controlled content to arbitrary filesystem paths, leading to potential privilege escalation or system compromise depending on file permissions and subsequent usage of those files.
The operational impact of this vulnerability is severe because fickling.load() explicitly chains the check_safety() function into pickle.loads() as a primary security gate. When the flawed logic determines that an import originates from a trusted standard library module, it returns LIKELY_SAFE with zero findings for payloads that actually reach code execution or file write operations through these callable primitives. This false positive verdict causes the tool to proceed with deserialization and execution of the payload under the privileges of the victim process. The root cause is identified as an over-reliance on module origin as a proxy for callable safety, which fails to account for the specific behaviors of individual functions within those modules. It is important to note that this issue is distinct from CVE-2026-118938, which addresses weaknesses in dotted _qualname_ resolution; while both relate to trust assumptions, they are separate vulnerabilities with different exploitation mechanisms and remediation paths.
Mitigation strategies must address the underlying heuristic flaw rather than merely patching specific instances. As of October 30, 2026, the vendor has added dataclasses._create_fn, mailcap.findmatch, and readline-related functions to the UNSAFE_IMPORTS denylist on the master branch in commit d011709, pending a formal release. However, released versions through 0.1.12 remain vulnerable because they lack these specific blacklists. Even with this update, the broader architectural issue persists: any callable reachable through a non-blocked standard library module inherits module-level trust regardless of its actual behavior. This means that related vulnerabilities like CVE-2026-118938 may still be exploitable on patched builds if they rely on similar trust assumptions about function origins. To fully mitigate this class of vulnerability, the analysis engine should implement deeper semantic analysis or taint tracking for standard library functions rather than relying solely on static module whitelisting. Security practitioners should upgrade to versions that include the updated UNSAFE_IMPORTS list and remain vigilant against new primitives in future Python releases that may expose similar risks through other standard library modules not yet identified as dangerous.