GHSA-r48f-3986-4f9c: fickling modules linecache, difflib and gc are missing from the unsafe modules blocklist
Our analysisAs stated in the [project's security policy](https://github.com/trailofbits/fickling/security/policy), we also don't consider `UnusedVariables` bypasses to be security issues. We added several unsafe modules mentioned by the reporter in advisory comments to the blocklist (https://github.com/trailofbits/fickling/commit/7f39d97258217ee2c21a1f5031d4a6d7343eb30d).Original reportTitle: UnusedVariables analysis bypass via BUILD opcode Arbitrary File Read through fickling.load()SummaryTwo independent bugs in fickling's AST-based static analysis combine to allow a malicious pickle file to execute arbitrary stdlib function calls - including reading sensitive files - while check_safety() returns Severity.LIKELY_SAFE and fickling.load() completes without raising UnsafeFileError.A server using fickling.load() as a security gate before deserializing untrusted pickle data (its documented use case) is fully bypassed. The attacker receives the contents of any file readable by the server process as the return value of fickling.load().DetailsInterpreter.unused_assignments() does not scan the result assignment's RHSFile: fickling/fickle.py, Interpreter.unused_assignments(), ~line 1242When the loop reaches result = _varN, it breaks immediately. The right-hand side of the result assignment is never walked for variable references. Any variable whose only reference is inside the result expression is therefore never added to the used set and is incorrectly flagged as unused - unless it also appears in an earlier non-assignment statement.The BUILD opcode generates exactly such a non-assignment statement:This makes _var2 (the result of the dangerous call) appear in the used set via the __setstate__ expression, so UnusedVariables never flags it.File: fickling/fickle.py, TupleThree.run(), and siblingsPython's ast module requires repeated fields (such as Tuple.elts) to be lists. When elts is a Python tuple, ast.iter_child_nodes() does not yield its elements, so ast.walk() never descends into them. Any variable reference stored inside such a tuple node is invisible to every analysis that uses ast.walk() - including UnusedVariables.Demo:Combined attack - arbitrary file readThe two bugs combine with the absence of `linecache` and `difflib` from `UNSAFE_IMPORTS`:check_safety() returns Severity.LIKELY_SAFE. fickling.load() calls pickle.loads(). At runtime, Differ().__dict__.update({0: 'root:x:0:0\n', ...}) succeeds and the file contents are returned to the caller.PoC`pip install fickling`ResultImpactVulnerability type: Static analysis bypass leading to arbitrary file read (and arbitrary stdlib code execution) through a security-gated deserialization API.Who is impacted: Any application or service that calls fickling.load() or fickling.loads() to validate untrusted pickle data before deserializing it. This is the primary documented use case of the fickling.loader module. The attacker supplies a pickle file; the server processes it through fickling.load(), receives LIKELY_SAFE, and unpickles the payload. File contents are returned directly in the deserialized object's attributes.Beyond file read, the same BUILD-opcode technique can be applied to any stdlib module absent from UNSAFE_IMPORTS (e.g., gc.get_objects() for full in-process memory inspection, inspect.stack() for call-frame local variable exfiltration, netrc.netrc() for credential theft).
CVSS v4.0: 6.9