GHSA-vvpj-8cmc-gx39

Summary

pkgutil.resolve_name() is a Python stdlib function that resolves any "module:attribute" string to the corresponding Python object at runtime. By using pkgutil.resolve_name as the first REDUCE call in a pickle, an attacker can obtain a reference to ANY blocked function (e.g., os.system, builtins.exec, subprocess.call) without that function appearing in the pickle's opcodes. picklescan only sees pkgutil.resolve_name (which is not blocked) and misses the actual dangerous function entirely.

This defeats picklescan's entire blocklist concept — every single entry in _unsafe_globals can be bypassed.

Severity

Critical (CVSS 10.0) — Universal bypass of all blocklist entries. Any blocked function can be invoked.

Affected Versions

• picklescan <= 1.0.3 (all versions including latest)

Details

How It Works

A pickle file uses two chained REDUCE calls:

1. STACK_GLOBAL: push pkgutil.resolve_name
2. REDUCE: call resolve_name("os:system") → returns os.system function object
3. REDUCE: call the returned function("malicious command") → RCE

picklescan's opcode scanner sees:

• STACK_GLOBAL with module=pkgutil, name=resolve_name → NOT in blocklist → CLEAN
• The second REDUCE operates on a stack value (the return of the first call), not on a global import → invisible to scanner

The string "os:system" is just data (a SHORT_BINUNICODE argument to the first REDUCE) — picklescan does not analyze REDUCE arguments, only GLOBAL/INST/STACK_GLOBAL references.

Decompiled Pickle (what the data actually does)

from pkgutil import resolve_name
_var0 = resolve_name('os:system')          # Returns the actual os.system function
_var1 = _var0('malicious_command')          # Calls os.system('malicious_command')
result = _var1

Confirmed Bypass Targets

Every entry in picklescan's blocklist can be reached via resolve_name:

| Chain | Resolves To | Confirmed RCE | picklescan Result |...