PickleScan's pkgutil.resolve_name has a universal blocklist bypass
Date: 2026-03
Affected: pip/picklescan
Attack vector: deserialization
Category: vulnerability-disclosure
Severity: Critical
CWE: CWE-183
CWE-693
CVSS: 10 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H)

## 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


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