yibai777/pytorch-audit

PyTorch Export Serialize Fallback RCE (Incomplete Fix Bypass)

Format: Torch Export (.pt2) – PyTorch
Project: PyTorch (pytorch/pytorch)
Version: 2.12.0
Severity: Critical – RCE on model load

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CVE Context

• CVE-2024-42480 (PyTorch < 2.5.1) was assigned to this vulnerability.
• The "fix" in 2.5.1 changed the first torch.load() attempt to use weights_only=True, but the except fallback still uses weights_only=False.
• This is an incomplete fix: an attacker can craft a payload that fails the weights_only=True attempt (e.g., using blocked globals) and the fallback loads it unsafely.
• Vulnerability is still present in PyTorch 2.12.0.

Description

deserialize_torch_artifact() in torch/_export/serde/serialize.py:422-445 first tries weights_only=True, and on any exception falls back to weights_only=False:

try:
    artifact = torch.load(buffer, weights_only=True)   # line 437
except Exception as e:
    buffer.seek(0)
    artifact = torch.load(buffer, weights_only=False)  # line 440 – FALLBACK!

An attacker crafts data/sample_inputs/model.pt in a .pt2 archive that references os.system (blocklisted). The weights_only=True path fails → falls back to weights_only=False → standard pickle.load() executes the payload.

Key Detail

When weights_only=False, torch.load() calls _legacy_load() which does pickle_module.load(f) at serialization.py:1887 — this is a full unpickling operation that executes the malicious pickle before checking the magic number.

Impact

Any user calling torch.export.load("malicious.pt2") gets RCE. This is a separate code path from the weights loading RCE in _package.py:877.

Steps to Reproduce

python poc_serialize_fallback.py --cmd "echo PWNED" --test-load

Output:

[!] weights_only=True → Exception → weights_only=False
[!] Post-exploitation error: RuntimeError: ...

But the command already executed (check PWNED output).

How It Works

1. Create a valid PT2 archive
2. Replace data/sample_inputs/model.pt with pickle using GLOBAL os system
3. torch.export.load() → deserialize_torch_artifact() → try weights_only=True → blocked
4. Fallback: weights_only=False → _legacy_load → pickle.load() → RCE

Files

File	Purpose
poc_serialize_fallback.py	Exploit PoC – generate & test malicious .pt2

Reference

• torch/_export/serde/serialize.py:437-440 – Fallback to weights_only=False
• torch/serialization.py:1887 – pickle_module.load() executes before magic check