Add README explaining the PoC

README.md

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+---
+tags:
+- security-research
+- vulnerability-poc
+---
+
+# msgpack-numpy RCE PoC (CWE-502)
+
+**DO NOT LOAD THIS FILE IN AN UNCONTROLLED ENVIRONMENT**
+
+This repository contains a proof-of-concept `.msgpack` file that demonstrates arbitrary code execution via `msgpack_numpy.unpackb()`.
+
+## Vulnerability
+
+`msgpack_numpy.decode()` at line 99-100 calls `pickle.loads(obj[b'data'])` with zero validation when `obj[b'kind'] == b'O'`. An attacker who distributes a crafted `.msgpack` file causes any victim process that loads it with `msgpack_numpy.unpackb()` to execute arbitrary code.
+
+## Reproduction
+
+```python
+import msgpack_numpy
+import msgpack
+
+data = open("malicious.msgpack", "rb").read()
+msgpack_numpy.unpackb(data, raw=False)  # RCE triggers here
+# Creates /tmp/msgpack-rce-proof.txt with content "EXPLOITED"
+```
+
+## Affected
+- msgpack-numpy 0.4.8 (latest, all versions affected)
+- 842,000 monthly PyPI downloads
+- Unmaintained since July 2024