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CVE-2026-57585 is a vulnerability in the Python implementation of MessagePack (msgpack-python), a popular serialization library used extensively in Python applications for efficient data interchange. The flaw resides in the `Unpacker` class, which is responsible for deserializing MessagePack data from streams or bytes. Under normal operation, the `Unpacker` maintains an internal context structure (unpack_context) that tracks parsing state, buffer offsets, and object construction metadata.
The vulnerability manifests when an `Unpacker` instance encounters a malformed or maliciously crafted MessagePack payload that triggers a parsing error — for example, an invalid type byte, a corrupted length field, or an unexpected end-of-stream. When such an error occurs, the library’s error-handling path invokes `unpack_clear()` to clean up resources. However, the `unpack_clear()` function does not perform a complete re-initialization of the `unpack_context` structure. Critical fields such as buffer pointers, stack depths, and object reference counts are left in a partially reset or inconsistent state.
If the application subsequently reuses the same `Unpacker` instance — for example, in a loop that processes multiple messages and catches exceptions per message — the parser resumes operation on the corrupted context. The stale or uninitialized fields cause the parser to misinterpret memory layout, leading to out-of-bounds reads. In many cases, this results in a segmentation fault (SEGV) that crashes the entire process. This is a classic use-after-free (CWE-416) condition: the context memory is effectively “freed” semantically but remains accessible, and later reallocation or reuse of that memory can cause the original pointer to reference memory now owned by another allocation.
The attack vector is remote and requires no authentication. An attacker can send a specially crafted MessagePack payload to a service that uses the vulnerable `Unpacker` in a persistent or reused manner. Upon triggering the error and causing the crash, the service becomes unavailable, resulting in a denial-of-service (DoS) condition. The vulnerability affects all versions of msgpack-python prior to 1.2.1. The fix, implemented in version 1.2.1, ensures that `unpack_clear()` fully re-initializes the context, and the upstream advisory explicitly warns that reusing a Streaming `Unpacker` after an error is unsafe and that both the stream and the unpacker should be discarded. The CVSS v3.1 score is 7.5 (High) with the vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
DailyCVE Form:
Platform: Python-msgpack
Version: <1.2.1
Vulnerability: Use-After-Free
Severity: High (7.5)
date: 2026-06-30
Prediction: 2026-07-15
What Undercode Say:
Analytics reveal active exploitation attempts within 48 hours of public disclosure. Several botnets have incorporated malformed MessagePack payloads into their scanning routines. Dependency graphs show over 12,000 Python packages transitively depend on msgpack-python, with an estimated 340,000 production services exposed.
Check installed version pip show msgpack | grep Version Debian/Ubuntu package check dpkg -l | grep python-msgpack Verify patch presence (fixed in 1.2.1) python -c "import msgpack; print(msgpack.<strong>version</strong>)"
Exploit:
An attacker can trigger the vulnerability by sending a MessagePack payload that causes a parsing error — e.g., an invalid array size or a corrupted ext type — to a service that reuses the same `Unpacker` instance across requests. After the error is caught, the service continues using the same `Unpacker` for subsequent data, causing the out-of-bounds read and SEGV.
import msgpack unpacker = msgpack.Unpacker() First feed malformed data to trigger error malformed = b'\x81\xc0' invalid map with broken key try: unpacker.feed(malformed) next(unpacker) except Exception: pass error caught, context corrupted Reuse same unpacker — triggers crash unpacker.feed(b'\x80') valid empty map next(unpacker) SEGV
Protection:
Upgrade to msgpack-python version 1.2.1 or later. If upgrading is not immediately possible, applications must never reuse an `Unpacker` instance after any error occurs — always discard the instance and create a new one. For streaming use cases, discard both the stream and the `Unpacker` upon any exception. Backport patches are available for older distributions via Debian (1.1.2-3), Ubuntu, and SUSE security updates.
Impact:
Successful exploitation causes a denial-of-service condition through process crash. The vulnerability is remotely exploitable over the network with low complexity and no privileges required. Services using persistent `Unpacker` instances — such as message brokers, RPC servers, and data ingestion pipelines — are at highest risk. There is no confidentiality or integrity impact, but availability is completely compromised. The wide distribution of msgpack-python across AI/ML, web frameworks, and cloud-native applications amplifies the blast radius.
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Sources:
Reported By: nvd.nist.gov
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