msgpack-python EUVD-2026-40423

The specific precondition is that the application must reuse the same Unpacker object after an unpacking error was raised and caught - i.e., catch-and-continue decoding on a shared or long-lived Unpacker (typical of streaming/persistent-connection designs). The vulnerability is limited to that usage pattern: applications that create a new Unpacker per message, or that abort/discard the Unpacker on any error, are not exposed. The CVSS vector is unauthenticated and needs no user interaction (PR:N/UI:N), but the impact is confined to a crash (A:H only, C:N/I:N), and exploitation depends on reaching code that feeds attacker-controlled bytes into the reused Unpacker. Affected only on msgpack-python versions before 1.2.1.

The CVSS 3.1 vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H yields 7.5 and isolates the impact to availability - no confidentiality or integrity loss - consistent with a crash/DoS rather than RCE. The network vector and lack of required privileges or user interaction make it easy to trigger in principle, but real-world risk is gated by an application-side precondition: the program must reuse the same Unpacker object after catching a decode error, which is a coding pattern rather than a default-on feature, arguably warranting higher attack complexity than AC:L suggests. There is no KEV listing and no POC referenced, and no EPSS value was supplied, so there is no evidence of active or widespread exploitation. Net: a credible, easy-to-trigger DoS for services that reuse Unpackers on long-lived connections, but lower priority than a CVSS-7.5 might imply for deployments that create a fresh Unpacker per message or do not catch-and-continue.

An attacker connected to a network service that streams MessagePack over a persistent connection sends a deliberately malformed payload to trigger a caught decode error, then sends additional bytes to the same long-lived Unpacker. The subsequent out-of-bounds read crashes the worker process with a SEGV, denying service; repeating this against a shared/reused Unpacker keeps the service down. No public POC is referenced in the available data.