msgpack-python CVE-2026-114613

Exploitable only when the application deserializes attacker-controlled MessagePack data using the compiled C extension (msgpack._cmsgpack), which is the default build; both unpackb() and the streaming Unpacker API are affected. The specific trigger is a MessagePack map whose key is neither str nor bytes while strict_map_key=True (the default): the key is rejected only after the map value has already been constructed, and the error path returns without releasing that value reference. Meaningful DoS requires many repeated malformed inputs to accumulate (~4 MiB leaked per failed parse, ~100 MiB after 25 per the reporter). Limiting factors: the pure-Python fallback is reported not to leak on this path and can serve as a temporary workaround, and request-size/rate limits reduce but do not eliminate cumulative leakage.

The input CVSS of 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) reflects a network-reachable, low-complexity, unauthenticated flaw, but the impact is availability-only (C:N/I:N) - there is no data disclosure or code execution, so despite the HIGH rating this is purely a denial-of-service issue. The signals are partly incomplete: no EPSS score is provided and the vulnerability is not listed in CISA KEV, so there is no confirmation of active or widespread exploitation; the only exploitation evidence is the reproduction snippet in the GHSA advisory (a public PoC reference, not verified as a weaponized exploit). The vuln.today Decision label is 'this_week', consistent with a credible but non-emergency DoS rather than an act_now/emergency item. Importantly, the DoS is cumulative, not instantaneous: each rejected parse leaks only ~4 MiB (reporter-measured, ~100 MiB after 25 failed parses), so exploitation requires sustained repeated malformed inputs rather than a single packet. Real-world risk is therefore moderate and concentrated in services that deserialize attacker-controlled MessagePack using the default C extension; it is meaningfully reduced where the pure-Python fallback is used (reported not to leak) or where request-size/rate limits are enforced. Net: a genuine but patchable DoS worth remediating this cycle, not a drop-everything emergency.

An attacker who can submit MessagePack to a service (for example an API endpoint that calls msgpack.unpackb() on request bodies using the default C extension) repeatedly sends maps whose key is neither str nor bytes, each of which is rejected by the default strict_map_key=True after the value has already been constructed, leaking roughly 4 MiB per request. Sustained malformed requests drive unbounded process memory growth until the service is killed or the host exhausts memory, causing denial of service. The GHSA advisory includes a reproduction snippet demonstrating the leak (a public PoC reference, not verified as working exploit code); the network/low-complexity/no-auth vector means no special access is needed beyond the ability to deliver msgpack input.