msgpack5 CVE-2026-107299

The affected application must use the msgpack5 streaming decoder (the Decoder/streaming decode path) on input controlled by a remote peer, and the attacker must be able to open or write to that stream starting with the reserved byte 0xc1. Exploitation is limited to deployments that feed untrusted, attacker-controlled MessagePack streams into the decoder; applications that only decode trusted or fully-buffered/validated input, or that enforce stream byte/time limits (an explicit documented workaround), are not realistically affected. No authentication or user interaction is required per the vector (PR:N/UI:N), but reaching the decoder's streaming input is the key prerequisite.

This is a moderate-severity availability issue (CVSS 5.9, AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H) confirmed by the GHSA-26wq-p25c-j6fv advisory and vendor patch. The streaming decoder treats the reserved MessagePack byte 0xc1 as incomplete rather than invalid input, so a remote peer can begin a stream with 0xc1 and then keep sending data that is buffered indefinitely, exhausting memory (CWE-228, improper handling of syntactically invalid input). No EPSS score, CISA KEV listing, or public POC is present in the supplied data, so this is not confirmed as actively exploited and there is no public exploit identified at time of analysis. The vector is network-reachable and unauthenticated (PR:N), which raises concern, but impact is limited strictly to availability (C:N/I:N) - there is no code execution or data exposure. One signal conflict exists: the input Tags label this 'Information Disclosure', which directly contradicts the CVSS vector (C:N, A:H) and the advisory text describing memory exhaustion - this is a memory-exhaustion denial of service, not information disclosure. Overall this is a real-but-bounded DoS priority: patch in internet-facing services that run the msgpack5 streaming decoder over untrusted input, but it does not rise to the urgency of an RCE.

A service uses the msgpack5 streaming decoder to parse MessagePack data arriving over a network socket from untrusted clients. A remote, unauthenticated attacker opens a connection and sends the reserved byte 0xc1 followed by a continuous stream of bytes; the decoder keeps buffering all of it while waiting for a value that can never become valid, driving process memory upward until the service is exhausted. No public POC is identified, but the trigger byte and streaming behavior are straightforward to reproduce from the advisory and patch.