A malformed MessagePack payload can cause out‑of‑bounds reads in the LZ4 decompression path of MessagePack-CSharp. The vulnerable implementation ignores the source‑length bound, allowing a remote attacker to craft token/length fields that trigger an AccessViolationException, which terminates the process. Under certain circumstances a brief unintended memory disclosure is also possible before failure.

The vulnerability exists in MessagePack-CSharp versions prior to 2.5.301 and 3.1.7. Applications that reference the MessagePack serializer for C# and enable Lz4Block or Lz4BlockArray compression modes are susceptible.

With a CVSS score of 8.2 the vulnerability is considered high severity. The EPSS score is not available and it is not listed in the CISA KEV catalog. A remote attacker can inject a crafted MessagePack payload over any network channel that the application accepts. Because the attack relies on deserialization of untrusted data, the attack vector is network‑based. Successful exploitation results in process termination and possible limited memory disclosure, making it a valuable tool for denial of service attacks against services using the affected library.