Quick answers
- Is This vulnerability exploited in the wild?
- No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
- How severe is This vulnerability?
- Medium severity, with a CVSS score of 5.3, published 29 May 2026.
- Which products and versions are affected?
- Andrew Arnott Nerdbank.MessagePack < 1.1.78.
- Is there a fix for This vulnerability?
- A patch was published on 2026-05-29: Version 1.1.78 released Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is This vulnerability attacked?
- It is attacked over the network, in a software library.
- Am I affected?
- Check whether you run Andrew Arnott Nerdbank.MessagePack at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
The vulnerability is a memory amplification DoS caused by untrusted input in collection deserializers. Multiple converters (including ArrayConverter, MutableDictionaryConverter, and JsonNodeConverter) trust the element count declared in MessagePack array and map headers to pre-allocate destination storage or rent buffers before consuming the actual elements. An attacker can provide a small payload (e.g., 1-2 bytes per element) that triggers a much larger managed heap allocation (e.g., 8 bytes per reference plus overhead). This can be exploited via any network endpoint (ASP.NET Core, SignalR, RPC) that accepts untrusted MessagePack data. The issue is fixed in version 1.1.78 by limiting pre-allocation and growing collections as data is encountered.
Affected products
- AArnott Nerdbank.MessagePack < 1.1.78
Timeline
- 2026-05-18: disclosed: Initial disclosure date
- 2026-05-29: advisory: Last updated date
- 2026-05-29: patched: Version 1.1.78 released