# CVE-2026-48511: MessagePack for C# denial of service in ExpandoObjectFormatter

Source: https://junglewise.ai/threats/cve-2026-48511-messagepack-for-c-denial-of-service-in-expandoobjectformatter  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-22. Updated: 2026-06-25  
License: CC BY 4.0 with a link back

CVE-2026-48511 · Severity: high · CVSS 7.5 · Published 2026-06-22

MessagePack-CSharp is a library used to serialize and deserialize data for high-performance communication. A vulnerability in how it handles dynamic objects allows an attacker to send a specially crafted message that consumes excessive CPU and memory. This can lead to a denial-of-service (DoS) condition, making the affected server or application unresponsive to legitimate users.

Technologies: [MessagePack-CSharp MessagePack for C#](https://junglewise.ai/threats/technologies/messagepack-for-c), [MessagePack-CSharp](https://junglewise.ai/threats/technologies/messagepack-csharp), [MessagePack (NuGet)](https://junglewise.ai/threats/technologies/messagepack). Vendors: [MessagePack-CSharp](https://junglewise.ai/threats/vendors/messagepack-csharp), [NuGet](https://junglewise.ai/threats/vendors/nuget).

## Quick answers

**Is CVE-2026-48511 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-48511 being exploited in the wild.

**How severe is CVE-2026-48511?** High severity, with a CVSS score of 7.5, published 22 June 2026.

**Which products and versions are affected?** MessagePack-CSharp MessagePack for C# < 2.5.301, >= 3.0 < 3.1.7; MessagePack-CSharp < 2.5.301, >= 3.0, < 3.1.7; MessagePack (NuGet) ECOSYSTEM:introduced=0,fixed=2.5.301.

**Is there a fix for CVE-2026-48511?** Yes. MessagePack (NuGet): upgrade to 2.5.301 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-48511 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-48511?** Check whether you run MessagePack-CSharp MessagePack for C#, MessagePack-CSharp or MessagePack (NuGet) 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.

## How to fix

- MessagePack (NuGet): upgrade to 2.5.301 or later

  ```sh
  dotnet add package MessagePack --version 2.5.301
  ```


## Technical details

The vulnerability is classified as CWE-407 (Inefficient Algorithmic Complexity). The ExpandoObjectFormatter.Deserialize method populates System.Dynamic.ExpandoObject by calling IDictionary.Add for each map entry. Because ExpandoObject internally uses array-like structures for member names, inserting many distinct keys results in repeated linear scans and array copies, leading to quadratic (O(n^2)) CPU and memory consumption. This occurs even when MessagePackSecurity.UntrustedData is enabled, as standard collision-resistant comparers do not mitigate ExpandoObject's internal insertion logic. Attackers can exploit this via the network without authentication by providing a large map with many distinct keys. Patches are available in versions 2.5.301 and 3.1.7.

## Affected products

- MessagePack-CSharp MessagePack-CSharp < 2.5.301, >= 3.0, < 3.1.7

## Identifiers

- CVE-2026-48511
- GHSA-2x83-8g95-xh59

## Timeline

- 2026-06-09: disclosed
- 2026-06-22: advisory: NVD publication
- 2026-06-25: patched: GitHub Advisory reviewed and published

## References

- [https://api.github.com/users/AArnott](https://api.github.com/users/AArnott)
- [https://github.com/AArnott](https://github.com/AArnott)
- [https://api.github.com/users/AArnott/gists%7B/gist_id%7D](https://api.github.com/users/AArnott/gists%7B/gist_id%7D)
- [https://api.github.com/users/AArnott/repos](https://api.github.com/users/AArnott/repos)
- [https://avatars.githubusercontent.com/u/3548?v=4](https://avatars.githubusercontent.com/u/3548?v=4)
- [https://api.github.com/users/AArnott/events%7B/privacy%7D](https://api.github.com/users/AArnott/events%7B/privacy%7D)

## Related threats

- [MessagePack-CSharp memory exhaustion in nested array deserialization](https://junglewise.ai/threats/cve-2026-92707-messagepack-nested-arrays-can-blow-past-buffer-length-limits)
- [MessagePack for C# security bypass in typeless deserialization](https://junglewise.ai/threats/cve-2026-48517-messagepack-for-c-security-bypass-in-typeless-deserialization)
- [MessagePack for C# hash-collision DoS in InterfaceLookupFormatter](https://junglewise.ai/threats/cve-2026-48516-messagepack-for-c-hash-collision-dos-in-interfacelookupformatter)
- [MessagePack for C# resource exhaustion in multi-dimensional array formatters](https://junglewise.ai/threats/cve-2026-48515-messagepack-for-c-resource-exhaustion-in-multi-dimensional-array)
- [MessagePack for C# denial of service in UnsafeBlitFormatterBase](https://junglewise.ai/threats/cve-2026-48514-messagepack-for-c-denial-of-service-in-unsafeblitformatterbase)