# CVE-2026-48509: MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies

> Insecure default security settings in MessagePack-CSharp allow attackers to cause denial of service via hash-collision attacks on ASP.NET Core request bodies.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Jun 2026
- **Updated:** 25 Jun 2026
- **Also known as:** ECHO-e60c-e8f0-b786, GHSA-2f33-pr97-265q

## What is CVE-2026-48509?

CVE-2026-48509 is a medium-severity denial of service vulnerability in MessagePack-CSharp, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks that MessagePackSecurity.UntrustedData is intended to mitigate, such as hash-collision attacks against dictionary-like model properties. This vulnerability is fixed in 2.5.301 and 3.1.7.

Weakness: CWE-1188 (Initialization of a Resource with an Insecure Default)

## Which versions are affected by CVE-2026-48509?

4 open source packages ship an affected version, including MessagePack. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `MessagePack` | NuGet | All versions | 2.5.301 |
| `MessagePack` | NuGet | All versions | 2.5.301 |
| `MessagePack` | NuGet | From 3.0 | 3.1.7 |
| `MessagePack` | NuGet | From 3.0 | 3.1.7 |

Products: Messagepack Messagepack.

## How do I fix CVE-2026-48509?

Upgrade MessagePack to a fixed version for your release line: 2.5.301 or 3.1.7.

## Is CVE-2026-48509 being exploited?

There is no known exploitation of CVE-2026-48509 in the wild and no public exploit code.

## Other vulnerabilities in MessagePack-CSharp

- [CVE-2026-48109](https://insights.sbomify.com/CVE-2026-48109/): MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input (high 8.2)
- [CVE-2026-48502](https://insights.sbomify.com/CVE-2026-48502/): MessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the process with stack and heap overflows (high 8.2)
- [CVE-2026-48506](https://insights.sbomify.com/CVE-2026-48506/): MessagePack-CSharp: MessagePackReader.Skip can recurse without enforcing maximum object graph depth (high 7.5)
- [CVE-2026-48510](https://insights.sbomify.com/CVE-2026-48510/): MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths (medium 6.3)
- [CVE-2026-48511](https://insights.sbomify.com/CVE-2026-48511/): MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps (medium 6.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-48509 and fix it if it is affected.

What it is: CVE-2026-48509 is a medium-severity denial of service vulnerability in MessagePack-CSharp, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-48509 in the wild and no public exploit code.

Affected packages and fixed versions:
- MessagePack (NuGet): affected all versions; fixed in 2.5.301
- MessagePack (NuGet): affected all versions; fixed in 2.5.301
- MessagePack (NuGet): affected from 3.0; fixed in 3.1.7
- MessagePack (NuGet): affected from 3.0; fixed in 3.1.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-48509/ (Markdown: https://insights.sbomify.com/CVE-2026-48509/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-48509
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-48509
- OSV: https://osv.dev/vulnerability/CVE-2026-48509

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-48509/
Summaries are generated and may contain mistakes; the official description above is the record.