# CVE-2026-48517: MessagePack-CSharp: Typeless deserialization type restrictions do not recurse into arrays or generic arguments

> Attackers can bypass type restrictions in MessagePack-CSharp by wrapping dangerous types in arrays or generics, potentially leading to code execution.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/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-b8c3-eda5-29b5, GHSA-qhmf-xw27-6rqr

## What is CVE-2026-48517?

CVE-2026-48517 is a medium-severity unsafe deserialization 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, MessagePack-CSharp's typeless deserialization includes MessagePackSerializerOptions.ThrowIfDeserializingTypeIsDisallowed(Type) as a safety check for dangerous types. The default implementation checks the outer type name, but it does not recursively inspect array element types or generic type arguments. As a result, a type that would be blocked directly can be wrapped inside an array or constructed generic type and pass the outer type check. The formatter machinery can then materialize formatters for the inner blocked type. This vulnerability is fixed in 2.5.301 and 3.1.7.

Weakness: CWE-502 (Deserialization of Untrusted Data), CWE-470 (Use of Externally-Controlled Input to Select Classes or Code)

## Which versions are affected by CVE-2026-48517?

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-2026-48517?

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

## Is CVE-2026-48517 being exploited?

There is no known exploitation of CVE-2026-48517 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-2026-48509](https://insights.sbomify.com/CVE-2026-48509/): MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies (medium 6.3)
- [CVE-2026-48510](https://insights.sbomify.com/CVE-2026-48510/): MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths (medium 6.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-48517 and fix it if it is affected.

What it is: CVE-2026-48517 is a medium-severity unsafe deserialization 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-2026-48517 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-2026-48517/ (Markdown: https://insights.sbomify.com/CVE-2026-48517/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-48517
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-48517
- OSV: https://osv.dev/vulnerability/CVE-2026-48517

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-48517/
Summaries are generated and may contain mistakes; the official description above is the record.