MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-48511: 0.40% estimated exploitation activity in the next 30 days; percentile 34.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals. Impact Applications are affected when they deserialize untrusted MessagePack maps into ExpandoObject using ExpandoObjectResolver or related resolver options. A hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load. This is not…

Published by the source

25 Jun 2026, 19:36 UTC

Last modified by the source

25 Jun 2026, 19:36 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-2x83-8g95-xh59

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# Patch Next advisory brief: CVE-2026-48511

Advisory: MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps
IDs: CVE-2026-48511, GHSA-2x83-8g95-xh59
Record: https://patchnext.com/advisories/cve-2026-48511 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: messagepack 3.1.7, 2.5.301. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet messagepack: affected < 2.5.301; >= 3.0, < 3.1.7; fixed in 2.5.301, 3.1.7

## Suggested inspection (read-only)
- NuGet messagepack: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on messagepack.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2x83-8g95-xh59 (GitHub-reviewed advisory GHSA-2x83-8g95-xh59 lists affected ranges: NuGet messagepack >=0 =3.0 <3.1.7)

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