MessagePack: Nested arrays can blow past buffer length limits

Exploitation

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

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-92707: 0.68% estimated exploitation activity in the next 30 days; percentile 52.37 (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.

Impact MessagePack-CSharp is vulnerable to denial of service through excessive memory allocation when deserializing attacker-controlled data into System.Object. When PrimitiveObjectFormatter encounters an array, it allocates an object[] using the declared array length before enforcing the configured maximum object-graph depth. A specially crafted payload containing deeply nested arrays can therefore keep many large arrays live simultaneously before deserialization is rejected for exceeding the depth limit. Because the array-header validation uses the total unread byte count as evidence that the declared elements may be present, nested array headers can reuse the same trailing bytes for this validation. This permits a relatively small payload to cause disproportionately large managed-heap allocations. The reported example uses an approximately 31.5 KiB payload to allocate roughly 114.7 MiB of reference storage on x64—an amplification factor of approximately 3,800—before the depth…

Published by the source

15 Oct 2026, 18:35 UTC

Last modified by the source

15 Oct 2026, 18:45 UTC

Source last checked

26 Oct 2026, 17:05 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-qhrr-8q5h-9q3h

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qhrr-8q5h-9q3h

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# Patch Next advisory brief: CVE-2026-92707

Advisory: MessagePack: Nested arrays can blow past buffer length limits
IDs: CVE-2026-92707, GHSA-qhrr-8q5h-9q3h
Record: https://patchnext.com/advisories/cve-2026-92707 (revision 2, source checked 26 Oct 2026, 17:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: messagepack 3.1.9, 2.5.303. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet messagepack: affected < 2.5.303; >= 3.0.0, < 3.1.9; fixed in 2.5.303, 3.1.9

## 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://osv.dev/vulnerability/GHSA-qhrr-8q5h-9q3h (OSV record GHSA-qhrr-8q5h-9q3h lists affected ranges: NuGet messagepack >=0 =3.0.0 <3.1.9 (fixed))
- Verified: https://github.com/advisories/GHSA-qhrr-8q5h-9q3h (GitHub-reviewed advisory GHSA-qhrr-8q5h-9q3h lists affected ranges: NuGet messagepack >=0 =3.0.0 <3.1.9)

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