Nerdbank.MessagePack has a memory amplification DoS in collection deserialization

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

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Nerdbank.MessagePack deserializers for many collection-shaped types trusted the element count declared in MessagePack array and map headers when allocating destination storage. A crafted payload could therefore force large arrays, pooled buffers, dictionaries, or collection instances to be allocated before the deserializer had consumed the corresponding elements. The same allocation pattern existed across strongly typed arrays, primitive arrays, mutable and immutable dictionaries, mutable enumerables, span-backed enumerable construction, JsonNode, MessagePackValue, and the object/dynamic primitive converters. Because MessagePack array and map headers carry an attacker-controlled element count, any converter that immediately allocates count elements or constructs a collection with capacity count can turn a payload that is merely large into a much larger managed heap allocation. The reader's residency checks reduce the most extreme header-only attack shape, but they do not remove the…

Published by the source

29 May 2026, 19:55 UTC

Last modified by the source

29 May 2026, 19:55 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qjvr-435c-5fjh

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# Patch Next advisory brief: GHSA-qjvr-435c-5fjh

Advisory: Nerdbank.MessagePack has a memory amplification DoS in collection deserialization
IDs: GHSA-qjvr-435c-5fjh
Record: https://patchnext.com/advisories/ghsa-qjvr-435c-5fjh (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 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nerdbank.messagepack 1.1.78. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet nerdbank.messagepack: affected < 1.1.78; fixed in 1.1.78

## Suggested inspection (read-only)
- NuGet nerdbank.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 nerdbank.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-qjvr-435c-5fjh (GitHub-reviewed advisory GHSA-qjvr-435c-5fjh lists affected ranges: NuGet nerdbank.messagepack >=0 <1.1.78)

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