How severe is CVE-2026-48513?

No CVSS score has been assigned to CVE-2026-48513 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-48513?

CVE-2026-48513 affects the following packages: MessagePack (NuGet), MessagePack (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2026-48513?

Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48513 is resolved across your whole dependency graph.

How do I detect CVE-2026-48513 in my NuGet dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-48513 if there is no patch (or I can't update yet)?

Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Is CVE-2026-48513 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-48513 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-48513?

CVE-2026-48513 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 35th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-48513?

CVE-2026-48513 is classified as CWE-674 (CWE-674). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-48513 published, and has it been updated?

CVE-2026-48513 was published on June 22, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET NuGet

Not in CISA KEV

CVE-2026-48513 — MessagePack

CVE-2026-48513 is a CWE-674 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.

MessagePack-CSharp: DynamicUnionResolver generated deserializers miss depth enforcement

Also known asGHSA-wfr3-xj75-pfwh

Published

Jun 22, 2026

Updated

Oct 28, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-48513.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

2 pkgs affected

.NETMessagePack.NETMessagePack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Runtime-generated union deserializers emitted by DynamicUnionResolver do not call MessagePackSecurity.DepthStep(ref reader) and do not decrement reader.Depth around recursive deserialization and skip paths.

This means union deserialization does not consistently participate in the maximum object graph depth enforcement that protects other recursive formatter paths. For unknown union keys, the emitted deserializer calls reader.Skip() on attacker-controlled data without an enclosing depth step.

Impact

Applications are affected when they deserialize untrusted payloads into object graphs containing [Union]-decorated interfaces or abstract classes handled by DynamicUnionResolver.

An attacker can provide a union payload with an unknown key and a deeply nested value. Because the generated union formatter does not enter the depth accounting scope before skipping or recursively processing the value, configured depth limits can be bypassed. In combination with recursive skip behavior, this can terminate the process with an uncatchable StackOverflowException.

This issue is narrower than the general TrySkip() recursion issue because it specifically concerns a formatter-generation path that fails to count union nesting. It remains independently fixable because the emitted IL should mirror the depth-step behavior used by source-generated union formatters and dynamic object formatters.

Affected components

Package: MessagePack
API: DynamicUnionResolver.BuildDeserialize
Data types: [Union]-decorated interface and abstract class hierarchies handled by the dynamic resolver
Finding ID: MESSAGEPACKCSHARP-070

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

Upgrade MessagePack to the patched version for your release line.
Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

The fix should emit DepthStep and matching reader.Depth-- cleanup in dynamic union deserializers, consistent with other recursive formatter implementations.

Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted payloads into dynamically resolved [Union] types. Prefer source-generated formatters that include depth checks, where applicable, and enforce outer message-size and schema constraints.

Resources

MESSAGEPACKCSHARP-070: dynamic union deserializer missing depth-step enforcement
CWE-674: Uncontrolled Recursion

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`MessagePack`	all versions	2.5.301`dotnet add package MessagePack --version 2.5.301`
.NETNuGet	`MessagePack`	≥ 3.0&&< 3.1.7	3.1.7`dotnet add package MessagePack --version 3.1.7`

Affected Products

1 product · 2 configurations

Application

messagepackmessagepack

≥ 3.0.3 && < 3.1.7

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48513 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Frequently Asked Questions

## Summary Runtime-generated union deserializers emitted by `DynamicUnionResolver` do not call `MessagePackSecurity.DepthStep(ref reader)` and do not decrement `reader.Depth` around recursive deserialization and skip paths. This means union deserialization does not consistently participate in the maximum object graph depth enforcement that protects other recursive formatter paths. For unknown union keys, the emitted deserializer calls `reader.Skip()` on attacker-controlled data without an enclosing depth step. ## Impact Applications are affected when they deserialize untrusted payloads int

O3 Security · Impact-Aware SCA

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