Vulnerability in MessagePack (CVE-2026-48509)
Summary
Vulnerability in MessagePack (CVE-2026-48509). Data can be tampered with by attackers. Exploitable via MessagePackSerializerOptions.Standard. Mitigation: upgrade to 3.1.7 or later.
Original description
The description as published by the source.
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks that MessagePackSecurity.UntrustedData is intended to mitigate, such as hash-collision attacks against dictionary-like model properties. This vulnerability is fixed in 2.5.301 and 3.1.7.
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-48509 has been found in MessagePack.
Attackers can target a specific entry point like MessagePackSerializerOptions.Standard over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 9.1/10.
What to do: upgrade MessagePack to 3.1.7 or later.
If unsure, ask your IT team or search "MessagePack CVE-2026-48509" on the vendor's site.
CVE-2026-48509 (MessagePack) — CWE-1188 / CVSS v3 9.1
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: MessagePackSerializerOptions.Standard / MessagePackSecurity.TrustedData / MessagePackSecurity.UntrustedData / MessagePack.AspNetCoreMvcFormatter
Patched: 3.1.7 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-1188) exists in MessagePack. Attackers reach the vulnerable code path via MessagePackSerializerOptions.Standard without authentication.
Check the vendor advisory for the affected versions of MessagePack. Attack surface: MessagePackSerializerOptions.Standard / MessagePackSecurity.TrustedData / MessagePackSecurity.UntrustedData / MessagePack.AspNetCoreMvcFormatter.
Severity: Critical (CVSS 9.1/10). Data can be tampered with by attackers.
Update to 3.1.7.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'MessagePack' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
grep -r 'MessagePack' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
MessagePackを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade MessagePack to 3.1.7ステージング環境で 3.1.7 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory github.com
- advisory nvd.nist.gov
- advisory github.com
- package github.com
- web github.com