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CVE-2026-48511— MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Quick assessment

Affected
MessagePack-CSharp MessagePack-CSharp
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

MessagePack-CSharp MessagePack-CSharp是MessagePack-CSharp团队的一款消息序列化工具。 MessagePack-CSharp 2.5.301之前版本和3.1.7之前版本存在资源管理错误漏洞，该漏洞源于ExpandoObjectFormatter.Deserialize在填充System.Dynamic.ExpandoObject时调用IDictionary<string, object>.Add，导致插入大量不同键时产生二次CPU和分配行为。

AI Predicted 6.5 Difficulty: Easy EPSS 0.40% · P33

Affected Version Matrix 2

VendorProduct Version RangeStatus
MessagePack-CSharp MessagePack-CSharp >= 3.0, < 3.1.7 affected
< 2.5.301 affected
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I. Basic Information for CVE-2026-48511

Vulnerability Information
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Vulnerability Title
MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps
Source: CVE Program / CVE List V5
Vulnerability Description
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, 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. This vulnerability is fixed in 2.5.301 and 3.1.7.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
算法复杂性
Source: CVE Program / CVE List V5
Vulnerability Title
MessagePack-CSharp 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
MessagePack-CSharp MessagePack-CSharp是MessagePack-CSharp团队的一款消息序列化工具。 MessagePack-CSharp 2.5.301之前版本和3.1.7之前版本存在资源管理错误漏洞，该漏洞源于ExpandoObjectFormatter.Deserialize在填充System.Dynamic.ExpandoObject时调用IDictionary<string, object>.Add，导致插入大量不同键时产生二次CPU和分配行为。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
MessagePack-CSharp MessagePack-CSharp >= 3.0, < 3.1.7 -

II. Public POCs for CVE-2026-48511

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III. Intelligence Information for CVE-2026-48511

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Vendor Advisories for CVE-2026-48511 (1)

Same Patch Batch · MessagePack-CSharp · 2026-06-22 · 12 CVEs total

CVE-2026-48109 8.2 HIGH MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after derefer
CVE-2026-48506 7.5 HIGH MessagePack-CSharp: MessagePackReader.Skip can recurse without enforcing maximum object gr
CVE-2026-48516 MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer setting
CVE-2026-48513 MessagePack-CSharp: DynamicUnionResolver generated deserializers miss depth enforcement
CVE-2026-48512 MessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcement
CVE-2026-48502 MessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the proce
CVE-2026-48509 MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTT
CVE-2026-48510 MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths
CVE-2026-48517 MessagePack-CSharp: Typeless deserialization type restrictions do not recurse into arrays
CVE-2026-48515 MessagePack-CSharp: Multi-dimensional array formatters allocate from unchecked dimensions
CVE-2026-48514 MessagePack-CSharp: Unity unsafe blit formatter allocates from unbounded byte length

IV. Related Vulnerabilities

Same product: MessagePack-CSharp
Same vendor: MessagePack-CSharp
Same weakness: CWE-407

V. Comments for CVE-2026-48511

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