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.
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History
Mon, 22 Jun 2026 22:00:00 +0000
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| 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. | |
| Title | MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps | |
| Weaknesses | CWE-407 | |
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cvssV4_0
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MITRE
Status: PUBLISHED
Assigner: GitHub_M
Published:
Updated: 2026-06-22T21:14:54.127Z
Reserved: 2026-05-21T16:18:10.618Z
Link: CVE-2026-48511
Vulnrichment
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NVD
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Redhat
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OpenCVE Enrichment
Updated: 2026-06-22T23:30:05Z