CVE-2026-48511
MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps
Impact
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.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| nuget | MessagePack | < 2.5.301 | 2.5.301 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Messagepack | Messagepack | before 2.5.301 from 3.0.3 before 3.1.7 |