CVE-2026-48511 is a medium-severity security vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7.
Summary 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. Impact Applications are affected when they deserialize untrusted MessagePack maps into ExpandoObject using ExpandoObjectResolver or related resolver options. A hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load. This is not a hash-collision attack against a configurable dictionary comparer. The super-linear behavior comes from ExpandoObject's insertion model, so MessagePackSecurity.UntrustedData does not eliminate the cost. Affected components Package: MessagePack APIs: ExpandoObjectFormatter.Deserialize, ExpandoObjectResolver Data type: System.Dynamic.ExpandoObject Finding ID: MESSAGEPACKCSHARP-102 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. Potential fixes include applying a map-entry count limit for ExpandoObject under untrusted-data settings, buffering into a security-aware dictionary before materializing a bounded ExpandoObject, or otherwise rejecting maps large enough to trigger quadratic behavior. Workarounds Patching is recommended. Until a patched version is available, avoid deserializing untrusted payloads into ExpandoObject. Prefer strongly typed DTOs or dictionaries with security-aware comparers and explicit count limits. Enforce request-size and map-entry limits at the transport or application layer. Resources MESSAGEPACKCSHARP-102: ExpandoObjectFormatter quadratic insertion behavior CWE-407: Inefficient Algorithmic Complexity
CVE-2026-48511 has a CVSS score of 7.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (2.5.301, 3.1.7). Upgrading removes the vulnerable code path.
nuget
MessagePack (< 2.5.301)MessagePack (>= 3.0, < 3.1.7)MessagePack → 2.5.301 (nuget)MessagePack → 3.1.7 (nuget)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Already deployed Kodem? See CVE-2026-48511 in your environment →Upgrade the following packages to resolve this vulnerability:
MessagePack to 2.5.301 or laterMessagePack to 3.1.7 or laterKodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-48511 is a medium-severity security vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7.
CVE-2026-48511 has a CVSS score of 7.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
MessagePack (nuget) versions < 2.5.301 is affected.
Yes. CVE-2026-48511 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
Whether CVE-2026-48511 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
MessagePack to 2.5.301 or laterMessagePack to 3.1.7 or later