# CVE-2026-48510: MessagePack — Fixed in 2.5.301 .NET NuGet Not in CISA KEV # CVE-2026-48510 — MessagePack CVE-2026-48510 is a CWE-409 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below. MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths Also known as[GHSA-v72x-2h86-7f8m](/vulnerability/GHSA-v72x-2h86-7f8m) Published Jun 22, 2026 Updated Oct 28, 2026 Affected 2 pkgs Patched 2 / 2 Exploits None indexed Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS) ## Exploitation Status ### No confirmed exploitation observed yet - CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment. Exploitation and automatability from CISA’s SSVC triage for CVE-2026-48510. ## EPSS Exploitation Probability [via FIRST.org ↗](https://www.first.org/epss/) 0.4%probability of exploitation in next 30 days Lower Risk0.00% Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk 0.00%0.30%0.60%0.90%0.2%0.4%Jul 26Oct 26Nov 26 Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/). ## Real-World Exposure 2 pkgs affected .NET`MessagePack`.NET`MessagePack` Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems. ## Description ### Summary When MessagePack-CSharp decompresses `Lz4Block` or `Lz4BlockArray` payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable. A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins. ### Impact Applications are affected when they deserialize attacker-controlled MessagePack payloads with `MessagePackCompression.Lz4Block` or `MessagePackCompression.Lz4BlockArray` enabled. In the `Lz4Block` case, an attacker-controlled integer is used to request the destination span. In the `Lz4BlockArray` case, per-block uncompressed lengths and their aggregate can be attacker-controlled. Without a cap, the declared output size can be disproportionate to the input size, producing out-of-memory exceptions, process termination on constrained hosts, or severe memory pressure. This advisory is about unbounded allocation from declared decompressed sizes. It is separate from the LZ4 source-buffer over-read issue, which concerns unsafe decoder reads beyond the compressed input buffer. ### Affected components - Package: `MessagePack` - Feature: LZ4 compressed MessagePack payloads - APIs: `MessagePackSerializer` with `WithCompression(MessagePackCompression.Lz4Block)` or `WithCompression(MessagePackCompression.Lz4BlockArray)` - Internal routine: `MessagePackSerializer.TryDecompress` - Finding ID: `MESSAGEPACKCSHARP-OPEN-004` ### Patches Fixes are prepared and will be released in coordinated patch versions. Upgrade guidance: 1. Upgrade `MessagePack` to the patched version for your release line. 2. Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions. The fix should reject negative and excessive uncompressed lengths before allocation. It should also cap aggregate decompressed size for block arrays and expose or honor an appropriate maximum decompressed length policy. ### Workarounds Patching is recommended. Until a patched version is available, do not enable MessagePack-CSharp's built-in LZ4 compression modes for untrusted inputs. If compression is required, enforce strict compressed and decompressed size limits outside MessagePack-CSharp before deserialization. ### Resources - `MESSAGEPACKCSHARP-OPEN-004`: LZ4 decompression allocation from unbounded uncompressed length - `MESSAGEPACKCSHARP-011`: duplicate decompression-bomb finding - CWE-409: Improper Handling of Highly Compressed Data - CWE-770: Allocation of Resources Without Limits or Throttling ## Affected Packages 2 total 2 fixed Ecosystem Package Vulnerable range Fix .NETNuGet `MessagePack` all versions 2.5.301`dotnet add package MessagePack --version 2.5.301` .NETNuGet `MessagePack` ≥ 3.0&&< 3.1.7 3.1.7`dotnet add package MessagePack --version 3.1.7` ## Affected Products 1 product · 2 configurations Application `messagepack`messagepack ≥ 3.0.3 && < 3.1.7 range ## Detection & mitigation playbook Open-source dependency 1. ### Detect Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version. 2. ### Fix Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48510 is resolved across your whole dependency graph. 3. ### Workarounds Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process. ## Frequently Asked Questions \## Summary When MessagePack-CSharp decompresses \`Lz4Block\` or \`Lz4BlockArray\` payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable. A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins. ## Impact Applications are affected when they deserialize attacker-controlled MessagePack payloads with \`MessagePackCompression.Lz4Block\` or \`MessagePackCompression.Lz4BlockArray\` enabled. O3 Security · Impact-Aware SCA ### Is CVE-2026-48510 in your dependencies? Find it across NuGet, including transitive dependencies. [Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)