Summary
MessagePack's LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input
Workarounds
Instead of upgrading, an application may take the following precautions:
- Disable LZ4 compression for untrusted input paths (
Lz4Block,Lz4BlockArray). - Only accept compressed payloads from strongly trusted producers.
- Isolate deserialization in a separate process/container with restart supervision to limit availability impact.
Resources
- MESSAGEPACKCSHARP-010
Impact
A vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes Lz4Block and Lz4BlockArray.
The decoder implementation is based on a deprecated fast-decompression algorithm that does not take a source-length bound. A remote attacker can send a crafted MessagePack payload with manipulated LZ4 token/length fields to force out-of-bounds reads from the compressed input buffer. In affected environments, this can trigger an AccessViolationException during decompression, causing process termination (denial of service). Under some conditions, limited unintended memory disclosure from over-read data may also be possible before failure.
This issue affects applications that deserialize untrusted data while LZ4 compression is enabled.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-48109 has a CVSS score of 8.2 (High). 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.
Affected versions
Security releases
Kodem intelligence
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.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The v2 versions are patched as of 2.5.301.
The v3 versions are patched as of 3.1.7.
Frequently Asked Questions
- What is CVE-2026-48109? CVE-2026-48109 is a high-severity improper input validation vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-48109? CVE-2026-48109 has a CVSS score of 8.2 (High). 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.
- Which versions of MessagePack are affected by CVE-2026-48109? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48109? Yes. CVE-2026-48109 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48109 exploitable, and should I be worried? Whether CVE-2026-48109 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
- What actually determines whether CVE-2026-48109 is exploitable, and how bad it is? 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.
- How do I fix CVE-2026-48109?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade