Summary
MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths
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.
Affected components
- Package:
MessagePack - Feature: LZ4 compressed MessagePack payloads
- APIs:
MessagePackSerializerwithWithCompression(MessagePackCompression.Lz4Block)orWithCompression(MessagePackCompression.Lz4BlockArray) - Internal routine:
MessagePackSerializer.TryDecompress - Finding ID:
MESSAGEPACKCSHARP-OPEN-004
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 lengthMESSAGEPACKCSHARP-011: duplicate decompression-bomb finding- CWE-409: Improper Handling of Highly Compressed Data
- CWE-770: Allocation of Resources Without Limits or Throttling
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.
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
CVE-2026-48510 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.
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.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePackto the patched version for your release line. - 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.
Frequently Asked Questions
- What is CVE-2026-48510? CVE-2026-48510 is a medium-severity allocation of resources without limits or throttling vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- How severe is CVE-2026-48510? CVE-2026-48510 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.
- Which versions of MessagePack are affected by CVE-2026-48510? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48510? Yes. CVE-2026-48510 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48510 exploitable, and should I be worried? Whether CVE-2026-48510 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-2026-48510 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-2026-48510?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade