8.2

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-48109

CVE-2026-48109: MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, 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 vulnerability is fixed in 2.5.301 and 3.1.7.

(GitHub Advisory)

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CVE-2026-48109

CVE-2026-48109:

8.2

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable YesTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
MessagePack	nuget	< 2.5.301	2.5.301
MessagePack	nuget	>= 3.0.214-rc.1, < 3.1.7	3.1.7

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is an out-of-bounds read in the LZ4 decompression logic, identified as CWE-125. The root cause is that the low-level, unsafe decompression functions (LZ4_uncompress_32 and LZ4_uncompress_64) did not receive the length of the compressed input buffer. As a result, they could be manipulated by a crafted payload to read past the buffer's boundary when parsing token and length fields, leading to a process crash (Denial of Service) or unintended memory disclosure.

The patch addresses this by modifying the public-facing MessagePack.LZ4.LZ4Codec.Decode function to pass the input buffer's length to the internal LZ4_uncompress_32 and LZ4_uncompress_64 functions. These internal functions were updated to accept the new length parameter and use it to perform explicit bounds checks before every read from the input buffer, thus preventing the out-of-bounds access. Any runtime profile capturing the exploitation of this vulnerability would show calls to these functions.

Vulnerable functions

MessagePack.LZ4.LZ4Codec.LZ4_uncompress_32

src/MessagePack/LZ4/LZ4Codec.Unsafe32.Dirty.cs

This function performs 32-bit LZ4 decompression. The original implementation did not have knowledge of the source buffer's length. A malicious actor could provide a crafted compressed payload that would cause the function to read past the end of the source buffer when parsing tokens and lengths. This out-of-bounds read could lead to a process crash (Denial of Service) or potential information disclosure.

MessagePack.LZ4.LZ4Codec.LZ4_uncompress_64

src/MessagePack/LZ4/LZ4Codec.Unsafe64.Dirty.cs

This is the 64-bit counterpart to `LZ4_uncompress_32` and contains the same vulnerability. It performs LZ4 decompression without validating that its reads are within the bounds of the source buffer. This allows an attacker to craft input that causes out-of-bounds memory access, resulting in a Denial of Service or potential data leakage.

MessagePack.LZ4.LZ4Codec.Decode

src/MessagePack/LZ4/LZ4Codec.Unsafe.cs

This public function is the entry point for the vulnerable LZ4 decompression logic. While the core out-of-bounds read occurs in the `LZ4_uncompress_*` methods, this function enables the vulnerability by invoking them without providing the source buffer's length. Any deserialization of LZ4-compressed data will pass through this function, making it a key indicator in a runtime profile during exploitation.

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WAF Protection Rules

WAF Rule

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Reasoning

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