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MessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the process with stack and heap overflows

High severity GitHub Reviewed Published Jun 9, 2026 in MessagePack-CSharp/MessagePack-CSharp • Updated Jun 25, 2026

Package

nuget MessagePack (NuGet)

Affected versions

>= 3.0, < 3.1.7

Patched versions

3.1.7

Description

Summary

MessagePackReader.ReadDateTime() can allocate stack memory based on an attacker-controlled MessagePack extension length. In the slow path for timestamp extension parsing, the computed tokenSize includes the extension body length from the wire and is used in a stackalloc operation before the extension length is validated as one of the valid timestamp sizes.

A very small payload can claim a large timestamp extension body and cause a stack allocation large enough to trigger an uncatchable StackOverflowException, terminating the host process.

Impact

Applications are affected when they deserialize untrusted payloads into types containing DateTime values. This path is available through the standard formatter set and does not require opting into typeless serialization, LZ4 compression, Unity-specific resolvers, or other specialized features.

MessagePackSecurity.UntrustedData and MaximumObjectGraphDepth do not mitigate this issue because the crash is caused by a single-frame stack allocation, not by object graph recursion.

An attacker can send a MessagePack timestamp extension header with an oversized body length and insufficient body bytes. The reader enters the slow path, attempts to stack-allocate a buffer sized from that declared length, and can terminate the process before a catchable serialization exception is thrown.

Affected components

  • Package: MessagePack
  • API: MessagePackReader.ReadDateTime
  • Data types: DateTime and formatter paths that call ReadDateTime
  • Finding IDs: MESSAGEPACKCSHARP-020, related stack allocation finding MESSAGEPACKCSHARP-CROW-MEM-001

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 validate timestamp extension lengths before any stack allocation. Valid MessagePack timestamp payload lengths are limited to the supported timestamp encodings, so oversized extension lengths should fail with a catchable MessagePack serialization exception before the slow path allocates a buffer.

Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted MessagePack payloads into schemas that contain DateTime or DateTimeOffset values. Where possible, enforce strict maximum message sizes and reject malformed extension payloads before they reach MessagePack-CSharp.

There is no complete workaround for applications that must deserialize attacker-controlled MessagePack data containing date/time fields with affected versions.

Resources

  • MESSAGEPACKCSHARP-020: ReadDateTime stack allocation from attacker-controlled extension length
  • MESSAGEPACKCSHARP-CROW-MEM-001: related attacker-controlled stack allocation finding in MessagePackReader
  • CWE-770: Allocation of Resources Without Limits or Throttling

CVE split rationale

This vulnerability is independently fixable in the DateTime extension parsing path by validating extension lengths before stack allocation. It is separate from recursive stack overflows, LZ4 issues, and collection allocation bugs.

References

@AArnott AArnott published to MessagePack-CSharp/MessagePack-CSharp Jun 9, 2026
Published by the National Vulnerability Database Jun 22, 2026
Published to the GitHub Advisory Database Jun 25, 2026
Reviewed Jun 25, 2026
Last updated Jun 25, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(17th percentile)

Weaknesses

Out-of-bounds Read

The product reads data past the end, or before the beginning, of the intended buffer. Learn more on MITRE.

Integer Overflow or Wraparound

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number. Learn more on MITRE.

Inefficient Algorithmic Complexity

An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached. Learn more on MITRE.

Improper Handling of Highly Compressed Data (Data Amplification)

The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. Learn more on MITRE.

Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')

The product uses external input with reflection to select which classes or code to use, but it does not sufficiently prevent the input from selecting improper classes or code. Learn more on MITRE.

Deserialization of Untrusted Data

The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid. Learn more on MITRE.

Uncontrolled Recursion

The product does not properly control the amount of recursion that takes place, consuming excessive resources, such as allocated memory or the program stack. Learn more on MITRE.

Memory Allocation with Excessive Size Value

The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. Learn more on MITRE.

Initialization of a Resource with an Insecure Default

The product initializes or sets a resource with a default that is intended to be changed by the administrator, but the default is not secure. Learn more on MITRE.

CVE ID

CVE-2026-48502

GHSA ID

GHSA-382j-8mxh-c7x2

Source code

Credits

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