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MessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcement

Moderate severity GitHub Reviewed Published Jun 9, 2026 in MessagePack-CSharp/MessagePack-CSharp

Package

MessagePack (NuGet)

Affected versions

< 2.5.301
>= 3.0, < 3.1.7

Patched versions

2.5.301
3.1.7

Description

Summary

MessagePack-CSharp's JSON conversion helpers contain multiple recursion paths that do not consistently enforce a depth limit. These paths are in the JSON conversion component rather than normal typed MessagePack deserialization.

Three related issues are covered by this advisory:

  1. MessagePackSerializer.ConvertFromJson recursively processes nested JSON arrays and objects in FromJsonCore() without consulting MessagePackSecurity.MaximumObjectGraphDepth.
  2. TinyJsonReader.ReadNextToken() recursively consumes comma and colon separator characters, allowing even malformed JSON with long separator runs to consume one stack frame per character.
  3. MessagePackSerializer.ConvertToJson applies depth checks to arrays and maps, but the typeless extension branch for ext-100 recursively calls ToJsonCore() without applying MessagePackSecurity.DepthStep(ref reader).

Each path can allow attacker-controlled input to exhaust the process stack and trigger an uncatchable StackOverflowException instead of failing with a catchable parse or serialization exception.

Impact

Applications are affected when they call MessagePack-CSharp JSON conversion APIs on attacker-controlled data. This includes gateways, diagnostics endpoints, migration tools, logging paths, and services that convert between external JSON and MessagePack payloads.

For JSON-to-MessagePack conversion, deeply nested JSON arrays or objects can recurse through FromJsonCore() without applying the configured object graph depth limit. Separately, long runs of comma or colon separator characters can recurse through TinyJsonReader.ReadNextToken() before normal structural validation rejects the input.

For MessagePack-to-JSON conversion, nested typeless extension wrappers can recurse through ToJsonCore() without the depth guard that the same function applies to arrays and maps.

MessagePackSecurity.UntrustedData does not fully mitigate these conversion paths because the missing checks occur inside JSON conversion and tokenization branches that do not consistently use the configured depth policy.

Affected components

  • Package: MessagePack
  • APIs: MessagePackSerializer.ConvertFromJson, MessagePackSerializer.ConvertToJson
  • Internal routines: FromJsonCore, ToJsonCore, TinyJsonReader.ReadNextToken
  • Data shapes: deeply nested JSON arrays/objects, long JSON separator runs, and nested typeless MessagePack extension values converted to JSON
  • Finding IDs: MESSAGEPACKCSHARP-090, MESSAGEPACKCSHARP-091, MESSAGEPACKCSHARP-092

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 JSON-to-MessagePack fix should add explicit JSON nesting-depth accounting to FromJsonCore, using the configured maximum object graph depth or an equivalent limit, or rewrite the conversion to use an iterative bounded stack.

The tokenizer fix should replace separator self-recursion in TinyJsonReader.ReadNextToken() with an iterative loop so consecutive commas, colons, and whitespace do not consume stack frames.

The MessagePack-to-JSON fix should apply DepthStep and matching reader.Depth-- cleanup around recursive ToJsonCore() calls made from the typeless extension branch, consistent with the existing array and map conversion branches.

Workarounds

Patching is recommended.

Until a patched version is available, do not pass untrusted JSON directly to ConvertFromJson, and do not call ConvertToJson on untrusted MessagePack payloads that may contain typeless extension values. Validate JSON nesting depth with a parser that enforces depth limits before calling MessagePack-CSharp, reject malformed JSON before conversion, and apply strict input-size limits.

Input-size limits reduce exposure but do not remove the recursive behavior in affected versions.

References

  • MESSAGEPACKCSHARP-090: ConvertFromJson unbounded structural recursion
  • MESSAGEPACKCSHARP-091: TinyJsonReader.ReadNextToken separator self-recursion
  • MESSAGEPACKCSHARP-092: ConvertToJson ext-100 branch missing depth enforcement
  • CWE-674: Uncontrolled Recursion

CVE split rationale

These issues are grouped because they affect the same JSON conversion feature area and share the same failure mode: recursive conversion/tokenization paths do not consistently enforce depth or iteration bounds for attacker-controlled input. They are distinct from normal binary MessagePack skip recursion, dynamic union formatter depth accounting, DateTime stack allocation, and allocation-oriented denial-of-service issues.

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

Severity

Moderate

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 Low
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:L/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.
(14th percentile)

Weaknesses

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.

CVE ID

CVE-2026-48512

GHSA ID

GHSA-cj9g-3mj2-g8vv

Source code

Credits

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