CVE-2026-53937

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Memory Leak in MCP Kotlin SDK

Publication date: 2026-09-09

Last updated on: 2026-09-14

Assigner: GitHub, Inc.

Description

MCP Kotlin SDK is the Kotlin Multiplatform software development kit for the Model Context Protocol. In versions 0.7.0 through 0.12.0, `ReadBuffer.append` in `kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/ReadBuffer.kt` writes every chunk of bytes received from the stdio transport into a `kotlinx.io.Buffer` with no size cap. Frames are extracted from that buffer only when a `\n` (0x0a) byte is observed. A peer that streams bytes without ever sending a newline causes the internal buffer to grow indefinitely until the JVM (or the surrounding host process) is OOM-killed. The leak is amplified by `StdioServerTransport` and `StdioClientTransport`, which both queue raw chunks through a `kotlinx.coroutines.channels.Channel<ByteArray>(Channel.UNLIMITED)` and then call `readBuffer.append(chunk)` without backpressure or size guard. This is a remote-pre-auth denial of service whenever an SDK stdio server's stdin is fed by an untrusted or attacker-controlled producer (for example: a host program that exec's the MCP server as a subprocess and pipes through bytes received from a network peer, or a sidecar wrapper that proxies bytes from an HTTP endpoint to the stdio transport). Version 0.13.0 fixes the issue.

CVSS Scores

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Meta Information

Published

2026-09-09

Last Modified

2026-09-14

Generated

2026-09-29

AI Q&A

2026-09-09

EPSS Evaluated

2026-09-27

NVD

CVE-2026-53937

Affected Vendors & Products

Vendor	Product	Version / Range
modelcontextprotocol	kotlin_sdk	From 0.7.0 (inc) to 0.12.0 (inc)
modelcontextprotocol	kotlin_sdk	0.13.0

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-400	The product does not properly control the allocation and maintenance of a limited resource.
CWE-770	The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

Attack-Flow Graph

Executive Summary

CVE-2026-53937 is a memory exhaustion vulnerability in the MCP Kotlin SDK's ReadBuffer class. It allows an attacker to cause a denial-of-service by sending continuous byte streams without newline characters, causing the internal buffer to grow indefinitely until the JVM or host process crashes due to out-of-memory conditions.

Detection Guidance

Monitor for processes using the MCP Kotlin SDK versions 0.7.0 to 0.12.0 with unbounded memory growth. Check for stdio transport processes consuming excessive memory or running OOM-killed. Use system monitoring tools like top, htop, or ps to observe memory usage spikes in Java or Kotlin processes linked to MCP servers.

Impact Analysis

This vulnerability can crash applications using vulnerable SDK versions by consuming all available memory. It affects any deployment where an MCP server's stdin or client's stdout is fed by untrusted input, such as network peers or containerized servers.

Compliance Impact

This vulnerability primarily causes denial-of-service (DoS) conditions by exhausting system memory, which could lead to service unavailability. While not directly violating GDPR or HIPAA, such disruptions may impact availability requirements under both regulations. GDPR mandates data availability controls, and HIPAA requires safeguards against service disruptions affecting protected health information access.

Mitigation Strategies

Upgrade the MCP Kotlin SDK to version 0.13.0 or later. Implement input validation and size limits for stdio streams. Configure monitoring for memory usage anomalies in MCP server processes. Restrict untrusted network access to MCP server stdin/stdout pipes.

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