CVE-2026-53937: Unbounded line buffer in StdioServerTransport/StdioClientTransport leads to memory exhaustion (DoS)
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
MCP Kotlin SDKto a version that resolves this vulnerability.Fixed in 0.13.0
Event History
Frequently Asked Questions
Which deployments are exposed to remote exploitation?
Remote pre-auth exploitation is possible when an SDK stdio server's standard input is supplied by an untrusted or attacker-controlled producer. Examples include a host that pipes network-peer data to an MCP subprocess or a sidecar that proxies an HTTP endpoint into the stdio transport.
What must an attacker do to trigger the denial of service?
The attacker must be able to cause bytes to be sent to the affected stdio transport and continuously stream data without a newline byte. No authentication or user interaction is required in the described exposed deployment scenario.
How can I determine whether my SDK version is affected?
MCP Kotlin SDK versions 0.7.0 through 0.12.0 are affected. The vulnerable path is used by StdioServerTransport and StdioClientTransport when they append received chunks to the shared ReadBuffer.
What is the expected operational impact if exploitation succeeds?
The internal buffer and unlimited raw-chunk queue can continue growing until the JVM or surrounding host process is killed for out-of-memory conditions. This results in denial of service rather than confidentiality or integrity impact.