The MCP Kotlin SDK versions 0.7.0 through 0.12.0 contain an uncontrolled resource consumption vulnerability (CWE-400) in the ReadBuffer.append function. Incoming byte chunks from stdio transport are appended to a kotlinx.io.Buffer without any size cap, and frames are only extracted upon receiving a newline byte (0x0a). If a peer continuously streams bytes without newlines, the internal buffer grows without bound, causing memory exhaustion and potential JVM or host process termination. The problem is worsened by StdioServerTransport and StdioClientTransport components that queue raw byte chunks through an unbounded channel and append them without backpressure or size checks. This results in a remote pre-authentication denial of service when the SDK stdio server's stdin is fed by untrusted sources, such as subprocesses or sidecar wrappers proxying network data. The vulnerability is resolved in version 0.13.0.

An attacker can cause a denial of service by sending a continuous stream of bytes without newline characters to the MCP Kotlin SDK stdio server, leading to unbounded memory growth and eventual out-of-memory termination of the JVM or host process. There is no impact on confidentiality or integrity, only availability is affected.

Upgrade to version 0.13.0 or later, which fixes the uncontrolled resource consumption issue. Until then, avoid exposing the SDK stdio server's stdin to untrusted or attacker-controlled input sources. Patch status is not explicitly stated in a vendor advisory, but version 0.13.0 is confirmed to fix the issue.