CVE-2026-53937: CWE-400: Uncontrolled Resource Consumption in modelcontextprotocol io.modelcontextprotocol:kotlin-sdk

The MCP Kotlin SDK (io.modelcontextprotocol:kotlin-sdk) versions 0.7.0 through 0.12.0 contain an uncontrolled resource consumption vulnerability (CWE-400) in the ReadBuffer.append function. This function appends every chunk of bytes received from stdio transport into a kotlinx.io.Buffer without any size cap. Frames are only extracted when a newline byte (0x0a) is observed. If a peer streams bytes continuously without sending a newline, the internal buffer grows without bound, leading to out-of-memory (OOM) conditions. The problem is worsened by StdioServerTransport and StdioClientTransport, which queue raw chunks in an unbounded channel and call readBuffer.append without backpressure or size guards. This results in a remote pre-auth denial of service when the SDK stdio server's stdin is fed by untrusted or attacker-controlled data, such as from a subprocess or sidecar proxy. The vulnerability is resolved in version 0.13.0.

An attacker who can send data to the stdio transport of the MCP Kotlin SDK server can cause unbounded memory growth by sending a continuous stream of bytes without newline characters. This leads to out-of-memory conditions and crashes of the JVM or host process, resulting in denial of service. There is no impact on confidentiality or integrity, only availability.

Upgrade to version 0.13.0 or later, which fixes the uncontrolled resource consumption issue. Until upgraded, avoid exposing the stdio transport to untrusted or attacker-controlled input sources. No other mitigations are specified.