CVE-2026-53937MCP Kotlin SDK's unbounded line buffer in StdioServerTransport/StdioClientTransport leads to memory exhaustion (DoS)

Official 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.

Affected products (as reported)

References

• https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493 x_refsource_CONFIRM
• https://github.com/modelcontextprotocol/kotlin-sdk/commit/6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab x_refsource_MISC
• https://github.com/modelcontextprotocol/kotlin-sdk/blob/6d5bac1/kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/ReadBuffer.kt x_refsource_MISC
• https://github.com/modelcontextprotocol/kotlin-sdk/releases/tag/0.13.0 x_refsource_MISC