{"id":"CVE-2026-53937","state":"PUBLISHED","title":"MCP Kotlin SDK's unbounded line buffer in StdioServerTransport/StdioClientTransport leads to memory exhaustion (DoS)","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.","publishedAt":"2026-09-08T23:21:46.554Z","updatedAt":"2026-09-14T12:09:30.392Z","severity":{"score":6.2,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","source":"GitHub_M"},"assigner":"GitHub_M","products":["modelcontextprotocol io.modelcontextprotocol:kotlin-sdk","modelcontextprotocol io.modelcontextprotocol:kotlin-sdk-client","modelcontextprotocol io.modelcontextprotocol:kotlin-sdk-core","modelcontextprotocol io.modelcontextprotocol:kotlin-sdk-server"],"affected":[{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk","versions":[{"version":"< 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-client","versions":[{"version":">= 0.7.0, < 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-core","versions":[{"version":"< 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-server","versions":[{"version":">= 0.7.0, < 0.13.0","status":"affected"}]}],"references":["https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493","https://github.com/modelcontextprotocol/kotlin-sdk/commit/6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab","https://github.com/modelcontextprotocol/kotlin-sdk/blob/6d5bac1/kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/ReadBuffer.kt","https://github.com/modelcontextprotocol/kotlin-sdk/releases/tag/0.13.0"],"weaknesses":["CWE-400","CWE-770"],"ingestedAt":"2026-10-29T20:12:16.098Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/53xxx/CVE-2026-53937.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-53937","state":"PUBLISHED","datePublished":"2026-09-08T23:21:46.554Z","dateUpdated":"2026-09-14T12:09:30.392Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-11T15:46:12.318Z"},"containers":{"cna":{"title":"MCP Kotlin SDK's unbounded line buffer in StdioServerTransport/StdioClientTransport leads to memory exhaustion (DoS)","descriptions":[{"lang":"en","value":"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":[{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk","versions":[{"version":"< 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-client","versions":[{"version":">= 0.7.0, < 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-core","versions":[{"version":"< 0.13.0","status":"affected"}]},{"vendor":"modelcontextprotocol","product":"io.modelcontextprotocol:kotlin-sdk-server","versions":[{"version":">= 0.7.0, < 0.13.0","status":"affected"}]}],"references":[{"url":"https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493","name":"https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/modelcontextprotocol/kotlin-sdk/commit/6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab","name":"https://github.com/modelcontextprotocol/kotlin-sdk/commit/6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab","tags":["x_refsource_MISC"]},{"url":"https://github.com/modelcontextprotocol/kotlin-sdk/blob/6d5bac1/kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/ReadBuffer.kt","name":"https://github.com/modelcontextprotocol/kotlin-sdk/blob/6d5bac1/kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/ReadBuffer.kt","tags":["x_refsource_MISC"]},{"url":"https://github.com/modelcontextprotocol/kotlin-sdk/releases/tag/0.13.0","name":"https://github.com/modelcontextprotocol/kotlin-sdk/releases/tag/0.13.0","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"LOCAL","availabilityImpact":"HIGH","baseScore":6.2,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-400","lang":"en","description":"CWE-400: Uncontrolled Resource Consumption","type":"CWE"}]},{"descriptions":[{"cweId":"CWE-770","lang":"en","description":"CWE-770: Allocation of Resources Without Limits or Throttling","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-09-08T23:22:04.726Z"},"source":{"advisory":"GHSA-74gp-qhv5-v493","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","references":[{"url":"https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493","tags":["exploit"]}],"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-09-14T12:09:07.813016Z","id":"CVE-2026-53937","options":[{"Exploitation":"poc"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-09-14T12:09:30.392Z"}}]}}}