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

{
  "id": "3132f5b019f2a1e19babd47109d33b9617b6700389fefebdecce0a168dd496c3",
  "url": "https://osv.dev/vulnerability/CVE-2026-53937",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "MCP Kotlin SDK's unbounded line buffer in StdioServerTransport/StdioClientTransport leads to memory exhaustion (DoS)",
    "summary": "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(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.",
    "evidence": "osv:CVE-2026-53937",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-53937",
      "aliases": [
        "CVE-2026-53937",
        "GHSA-74gp-qhv5-v493"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/modelcontextprotocol/kotlin-sdk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "9a70ceaaba99adab484655679ea12646e36c20eb"
                },
                {
                  "fixed": "6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab"
                },
                {
                  "fixed": "b7eeeb12058099f980fa98d1468506489a6b5f32"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0.7.0"
                      },
                      {
                        "last_affected": "0.12.0"
                      }
                    ],
                    "source": "DESCRIPTION"
                  },
                  {
                    "range": [
                      {
                        "fixed": "6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "b7eeeb12058099f980fa98d1468506489a6b5f32"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "0.13.0"
                  }
                ]
              }
            }
          ],
          "versions": [
            "0.12.0",
            "0.11.1",
            "0.11.0",
            "0.9.0",
            "0.8.3",
            "0.8.1",
            "0.8.0",
            "0.7.7",
            "0.7.6",
            "0.7.5",
            "0.7.4",
            "0.7.2",
            "0.7.2-alpha3",
            "0.7.2-alpha2",
            "0.7.2-alpha1",
            "0.7.1",
            "v0.7.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-53937.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:32.327269090Z",
      "provider": "osv",
      "references": [
        "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",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53937.json",
        "https://github.com/modelcontextprotocol/kotlin-sdk/security/advisories/GHSA-74gp-qhv5-v493",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-53937",
        "https://github.com/modelcontextprotocol/kotlin-sdk/commit/6e6f80512fb8fcc9f3c031cfd693ccbcf9c4aaab"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-400",
          "CWE-770"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53937.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-53937",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 499400,
  "published_at": "2026-09-08T23:21:46.554+00:00"
}