Vulnerability · CVE-2026-59951
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP).
Published Oct 14, 2026 · catalog updated Oct 15, 2026 · The MCP Python SDK,
Check if you run it
No confirmed exploitation is on record for CVE-2026-59951. EPSS is 0.46%. Act only if a listed version of The MCP Python SDK, is installed.
Gaps on this record
- The stored product name is a package slug. The name on this page is taken from the title (The MCP Python SDK,).
- No vendor fix or required action is stored. Compare the linked advisory before you change production.
- Severity
- High7.5
- Known exploitation
- Not in CISA/VulnCheck KEV
- EPSS
- 0.46%
- Remediation
- Advisory link
- Affected products
- PyPI mcp · pypi mcp
### Summary In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state: - sessions did not expire; - a client ended the session but left its entry registered; - a session whose first request was refused stayed registered with a running task. Each session also kept its parsed parameters, and the number of sessions was unlimited. A client that could reach the endpoint could grow server memory without bound using ordinary short requests; only a restart reclaimed it. ### Am I affected? Yes, if you serve MCP over Streamable HTTP in stateful mode and clients you do not fully trust can reach the endpoint. Stateful mode is the default for: - (1.x) - and (2.x) - Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1. Not affected: - servers on 1.12.0 or later passing (no sessions are kept) - servers using only the stdio or SSE transports Authentication limits who can open sessions, not how many a token holder can open. ### Fix Upgrade to 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions: - a session with no request in flight for seconds (default 1800) is closed and its ID then answers ; open streams and in-progress requests keep it alive; - at most sessions exist at once (default 10000); a request that would open another is answered ; - a session is discarded as soon as it ends, whether by , expiry, or a refused opening request. Both settings are keyword arguments on (1.x), / (2.x) and ; disables them. This bounds retention rather than eliminating it: a live session still holds its parameters (up to roughly twice the 4 MiB body limit), and an with invalid parameters still opens a session that expires like any other. On a small host, lower , and to fit. If you cannot upgrade yet, pass (1.12.0 or later) where you need no per-session state, or restrict who can reach the endpoint.
Are you affected?
Enter the version you run. The result uses only the ranges stored on this record.
The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.
- PyPI mcp1.8.0 → 1.30.0
- PyPI mcp2.0.0 → 2.2.0
- pypi mcpversion unspecified
How to fix CVE-2026-59951
Recommended remediation
A patch or advisory reference is linked. Open the original publisher for availability by product version.
Official advisories
What to do
Three questions, each answered from this record only. Confirm the vendor advisory before you patch.
Should you patch?
No confirmed exploitation is on record for CVE-2026-59951. EPSS is 0.46%. Act only if a listed version of The MCP Python SDK, is installed.
Which versions are affected?
Compare what you have installed with 1.8.0 → 1.30.0, 2.0.0 → 2.2.0. Compare the installed version with the linked vendor advisory before you change production.
What should an analyst verify?
CWE-400: Uncontrolled Resource Consumption; CWE-770: Allocation of Resources Without Limits or Throttling; CWE-772: Missing Release of Resource after Effective Lifetime. EPSS 0.46%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.
Code
The project this record applies to. Exploit listings stay in References.
Timeline
Detection, updates, and resolution for this record.
Oct 14, 2026 · NIST NVD
Detected
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed s
Oct 15, 2026 · NIST NVD
Updated
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain session entries without an idle expiration or aggregate session-count limit. DELETE, refused opening requests, and completed s
Oct 15, 2026 · GitHub Advisory Database
Fix available
mcp fixed in 1.30.0; mcp fixed in 2.2.0
Oct 15, 2026 · GitHub Advisory Database
Fix available
mcp fixed in 1.30.0
Oct 15, 2026 · GitHub Advisory Database
Fix available
mcp fixed in 2.2.0
Oct 15, 2026 · GitHub Advisory Database
Updated
### Summary In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state: - sessions did not expire; - a client ended the session but left its entry registered; - a session whose first request was refused stayed registered with a running task. Each session also kept its parsed parameters, and the number of sessions was unlimited. A client that could r
Sources
- CVE.orgVulnerability recordCVE-2026-59951
Official CVE identifier
cve.org - FIRST EPSSExploitation probabilityCVE-2026-59951
30-day exploitation probability 0.5%
first.org - GitHub Advisory DatabasePackage advisoryGHSA-84m7-p3x7-pcfv
GitHub Advisory Database record mirrored by OSV.
github.com - GitLab Advisory DatabasePackage advisoryCVE-2026-59951
In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state: sessions did not expire; a client DELETE ended the session but left its entry registered; a session whose first request was refused stayed registered with a running task. Each session also kept its parsed initialize parameters, and the number of sessions was unlimited. A client that could r
advisories.gitlab.com - NIST NVDVulnerability recordCVE-2026-59951
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). From 1.8.0 until 1.30.0 and 2.2.0, stateful Streamable HTTP servers, including FastMCP, MCPServer, Server.streamable_http_app(), and StreamableHTTPSessionManager, retain sessio
nvd.nist.gov - OpenSSF OSV / OSV ScannerPackage advisoryGHSA-84m7-p3x7-pcfv
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
osv.dev - PyPA Advisory DatabasePackage advisoryGHSA-84m7-p3x7-pcfv
MCP Python SDK: Unbounded Session Accumulation in StreamableHTTP Transport Enables Resource Exhaustion
osv.dev
References
Freshness
Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.