MCP Ruby SDK Session Hijacking via Unbound Session IDs
First seen Jul 30, 2026 · Updated Jul 30, 2026
The MCP Ruby SDK's StreamableHTTP transport failed to bind session IDs to the client/owner that created them, so anyone who obtains a valid session ID (via theft, leakage in logs/URLs, or interception) could issue tools/call requests that execute with the victim's authenticated session context. This allows an attacker to impersonate a legitimate user or agent and trigger tool actions on their behalf without needing credentials.
Technical Analysis
MCP servers using StreamableHTTPTransport generate session IDs to track client state across requests, but prior to 0.23.0 the SDK did not associate each session ID with a specific owner/origin (e.g., no binding to a client certificate, auth token, or connection fingerprint). An attacker who acquires a session ID—through network sniffing, log exposure, referrer leakage, or a shared/misconfigured proxy—can replay it in their own tools/call request and have it processed as if it originated from the legitimate session owner. This crosses the trust boundary between the MCP client and server: the server treats the bearer of a session ID as fully authorized, enabling unauthorized invocation of tools with the victim's permissions and potential access to any resources those tools can reach. The vulnerability is a session-management/authorization flaw rather than a flaw in any specific tool, but its impact scales with whatever tools are exposed via the compromised session.
Affected Systems
mcp (Ruby SDK / mcp gem); protocols: MCP
Detection Signatures
- Unexpected tools/call requests from new/unexpected IP addresses reusing a known session ID
- Session IDs appearing in server logs, proxy logs, or URLs (potential leakage vector)
- Multiple concurrent connections using an identical MCP session ID from differing client fingerprints/user agents
- Anomalous tool invocation patterns inconsistent with the historical behavior of a given session owner
Remediation Steps
- 1
Upgrade mcp gem
Update the Ruby MCP SDK to version 0.23.0 or later, which binds session IDs to their owning session/connection.
- 2
Rotate and invalidate existing sessions
Force expiration of all active session IDs issued by pre-patch servers to eliminate any already-leaked or stolen tokens.
- 3
Bind sessions to strong client context
Where possible, layer additional binding such as mTLS client certs, IP pinning, or short-lived signed tokens tied to the session to reduce impact of ID leakage even post-patch.
- 4
Avoid session ID exposure
Ensure session IDs are never logged, placed in URLs, or sent via referrer-leaking channels; use secure, HttpOnly cookies or headers instead.
- 5
Monitor for anomalous session reuse
Implement detection for a single session ID being used from multiple distinct network origins or client fingerprints in a short window.
CVE / Advisory IDs
Industries Most Exposed
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