CVE-2026-67431 Overview
CVE-2026-67431 is a broken access control vulnerability [CWE-284] in the MCP Ruby SDK, the official Ruby implementation for Model Context Protocol (MCP) servers and clients. The flaw exists in MCP::Server::Transports::StreamableHTTPTransport within the mcp gem prior to version 0.23.0. The transport does not bind a session identifier to a specific session owner. An attacker who obtains a valid session ID can send tools/call requests that execute inside the victim's session context. Version 0.23.0 fixes the issue by enforcing session ownership validation.
Critical Impact
A stolen or leaked session ID allows an unauthenticated network attacker to invoke tools and take actions as the legitimate session owner, undermining the integrity of MCP tool execution.
Affected Products
- MCP Ruby SDK (mcp gem) versions prior to 0.23.0
- Ruby applications using MCP::Server::Transports::StreamableHTTPTransport
- Model Context Protocol servers and clients built on the affected SDK
Discovery Timeline
- 2026-07-29 - CVE-2026-67431 published to NVD
- 2026-07-30 - Last updated in NVD database
Technical Details for CVE-2026-67431
Vulnerability Analysis
The MCP Ruby SDK exposes an HTTP-based streamable transport that maintains state across requests using session identifiers. Sessions carry authentication context and tool invocation privileges. The StreamableHTTPTransport class treats possession of a session ID as sufficient proof of identity, without binding that ID to the principal who originally established the session.
An attacker who obtains a session ID through log exposure, referrer leakage, network interception, or client-side theft can present it to the server. The server accepts the identifier and honors any tools/call request the attacker sends, executing the tool invocation under the victim's session and privileges. This effectively converts session ID confidentiality into the sole access control boundary.
Root Cause
The root cause is missing authorization enforcement in the session lookup path. The transport locates the session by ID and proceeds without verifying that the requesting client matches the owner recorded at session creation. There is no cryptographic binding between the session token and an authenticated principal, transport-level identity, or client fingerprint. The patch in version 0.23.0 introduces owner binding so requests using a session ID are validated against the original session owner.
Attack Vector
Exploitation is remote and requires no privileges or user interaction beyond obtaining a session ID. The attacker sends a crafted HTTP request containing the stolen session ID and a JSON-RPC tools/call payload to the MCP server endpoint. The server dispatches the tool invocation as if the legitimate user submitted it. Consult the GitHub Security Advisory GHSA-5p9g-j988-pcwv and the remediation commit for technical specifics.
Detection Methods for CVE-2026-67431
Indicators of Compromise
- tools/call requests arriving with a valid session ID from a source IP or user-agent that differs from the session's original establishment context.
- Repeated session ID reuse across geographically or network-topologically inconsistent client endpoints within short time windows.
- Unexpected tool invocations in MCP server logs that do not correlate with legitimate user activity in the same session.
Detection Strategies
- Enable verbose logging on StreamableHTTPTransport and capture source IP, user-agent, and timestamps alongside every session ID reference.
- Correlate session establishment events with subsequent tools/call invocations and alert on client identity drift within a session.
- Instrument tool invocation handlers to record the principal executing each call and flag mismatches against the session owner.
Monitoring Recommendations
- Ingest MCP server access logs into a centralized SIEM and build detections for session ID reuse from divergent clients.
- Monitor gem inventory using dependency scanning to identify Ruby projects still pinned to mcp versions below 0.23.0.
- Alert on outbound actions initiated by MCP tools that fall outside baseline usage patterns for a given user.
How to Mitigate CVE-2026-67431
Immediate Actions Required
- Upgrade the mcp gem to version 0.23.0 or later in every Ruby application using the streamable HTTP transport.
- Invalidate all active MCP session IDs after upgrading to prevent reuse of tokens issued under the vulnerable code path.
- Audit MCP server logs for unexpected tools/call activity that may indicate prior exploitation attempts.
Patch Information
The fix is available in MCP Ruby SDK v0.23.0. The remediation commit 35466605 adds session owner binding so requests carrying a session ID are validated against the principal that originally created the session. Update the gem via bundle update mcp and confirm the resolved version in Gemfile.lock.
Workarounds
- Place the MCP server behind an authenticating reverse proxy that enforces per-user mutual TLS or bearer token validation before requests reach the transport.
- Restrict session ID transport to secure, HttpOnly, SameSite=Strict cookies and terminate sessions rapidly to reduce the window for theft.
- Disable the StreamableHTTPTransport where feasible and use an alternate transport until the upgrade is deployed.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
