CVE-2026-67432 in ruby-sdk
Summary
by MITRE • 07/29/2026
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 reads and parses an entire JSON-RPC POST body without a size limit, allowing an unauthenticated remote attacker to exhaust process memory. This issue is fixed in version 0.23.0.
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Analysis
by VulDB Data Team • 07/29/2026
The MCP Ruby SDK represents a critical component in implementing Model Context Protocol servers and clients, serving as the official ruby implementation for handling communication between various AI model contexts. This particular vulnerability affects the StreamableHTTPTransport class within the mcp gem, which processes incoming JSON-RPC POST requests through the HTTP transport mechanism. The flaw resides in the transport layer's handling of request bodies where it reads and parses entire JSON payloads without implementing any size restrictions or memory limits. This design oversight creates a fundamental security weakness that allows remote attackers to exploit the system by submitting excessively large JSON-RPC payloads.
The technical implementation of this vulnerability stems from the absence of input validation mechanisms specifically designed to limit the size of incoming HTTP request bodies. When the StreamableHTTPTransport processes POST requests, it consumes the complete request body into memory before performing any parsing operations, leaving the application susceptible to memory exhaustion attacks. This behavior directly violates secure coding practices that mandate input sanitization and resource limitation controls. The vulnerability manifests as a denial of service condition where an attacker can submit malformed payloads designed to consume excessive system resources, potentially leading to process termination or system instability.
The operational impact of this vulnerability extends beyond simple denial of service scenarios into more serious security implications for systems relying on the MCP Ruby SDK. Remote unauthenticated attackers can exploit this weakness without requiring any credentials or prior access to the system, making it particularly dangerous in publicly accessible environments. The memory exhaustion attack can cause significant disruption to services that depend on the MCP protocol, potentially affecting multiple concurrent users or applications that utilize the affected transport mechanism. This vulnerability directly maps to CWE-400 which addresses "Uncontrolled Resource Consumption" and aligns with ATT&CK technique T1499.200 focusing on "Resource Hijacking" through excessive memory consumption.
Organizations utilizing versions of the mcp gem prior to 0.23.0 should immediately implement mitigation strategies including upgrading to the patched version that introduces proper size limiting for JSON-RPC request bodies. The fix implemented in version 0.23.0 addresses the core issue by introducing configurable limits on the maximum size of incoming JSON payloads, preventing attackers from submitting excessively large requests that could exhaust system resources. Additional defensive measures include implementing network-level rate limiting and monitoring for unusual request sizes, though these represent supplementary protections rather than primary fixes to the vulnerability itself. System administrators should also consider deploying intrusion detection systems capable of identifying and blocking suspicious request patterns that may indicate exploitation attempts against this specific vulnerability.