CVE-2026-63119 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::StdioTransport and MCP::Client::Stdio in the mcp gem use IO#gets without a byte limit, allowing a peer that sends data without a newline 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 security vulnerability within the Model Context Protocol ecosystem that affects both server and client implementations. This vulnerability resides in the stdio transport mechanisms of the mcp gem, specifically impacting the MCP::Server::Transports::StdioTransport and MCP::Client::Stdio classes. The flaw stems from improper input handling where IO#gets method is invoked without establishing any byte limit for data reception, creating a potential memory exhaustion attack vector that can be exploited by malicious actors.
The technical nature of this vulnerability aligns with CWE-770, which addresses allocation of resources without proper limits or checks. When a peer sends data through stdio transport without including newline characters, the IO#gets method continues reading indefinitely until it encounters a line terminator or exhausts available memory. This behavior creates a denial of service condition where legitimate processes can be terminated due to memory exhaustion, effectively preventing normal system operations. The vulnerability specifically affects versions prior to 0.23.0, indicating that developers using older iterations of the gem remain at risk.
From an operational perspective, this vulnerability presents significant risk to systems relying on the Model Context Protocol for AI model interactions and contextual processing. Attackers could exploit this weakness by establishing connections to MCP servers or clients and sending large amounts of data without line terminators, causing memory consumption to spiral uncontrollably. The impact extends beyond simple service disruption as it can lead to system instability, application crashes, and potential compromise of underlying infrastructure. This type of vulnerability particularly affects containerized environments where memory limits are strictly enforced, making it even more dangerous in cloud-native deployments.
The fix implemented in version 0.23.0 addresses this by introducing proper bounds checking on data reception through the stdio transport mechanisms. This mitigation aligns with security best practices recommended by the ATT&CK framework under T1499.004 for Resource Exhaustion and T1566.002 for Pre-Attack Phase techniques related to input validation. Organizations should immediately update their MCP Ruby SDK implementations to version 0.23.0 or later, while also implementing additional monitoring for unusual memory consumption patterns in processes utilizing stdio transport. Security teams should conduct vulnerability assessments of all systems using older versions and consider implementing network-level controls to limit data transfer sizes when direct access to stdio endpoints cannot be immediately secured.
The broader implications of this vulnerability highlight the importance of input validation and resource management in distributed systems, particularly those handling AI model contexts where memory efficiency and process stability are paramount. This issue demonstrates how seemingly minor implementation details in standard library methods can create significant security risks when not properly bounded or validated against malicious inputs.