CVE-2026-45018 in Chainlit
Summary
by MITRE • 08/25/2026
Chainlit is a Python framework for building production-ready conversational AI applications. From 2.4.0rc0 until 2.12.0, Chainlit deployments with features.mcp.enabled set to true in .chainlit/config.toml expose the POST /mcp endpoint without requiring authentication. For stdio transport, the endpoint accepts a user-controlled fullCommand string. The validate_mcp_command() function in backend/chainlit/mcp.py checks only the executable name against config.features.mcp.stdio.allowed_executables and passes unchecked arguments to StdioServerParameters in backend/chainlit/server.py. Because npx supports the -c argument, an attacker can execute arbitrary shell commands with the privileges of the Chainlit process. If allowed_executables is unset, its None default is treated as allowing every executable. This issue is fixed in version 2.12.0.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in Chainlit versions from 2.4.0rc0 through 2.12.0 represents a critical authentication bypass and command injection flaw within the Model Context Protocol (MCP) integration feature. When the configuration flag features.mcp.enabled is set to true, the application exposes an unauthenticated POST endpoint at /mcp. This lack of access control allows any external actor to interact with the backend service without providing valid credentials or session tokens. The severity of this exposure is compounded by how the system processes incoming requests for stdio transport mechanisms. Specifically, the endpoint accepts a user-controlled parameter named fullCommand, which is intended to define the execution context for MCP tools but lacks sufficient validation constraints on its contents beyond basic structural checks.
The core technical flaw resides in the validate_mcp_command function located within backend/chainlit/mcp.py. This function performs an allowlist check that verifies only the executable name against a configuration-defined list of allowed executables found at config.features.mcp.stdio.allowed_executables. Crucially, while the binary path is validated, all subsequent arguments passed via the fullCommand string are forwarded directly to StdioServerParameters without any sanitization or validation. This design oversight creates a classic command injection vector because many standard utilities, particularly npx (Node Package Execute), support flags that allow for arbitrary code execution. For instance, npx accepts the -c argument, which instructs it to execute a provided string as a shell command rather than running a specific package script. By passing unchecked arguments through this parameter, an attacker can leverage such features to inject and execute arbitrary operating system commands with the same privileges as the Chainlit process itself.
The impact of this vulnerability is severe, effectively granting remote code execution capabilities to unauthenticated attackers on systems where the vulnerable configuration is active. If the allowed_executables list remains unset or defaults to None, the validation logic treats this absence as an implicit allow-all policy for executable names. This means that even if a developer attempts to restrict access by not specifying an allowlist, they inadvertently enable unrestricted command execution capabilities through any supported transport mechanism like npx. The attacker can exploit this to exfiltrate sensitive data stored within the application environment, pivot into internal networks accessible from the host machine, or compromise other services running on the same infrastructure. This aligns with CWE-78 Improper Neutralization of Special Elements used in an OS Command and CWE-287 Improvement of Privileges due to Incorrect Authentication, as well as MITRE ATT&CK techniques related to Remote Code Execution via Client-Side Scripting or System Command Injection depending on the specific deployment context.
To mitigate this risk, organizations must immediately upgrade Chainlit to version 2.12.0 or later, where these validation gaps have been addressed and proper input sanitization is enforced for all command arguments. In environments where upgrading is not immediately feasible, a temporary mitigation involves explicitly configuring features.mcp.stdio.allowed_executables with a strict allowlist of only the specific binaries required for legitimate operations, ensuring that dangerous utilities like npx are excluded from this list. Additionally, deploying the application behind an API gateway or reverse proxy that enforces authentication and rate limiting on the /mcp endpoint can provide a layer of defense in depth, although it does not replace the need to patch the underlying vulnerability within the framework itself.