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Security: Unsanitized client name used in User-Agent construction #379

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Security: Unsanitized client name used in User-Agent construction #379

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23 changes: 22 additions & 1 deletion tools/modelcontextprotocol/src/userAgent.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -25,12 +25,33 @@ export function extractClientName(message: {
return undefined;
}

const MAX_CLIENT_NAME_LENGTH = 128;
const CLIENT_NAME_ALLOWED = /[^a-zA-Z0-9 ._/-]/g;

/**
* Build the User-Agent string, appending the MCP client name if available.
*/
function sanitizeClientName(clientName: string): string | undefined {
const sanitized = clientName
.replace(/[\r\n]/g, '')
.replace(/[^\x20-\x7E]/g, '')
.replace(CLIENT_NAME_ALLOWED, '')
.trim()
.slice(0, MAX_CLIENT_NAME_LENGTH);

if (!sanitized) {
return undefined;
}

return sanitized;
}

export function buildUserAgent(clientName?: string): string {
if (clientName) {
return `${BASE_USER_AGENT} (${clientName})`;
const sanitizedClientName = sanitizeClientName(clientName);
if (sanitizedClientName) {
return `${BASE_USER_AGENT} (${sanitizedClientName})`;
}
}
return BASE_USER_AGENT;
}
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