Checkmate is an open-source, self-hosted tool designed to track and monitor server hardware, uptime, response times, and incidents in real-time with beautiful visualizations. Prior to 3.9.1, the public POST /api/v1/auth/register route in server/src/api/routes/authRoutes.ts passes multipart profileImage uploads through in-memory Multer parsing before registration validation, without file-size, file-count, or MIME-type limits in server/src/api/middleware/upload.ts. An unauthenticated attacker can submit concurrent oversized files that are buffered before invalid registration or invite-token checks reject the request, exhausting memory and crashing or destabilizing the backend. This issue is fixed in version 3.9.1.

CVE-2026-55241 affects Checkmate, an open-source self-hosted server monitoring tool, prior to version 3.9.1. The vulnerability exists in the public POST /api/v1/auth/register endpoint, which processes multipart profileImage uploads via in-memory Multer parsing before any registration validation occurs. The upload middleware in server/src/api/middleware/upload.ts lacks file-size, file-count, and MIME-type restrictions. An unauthenticated attacker can exploit this by submitting concurrent oversized file uploads, causing files to be buffered in memory before invalid registration or invite-token checks reject the request. This leads to memory exhaustion, potentially crashing or destabilizing the backend server. The vulnerability is fixed in Checkmate version 3.9.1.