CVE-2026-66063

## Summary The multipart upload filename fix splits on the path separator but never rejects dot-dot, allowing a write outside the served tree. ## Finding (Medium): upload filename escapes the served tree (residual of CVE-2026-35393) The multipart filename fix (updown.go lines 135-136) splits on the path separator but never rejects "..". Uploading with filename=".." results in os.Create against the parent of the upload folder with a trailing marker character, outside the served tree, and the subsequent failed rename leaves that file behind. Verified: a file containing ESCAPED_WRITE_PROOF was written outside the webroot, unauthenticated, with the default configuration. Not claimed: a Windows-specific variant (Go's Part.FileName() already applies filepath.Base). ## Suggested fixes 1. Reject any upload filename that is "..", is empty after sanitisation, or resolves outside the upload folder; validate the final resolved destination rather than only transforming the input. ## Tooling AI assistance was used while investigating. The finding was reproduced against a running server on loopback.