RustFS is a distributed object storage system built in Rust. In 1.0.0-beta.4, authenticated users with only PutObject permission on their own bucket can exploit a path traversal vulnerability in the Snowball auto-extract feature to write arbitrary objects into other users' buckets, completely breaking multi-tenant isolation. The vulnerability chains three flaws: No ../ sanitization in tar entry key normalization; IAM wildcard matching uses raw (uncleaned) paths; and Filesystem path cleaning resolves ../ across bucket boundaries.