Reflink is a copy-on-write filesystem feature that lets two files share the same disk blocks until one of them changes. Instead of duplicating data immediately, the filesystem points both files at the same stored blocks and creates a private copy only when a write occurs.
This matters for security because reflink depends on careful bookkeeping. If reference counts, copy-on-write handling, or race conditions are wrong, a write can land on the wrong block or corrupt shared data. That can become a privilege escalation, data tampering, or integrity-loss issue on filesystems that support reflinks. Defenders should know whether reflink is enabled on production volumes, because the attack surface is tied to filesystem layout as much as to the kernel version. In practice, reflink bugs are often triggered by concurrent writes, direct I/O, or other paths that stress block sharing logic, so patching and filesystem-aware testing are important.