AI-Generated Path Traversal Fix Leaves Symlink Vulnerability Intact, Researchers Warn

A developer used the Cursor AI coding assistant to fix a path traversal vulnerability (CWE-22) in a Node.js file download endpoint, receiving a solution that used path.basename, path.resolve, and a startsWith containment check. While the fix looks correct, it operates entirely on strings and never consults the filesystem, meaning it cannot detect symbolic links pointing outside the intended directory. If a file inside the upload folder is a symlink to a sensitive path like /etc/passwd, the handler will serve it while all security checks pass. This same flaw was exploited in the real-world CVE-2026-40931 vulnerability in the compressing npm package, which had a structurally identical patch that researchers bypassed using a pre-existing symlink. The recommended fix is to use fs.realpath before the containment check, ensuring the comparison reflects the file's actual resolved location on disk.