File Upload Security Check (A04:2025)

What this checks

Protects against unrestricted file upload attacks where an attacker uploads executable files (web shells, scripts, HTML with embedded JS) that the server later serves or executes. Exploitation leads to remote code execution, stored XSS, or full server compromise.

Vulnerable patterns

• Upload handler that writes the user-supplied filename directly to disk, enabling path traversal and extension bypass
• Upload destination inside the web-accessible document root, so uploaded content is served directly by the web server
• No extension allowlist, or an extension denylist that misses novel or case-varied extensions
• No upload-size cap enforced at the framework or reverse-proxy level
• MIME type taken from the client-supplied Content-Type header rather than the file's magic bytes

Fix immediately

Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties:

1. Extension is validated against an allowlist, never a denylist. Denylists miss novel extensions (.phtml, .phar, .svg) and case variations. An allowlist of expected types (.png, .jpg, .pdf) fails closed.
2. The user-supplied filename is never used as the storage path. Rename the file to a CSPRNG identifier, preserving only the validated extension. This defeats path traversal (../../etc/passwd), overwrites of existing files, and filename-based XSS on download.
3. Uploads are written outside the webroot. The server must not serve them directly — files are handed out through an application route that sets a safe Content-Type and Content-Disposition: attachment, never inferred from the filename.
4. Size is capped at the framework or proxy level, not just inside the handler. A handler-only check lets a multi-gigabyte upload exhaust memory before the check runs.
5. MIME type is validated against the file's magic bytes, not the Content-Type header the client sent — the header is attacker-controlled.

Translate these principles to the audited file's language and framework. Use the framework's documented upload-size limit, storage path configuration, and content-type sniffing helper — do not hand-roll path joining or extension parsing.

Verification

Confirm the following properties hold (language-agnostic):

• Only explicitly-allowed file extensions are accepted — enforced via an allowlist, not a denylist
• The user-supplied filename is never used as the storage path — files are renamed to a random or hashed identifier
• Uploaded files are stored outside the web-accessible document root
• A maximum upload size is enforced at the framework or reverse-proxy level
• Files are served with Content-Disposition: attachment and a safe, explicit Content-Type — never inferred from the filename

References

• CWE-434 (Unrestricted Upload of File with Dangerous Type)
• CWE-22 (Path Traversal)
• OWASP A04:2025 Insecure Design