Security Review Pattern for File Upload Endpoints
A practical review sequence for extension filtering, MIME validation, content sniffing, and asynchronous malware scanning.
Summary
File upload features fail in predictable ways. This guide presents a repeatable review pattern that surfaces dangerous parser paths early.
Review Sequence
- Map upload entry points and storage destinations.
- Test extension allowlist bypasses (
.jpg.php, double extensions, Unicode tricks). - Compare declared MIME type, sniffed MIME type, and storage metadata.
- Validate post-upload processing jobs for command injection risks.
Minimal Validation Snippet
allowed = {"image/png", "image/jpeg"}
if detected_mime not in allowed:
raise ValidationError("Unsupported file type")Always validate content, not just extension strings.