Upload Insecure Files

SKILL: Upload Insecure Files — Validation Bypass, Storage Abuse, and Processing Chains

AI LOAD INSTRUCTION: Expert file upload attack playbook. Use when the target accepts files, imports, avatars, media, documents, or archives and you need the full workflow: validation bypass, storage path abuse, post-upload access, parser exploitation, multi-tenant overwrite, and chaining into XSS, XXE, CMDi, traversal, or business logic impact. For web server parsing vulnerabilities, PUT method exploitation, and specific CVEs (WebLogic, Flink, Tomcat), load the companion SCENARIOS.md.

0. RELATED ROUTING

Extended Scenarios

Also load SCENARIOS.md when you need:

• IIS parsing vulnerabilities — x.asp/ directory parsing, ; semicolon truncation (shell.asp;.jpg)
• Nginx parsing misconfiguration — avatar.jpg/.php with cgi.fix_pathinfo=1
• Apache parsing — multiple extensions, AddHandler, CVE-2017-15715 \n (0x0A) bypass
• PUT method exploitation — IIS WebDAV PUT+COPY, Tomcat CVE-2017-12615 readonly + .jsp/ bypass
• WebLogic CVE-2018-2894 arbitrary file upload via Web Service Test Page
• Apache Flink CVE-2020-17518 file upload with path traversal
• Upload + parsing vulnerability chain — EXIF PHP code + Nginx /.php path info
• Full extension bypass reference table (PHP/ASP/JSP alternatives, case variations, null bytes)

Use this file as the deep upload workflow reference. Also load:

• path traversal lfi when filename, extraction path, or include path becomes file-system control
• xss cross site scripting when uploads are rendered in browser contexts
• xxe xml external entity when SVG, OOXML, or XML imports are accepted
• cmdi command injection when a processor, converter, or media pipeline executes system tools
• business logic vulnerabilities when quotas, overwrite rules, approvals, or storage paths create logic bugs
• ghost-bits-cast-attack when the server is Apache Tomcat and the WAF blocks .jsp in filename* — Tomcat's RFC2231Utility narrows each char to byte, so 1.陪sp (U+966A low byte = j) writes 1.jsp to disk while the WAF sees no .jsp literal

1. CORE MODEL

Every upload feature should be tested as four separate trust boundaries:

1. Accept: what validation happens before the file is stored?
2. Store: where is the file written and under what name and permissions?
3. Process: what background tools, converters, scanners, parsers, or extractors touch it?
4. Serve: how is it later downloaded, rendered, transformed, or shared?

Many targets validate only one stage. The bug usually appears in a different stage than the one where the file was uploaded.

2. RECON QUESTIONS FIRST

Before payload selection, answer these:

• Which extensions are allowed, denied, or normalized?
• Does the backend trust extension, MIME type, magic bytes, or all three?
• Is the file renamed, transcoded, unzipped, scanned, or re-hosted?
• Is retrieval direct, proxied, signed, or served from a CDN?
• Can one user predict or overwrite another user's file path?
• Do filenames, metadata, or previews reflect back into HTML, logs, admin consoles, or PDFs?

3. VALIDATION BYPASS MATRIX

Representative bypass families:

shell.php.jpg
avatar.jpg.php
file.asp;.jpg
file.php%00.jpg
file.svg
archive.zip

This small sample set already covers the main use cases of the former standalone upload payload helper, so no extra entry is needed for first-pass selection.

Do not stop at upload success. Successful upload without dangerous retrieval or processing is not enough.

4. STORAGE AND RETRIEVAL ABUSE

Predictable or controllable paths

Look for patterns like:

/uploads/USER_ID/avatar.png
/files/org-slug/report.pdf
/cdn/tmp/<uuid>/<filename>

Test for:

• cross-tenant read by guessing IDs, slugs, or UUID patterns
• overwrite by reusing another user's filename
• path normalization bugs in filename or archive members
• private file exposed through direct object URL despite UI-level access control

Filename-based injection surfaces

A safe file can still be dangerous if the filename is reflected into:

• gallery HTML
• admin moderation panels
• PDF/CSV export jobs
• logs, audit views, or email notifications

If filename is reflected, treat it like stored input, not like passive metadata.

5. PROCESSING-CHAIN ATTACKS

The highest-value upload bugs often live in asynchronous processors.

Common processor classes

What to prove

1. The file is touched by a processor.
2. The processor behaves differently from the upload validator.
3. That difference creates impact: read, execute, overwrite, SSRF, or stored client-side execution.

6. HIGH-VALUE EXPLOITATION PATHS

Browser execution

• SVG served as active content
• HTML or text uploads rendered inline
• EXIF or filename reflected into an HTML page

XML and document parsing

• SVG XXE for file read or SSRF
• OOXML import for XML entity or parser abuse
• CSV import for formula execution in analyst workflows

Server-side execution or file-system impact

• image or document converter invoking shell tools
• zip slip writing outside intended directory
• upload-to-LFI chain where uploaded content later becomes includable

Access-control and sharing bugs

• private upload accessible via predictable URL
• moderation or quarantine path still publicly reachable
• one user replacing another user's public asset

7. AUTHORIZATION AND BUSINESS LOGIC CHECKS

Upload features frequently hide non-parser bugs:

• upload quota enforced in UI but not API
• plan restrictions checked on upload page but not on import endpoint
• file ownership checked on list view but not on direct download or replace endpoint
• approval workflow bypassed by calling the final storage endpoint directly
• delete or replace action missing object-level authorization

When the upload path includes account, project, or organization identifiers, always run an A/B authorization test.

8. TEST SEQUENCE

1. Upload one benign marker file and map rename, path, and retrieval behavior.
2. Try one validation-bypass sample and one active-content sample.
3. Check whether retrieval is attachment, inline render, transformed preview, or background processing.
4. If processing exists, pivot by processor family: XSS, XXE, CMDi, zip slip, or SSRF.
5. Run tenant-boundary and overwrite tests on file IDs, replace endpoints, and public URLs.

9. CHAINING MAP