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 — directory parsing, semicolon truncation ()
- Nginx parsing misconfiguration — with
- Apache parsing — multiple extensions, , CVE-2017-15715 (0x0A) bypass
- PUT method exploitation — IIS WebDAV PUT+COPY, Tomcat CVE-2017-12615 + 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 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
1. CORE MODEL
Every upload feature should be tested as four separate trust boundaries:
- Accept: what validation happens before the file is stored?
- Store: where is the file written and under what name and permissions?
- Process: what background tools, converters, scanners, parsers, or extractors touch it?
- 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
| Validation Style | What to Test |
|---|
| extension blacklist | double extension, case toggles, trailing dot, alternate separators |
| content-type only | mismatched multipart , browser vs proxy rewrite |
| magic-byte only | polyglot files or valid header plus dangerous tail content |
| server-side rename | whether dangerous content survives rename and later rendering |
| image-only policy | SVG, malformed image plus metadata, parser differential |
| archive or import only | zip contents, nested path names, XML members, decompression behavior |
Representative bypass families:
text
shell.php.jpg
avatar.jpg.php
file.asp;.jpg
file.php%00.jpg
file.svg
archive.zip
这组小样本已经覆盖了原本单独 upload payload helper 的主要用途，不再需要额外入口来做第一轮选型。
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:
text
/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
| Processor | Risk |
|---|
| image resizing or thumbnailing | parser differential, ImageMagick or library bugs, metadata reflection |
| video or audio transcoding | FFmpeg-style parsing and protocol abuse |
| archive extraction | zip slip, overwrite, decompression bombs |
| document import | CSV formula injection, office XML parsing, macro-adjacent workflows |
| XML or SVG parsing | XXE, SSRF, local file disclosure |
| HTML to PDF or preview rendering | SSRF, script execution, local file references |
| AV or DLP scanning | unzip depth, hidden nested content, race conditions |
What to prove
- The file is touched by a processor.
- The processor behaves differently from the upload validator.
- 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
- Upload one benign marker file and map rename, path, and retrieval behavior.
- Try one validation-bypass sample and one active-content sample.
- Check whether retrieval is attachment, inline render, transformed preview, or background processing.
- If processing exists, pivot by processor family: XSS, XXE, CMDi, zip slip, or SSRF.
- Run tenant-boundary and overwrite tests on file IDs, replace endpoints, and public URLs.
9. CHAINING MAP
| Observation | Pivot |
|---|
| SVG or XML accepted | xxe xml external entity |
| filename or metadata reflected | xss cross site scripting |
| converter or processor shells out | cmdi command injection |
| extraction path looks controllable | path traversal lfi |
| overwrite, quota, approval, or tenant bug | business logic vulnerabilities |
10. OPERATOR CHECKLIST
text
[] Confirm accept/store/process/serve stages separately
[] Test one extension bypass and one content-based payload
[] Check inline render vs forced download
[] Inspect filenames, metadata, and preview surfaces for reflection
[] Probe processing chain: image, archive, XML, document, PDF
[] Run A/B authorization on read, replace, delete, and share actions
[] Map predictable paths and public/private URL boundaries
11. UPLOAD SUCCESS RATE MODEL & ADVANCED METHODOLOGY
Success Rate Formula
P(RCE via Upload) = P(bypass_detection) × P(obtain_path) × P(execute_via_webserver)
Many testers focus only on bypassing file type checks, but forget:
- Path discovery: Without knowing the upload path, even a successful bypass is useless
- Server parsing: Even with a file uploaded, if the web server doesn't parse it as PHP, no RCE
Rich Text Editor Path Matrix
| Editor | Common Upload Path | Version Indicator |
|---|
| FCKeditor | /fckeditor/editor/filemanager/connectors/
| /fckeditor/_whatsnew.html
|
| CKEditor | | |
| eWebEditor | | Admin: /ewebeditor/admin_login.asp
|
| KindEditor | | /kindeditor/kindeditor.js
|
| UEditor | or | /ueditor/ueditor.config.js
|
Validation Defect Taxonomy (5 Dimensions)
| Dimension | Flaw Examples |
|---|
| Location | Client-side only, inconsistent front/back |
| Method | Extension blacklist (incomplete), MIME check only, magic bytes only |
| Logic order | Renames AFTER execution check, validates BEFORE full upload |
| Scope | Checks filename but not file content, checks first bytes only |
| Execution context | Upload succeeds but different vhost/handler processes the file |
Response Manipulation Bypass
# If server returns allowedTypes in response for client-side validation:
# Intercept response → modify allowedTypes to include .php → upload .php
# The server never actually validates — it trusts client filtering
IIS Semicolon Parsing
# IIS treats semicolon as parameter delimiter in filenames:
shell.asp;.jpg → IIS executes as ASP
# NTFS Alternate Data Stream:
shell.asp::$DATA → Bypasses extension check, IIS may execute
Apache Multi-Extension
# Apache parses right-to-left for handler:
shell.php.jpg → May execute as PHP if AddHandler php applies
# Newline in filename (CVE-2017-15715):
shell.php\x0a → Bypasses regex but Apache still executes as PHP
Nginx cgi.fix_pathinfo
# With cgi.fix_pathinfo=1 (PHP-FPM):
/uploads/image.jpg/anything.php → PHP processes image.jpg as PHP!
# Upload legitimate-looking JPG with PHP code embedded