# Document-Converter > Free converter for everyone.\ > You find the flag at : /flag\ > **Link:** We are given a black-box web challenge. This application allows us to upload files in various formats (.doc, .jpg, etc.) and converts them into a PDF for us to download. ![](/files/yPzkpHWQr5f9wtDVAMma) The first thing that came to mind was whether I can upload arbitrary HTML, since HTML has plenty of potential SSRF / file inclusion vectors. Sure enough, when I uploaded the following HTML file, I got a callback to my server. ```markup Hello world. ``` The `User-Agent` showed that LibreOffice was making the callback. ```http OPTIONS /exploit.css HTTP/1.1 Host: a255-42-60-216-15.ngrok.io User-Agent: LibreOffice Cache-Control: no-cache Pragma: no-cache X-Forwarded-For: 20.233.9.240 X-Forwarded-Proto: http Accept-Encoding: gzip ``` Interesting! So LibreOffice is being used to convert the documents. I searched around a bit and came across [this writeup](https://www.l0l.xyz/sec/2021/01/05/1-webdesktop-root-ssrf.html) on SSRF using LibreOffice documents. We create a sample LibreOffice word document, `poc.odt`. After unzipping the ODT file, we can modify the `content.xml` file to include our payload. We create a `text:section` tag that links to the `/flag` file. ```markup ...

``` Then, zipping the files again into a `modified.odt` gives us our payload. Uploading this to the server gives us the flag! ![](/files/fduBgeA2uDNml0ZcQ3zN) --- # 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://ctf.zeyu2001.com/2022/securinets-ctf-quals-2022/document-converter.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.