add readme for content-type bypass and input validation

content-type_bypass/README.md

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+# Content-Type Bypass 
+
+## How We Found It
+We found this page on the home page, at the bottom there is an Add image button. on this page we can upload a file.  
+what happens if we try to upload a normal image ?  
+```
+/tmp/JavaScript-logo.jpg succesfully uploaded.
+```
+intresting. the site tells me where the file is uploaded which might not be a good idea. what if I upload a script and somehow manage to make the server execute it.
+Let's try to send a random file I have.
+```
+Your image was not uploaded.
+```
+:( why doesn't it accept my file. is it the extention ? let's try to rename it  
+```
+/tmp/output.jpg succesfully uploaded.
+```
+it is uploaded but I don't get anything. maybe I need to do something else, how would it be executed if it doesn't have the right extention.
+let's modify the headers and request content using curl.
+
+first, we're going to try to upload a file normally using curl with this command  :
+```
+curl -X POST -F "uploaded=@JavaScript-logo.jpg" -F Upload=Upload "http://192.168.56.101/?page=upload"
+```
+it worked, nice now let's try to modify the content-type settings in the `uploaded` field with this command
+```
+curl -X POST -F "uploaded=@output.path;type=image/jpeg" -F Upload=Upload "http://192.168.56.101/?page=upload"
+```
+it worked and I got the flag.
+
+## Utility of It
+- Allows an attacker to upload and possibly execute malicious files (e.g., scripts or binaries) on the server.
+- Can lead to Remote Code Execution (RCE) if the file is interpreted by the server.
+- Can expose internal paths or enable path traversal, depending on how uploaded files are handled.
+- Dangerous in CTFs and real-world apps alike.
+
+## How Can We Patch It
+- Strictly validate file types server-side, not just by extension or content-type header.
+- Use MIME sniffing + magic byte checks to confirm file content matches expected type.
+- Rename uploaded files and store them in non-executable directories.
+- Disallow direct access to uploaded files unless absolutely necessary.
+- If uploads are needed, serve them via a proxy or from a CDN that does not allow execution.