Unrestricted File Upload
Summary#
Invicti detected an unrestricted file upload, which allows users to upload files to the web server.
If one of the uploaded files result a code execution, Invicti will report it as a separate issue.
Impact#
The consequences of unrestricted file upload can vary, including complete system takeover, an overloaded file system or database, forwarding attacks to back-end systems, and simple defacement. It depends on what the application does with the uploaded file and especially where it is stored. Here is the list of attacks that the attacker might do:
- Compromise the web server by uploading and executing a web-shell which can run commands, browse system files, browse local resources, attack other servers, and exploit the local vulnerabilities, and so forth.
- Put a phishing page into the website.
- Put a permanent XSS into the website.
- Bypass cross-origin resource sharing (CORS) policy and exfiltrate potentially sensitive data.
- Upload a file using malicious path or name which overwrites critical file or personal data that other users access. For example; the attacker might replace the
.htaccessfile to allow him/her to execute specific scripts.
Remediation#
- Never accept a filename and its extension directly without having a white-list filter.
- If there is no need to have Unicode characters, it is highly recommended to only accept alpha-numeric characters and only one dot as an input for the file name and the extension.
- Limit the file size to a maximum value in order to prevent denial of service attacks.
- Uploaded directory should not have any "execute" permission.
- Don't rely on client-side validation only.
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