Bypassing File Upload Filters with Garbage Character Padding to Get a Web Shell

Information Gathering

Using a search on Xiaohongshu for “某大学 xx 系统的密码是多少呀”, the author discovered a university system where the account is a student ID; logging in with a valid ID results in successful authentication.

Arbitrary File Upload

After logging in, the attacker navigates to the file upload page.

By uploading a .txt file that contains only garbage characters, the front‑end validation is bypassed.

The file is renamed to something like dddd…ddd.aspx, allowing it to be interpreted as an ASP page.

Further steps show how detection is evaded, the upload succeeds, and the file can be accessed and parsed to obtain a web shell.

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