Inappropriate file type control in Zscaler Proxy versions 3.6.1.25 and prior allows local attackers to bypass file download/upload restrictions.

During the summer of 2022, I have found a vulnerability affecting the ZScaler proxy (versions 3.6.1.25 and prior). This vulnerability would allow local attackers to bypass the restriction on downloads/uploads of password-protected archives using tools like Burp or even native Microsoft utilities like Bitsadmin, which relies on the Background Intelligent Transfer Service (BITS) protocol.

Per Microsoft’s documentation, the BITS protocol “defines a way to transfer large payloads from a client to an HTTP server or vice versa, even in the face of interruptions, by sending the payload in multiple fragments”. This allows to bypass restrictions based on file type as Zscaler is not able to properly reconstruct the file across multiple requests.

Although this proof of concept will focus only on the download aspect, this vulnerability applies to the uploads as well.