Uploaded files pose a serious threat to apps. The initial stage in many attacks is to inject code into the system being targeted. The attack then simply has to find a means to execute the code. The use of a file upload aids the attacker in completing the first stage.
Unrestricted file upload can result in a variety of outcomes, including total system takeover, an overloaded file system or database, forwarding assaults to back-end systems, client-side attacks, or simple defacement. It is determined by what the program does with the submitted file and, more importantly, where it is kept.
There are two types of difficulties here. The first issue is with file information, such as the location and file name. HTTP multi-part encoding, for example, is often given by the transport. This data may lead to the program overwriting a vital file or storing the file in an incorrect location. Before utilizing the metadata, it must be thoroughly validated.
The other type of issue is one with file size or content. The spectrum of issues here is totally dependent on what the file is used for. See the samples below for some instances of how files might be abused. To defend against this form of attack, you should carefully examine everything your application does with files and consider what processors and interpreters are involved.
Factors of Risk
- This vulnerability has a large effect because the alleged code may be run either on the server or on the client side. The offender faces a significant risk of being discovered. The frequency is widespread. As a result, the seriousness of this form of vulnerability is severe.
- To fully assess the risks, it is critical to study the access controls of a file upload module.
- Server-side attacks: A web server can be exploited by uploading and executing a web-shell that can perform commands, view system files, browse local resources, attack other servers, or exploit local vulnerabilities, among other things.
- Client-side assaults: When malicious files are uploaded, the website becomes subject to client-side attacks like XSS or Cross-site Content Hijacking.
- When a file on the same or a trustworthy server is required, uploaded files might be utilized to exploit other weak areas of an application (can again lead to client-side or server-side attacks)
- Client-side vulnerabilities in broken libraries/applications may be triggered via uploaded files (e.g. iPhone MobileSafari LibTIFF Buffer Overflow).
- Uploaded files may cause vulnerabilities in broken libraries/applications on the server side (for example, the ImageMagick issue known as ImageTragick!).
- Uploaded files may exploit flaws in out-of-date real-time monitoring programs (e.g. Symantec antivirus exploit by unpacking a RAR file)
- A malicious file, such as a Unix shell script, a Windows virus, an Excel file containing a harmful formula, or a reverse shell, might be uploaded to the server in order for an administrator or webmaster to execute code later — on the victim’s PC.
- An attacker may be able to insert a phishing page or deface the website.
- The file storage server might be misused to store harmful files such as viruses, illicit software, or pornographic material. Uploaded files may also contain malware command and control data, messages of violence and harassment, or steganographic data that criminal organizations can employ.
- Unauthorized individuals may have access to sensitive files that have been uploaded.
- Internal information, such as server internal routes, may be disclosed in error messages by file uploaders.
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