How Hackers Exploit File Upload Vulnerabilities in Web Applications — Sai Kumar

Hackers often target file upload functionalities because they can use them to upload malicious files into the system. If the application does not properly validate or restrict file uploads, attackers can exploit this weakness and gain unauthorized access.

In today's web applications, file upload features are very common. We use them to upload profile pictures, documents, resumes, and many other files. While this feature looks simple from a user's perspective, it can become a serious security risk if not implemented properly.

File upload vulnerabilities are critical because they give attackers a direct entry point into the system. This makes it important for developers and security teams to handle file uploads carefully.

HOW IT WORKS

File upload vulnerabilities happen when an application does not properly check the type, size, or content of uploaded files.

Normally, a secure system should:

• Allow only specific file types (like .jpg, .png, .pdf)
• Scan files for malicious content
• Store files safely

However, when these checks are missing or weak, hackers take advantage of it. One common method is uploading a malicious script file (like a .php file). If the server executes this file, the attacker can run commands on the system. One common method is uploading a malicious script file (like a .php file). If the server executes this file, the attacker can run commands on the system.

Another method is bypassing file type restrictions. For example, an attacker might rename a file from shell.php to image.jpg.php or use special encoding tricks so the system fails to detect it.

Hackers can also upload files containing malware or reverse shells, which allow them to control the server remotely.

From what I understood, the main issue is not the upload feature itself, but how poorly it is implemented. Even a small mistake in validation can lead to serious consequences.

Case Study

1. Image Upload Exploit in Web Applications

Many web applications allow users to upload profile pictures. In some cases, developers only check the file extension and not the actual file content. Attackers take advantage of this by uploading a malicious script disguised as an image file. Once uploaded, they can access the file through a URL and execute it.

The system trusted the file extension without verifying the content. Just checking the file name is not enough. Proper validation and content checking are necessary.

2. Unrestricted File Upload in Vulnerable Websites

Some websites allow users to upload files without any restrictions. This is known as an unrestricted file upload vulnerability.

Attackers upload backdoor scripts or web shells, which give them full control over the server.

Response:

Security teams usually fix this by:

• Restricting file types
• Adding server-side validation
• Disabling execution in upload directories

Key Insight:

From this example, the unrestricted uploads are one of the easiest ways for attackers to compromise a system.

3: CVE-2025–23921 — WordPress Gravity Forms Multi Up loader

CVSS Score: 8.8 (High)

Product: Multi Uploader for Gravity Forms (WordPress Plugin)

Vulnerability Type: Unrestricted File Upload Attack

Vector: Remote, No Authentication Required

Discovered In: August 2024 (actively exploited before CVE was assigned) Official

Patch: Yes plugin update available

Step 1: Finding Vulnerable Sites

Attackers begin by identifying websites that may be susceptible to compromise. They often look for outdated software, vulnerable plugins, insecure file upload functionality, or misconfigured web applications that could provide an initial entry point into the server.

Step 2: Crafting the Webshell

Once a potential target is identified, attackers prepare a malicious script (webshell) designed to blend in with legitimate files. They may disguise or obfuscate the code to evade security controls and increase the chances of successful deployment on the target server.

Step 3: Uploading the Webshell

The attacker attempts to upload the malicious file by exploiting weaknesses in file upload mechanisms or vulnerable applications. If the upload succeeds and the file is stored in an executable location, it can become an entry point for unauthorized access to the server.

Step 4: Executing Commands

After gaining initial access, the attacker uses the webshell to interact with the compromised server, gathering information about the operating system, user permissions, installed applications, and system configuration. This reconnaissance helps determine the extent of access and identify opportunities for further compromise.

Step 5: Full Server Compromise

With persistent access established, the attacker may attempt to expand control over the environment by accessing sensitive configuration files, extracting application or database information, establishing additional persistence mechanisms, abusing server resources for malicious activities, moving laterally to other connected systems, and attempting to evade detection. At this stage, the impact can extend beyond a single application, potentially affecting the confidentiality, integrity, and availability of the entire server environment.

Why This Earned CVSS 8.8

The factors that made this critical:

Anyone on the internet could reach the upload endpoint. No account needed. No special tools needed just curl or a browser. The uploaded file could be anything, including executable code. And once the webshell was uploaded, the attacker had the same access level as the web server itself enough to read, write, and execute files.

The only thing preventing a 9.8 or 10.0 score was that the attacker needed to know the upload endpoint existed (which is not hard to find with scanning tools like WPScan)

Conclusion

File upload vulnerabilities may look like small issues, but they can lead to serious security problems if not handled properly. Attackers can use these vulnerabilities to upload malicious files, gain access to systems, and even take full control of servers.

From this topic, I understood that proper validation, secure storage, and regular security testing are very important in web applications. Developers should never trust user input and must always verify uploaded files carefully.

In my opinion, preventing such vulnerabilities is much easier than dealing with the consequences of an attack. With proper security practices, organizations can avoid many risks related to file uploads.

Key Points

• File upload features can be risky if not properly secured
• Hackers often upload malicious scripts to exploit systems
• Weak validation is the main reason for these vulnerabilities
• Proper checks and restrictions can prevent most attacks
• Security awareness is important for both developers and organizations.

Written By — Sai Kumar ( KIIS Intern)

Start your journey in Cybersecurity with KIIS call us now +919398514034.