What is unsanitized filename handling?

According to The Open Worldwide Application Security Project’s (OWASP's) analysis of unrestricted file upload vulnerabilities, file metadata like paths and filenames can "trick the application into overwriting a critical file or storing the file in a bad location" when not validated properly. This class of vulnerability, commonly known as unsanitized filename handling, represents a security gap that can lead to directory traversal attacks, code execution, and data breaches, particularly dangerous for healthcare organizations handling protected health information (PHI).

Learn more: What is a data breach?

Why filenames become weapons

Unsanitized filename handling vulnerabilities arise when developers trust user-provided filenames without implementing proper security controls. Healthcare applications frequently handle file uploads for patient records, medical imaging, lab results, and insurance documentation, creating numerous opportunities for exploitation if filename inputs aren't properly validated and sanitized.

OWASP consistently ranks improper input validation among the top security risks facing web applications. These vulnerabilities persist across platforms, with OWASP documenting extensive lists of bypass techniques including double extensions, null byte injection, and alternate data streams that attackers use to circumvent basic filename filters. For healthcare organizations, these vulnerabilities can expose patient data and violate HIPAA compliance requirements.

Go deeper: What is required for HIPAA compliance?

How filename attacks appear

When healthcare applications fail to sanitize filenames, attackers can craft malicious inputs that compromise patient data security:

• Directory traversal sequences: Attackers insert characters like "../" or ".." to navigate outside intended directories, potentially accessing sensitive patient files, configuration data, or HIPAA audit logs. According to CISA's 2024 Secure by Design alert, directory traversal vulnerabilities enabled threat actors to compromise over 2,000 internet-exposed instances across multiple campaigns, with attackers stealing "usernames, password hashes, and authentication tokens" from vulnerable systems. For healthcare organizations, such breaches could expose entire patient databases and authentication systems.
• Special characters injection: Malicious users embed characters that have special meaning to operating systems or applications, such as null bytes, semicolons, or pipe symbols that can alter command execution and expose PHI.
• Unicode and encoding tricks: Sophisticated attacks use alternative character encodings or Unicode normalization, which are special characters from different alphabets that appear identical to standard letters but have completely different computer codes, to bypass basic filtering mechanisms while accessing restricted medical records.
• File extension manipulation: Attackers may use double extensions, MIME type confusion, or case sensitivity differences to upload executable files disguised as harmless medical documents or DICOM images.
• Path length exploitation: Excessively long filenames can trigger buffer overflows or cause applications to shorten names in predictable ways that enable unauthorized access to patient databases.

The 2023 MOVEit Transfer data breach demonstrates the potential of file handling vulnerabilities in healthcare settings. According to the UK's National Cyber Security Centre (NCSC), criminals exploited a vulnerability in Progress Software's MOVEit file transfer application, which is "used by thousands of organisations around the world." The Cl0p ransomware group's attack critically relied on both SQL injection and unsanitized filename handling to achieve code execution.

The NCSC confirmed that "a number of organisations whose supply chains use the MOVEit app have suffered a data breach as a result, with customer and/or employee data being stolen." For healthcare organizations, this supply chain attack was particularly devastating, as many relied on MOVEit for transferring sensitive patient files between providers, insurers, and business associates. The attack's use of carefully crafted filenames that bypassed security filters allowed attackers to upload web shells and establish persistent backdoor access.

The MOVEit incident shows how seemingly minor oversights in filename validation can cascade into massive HIPAA breaches. As the NCSC noted, "Cyber attacks like this that target organisations' supply chains (rather than the organisation directly) are increasingly common," making secure filename handling a requirement for healthcare organizations that must share PHI across complex provider networks.

FAQs

What are web shells?

Web shells are malicious scripts that act like a remote control for a compromised server. Once uploaded through a file handling vulnerability, they give attackers a hidden backdoor to execute commands, steal data, or install ransomware, all through a web browser interface that looks like a normal administrative tool.

What is SQL injection?

SQL injection occurs when attackers insert malicious database commands into input fields (like filename fields) that aren't properly secured. It's like slipping extra instructions into a form that tricks the database into revealing or modifying data it shouldn't, potentially exposing entire patient record systems.

What is remote code execution (RCE)?

Remote code execution means attackers can run malicious programs on your server from anywhere in the world. In the context of filename vulnerabilities, RCE occurs when a poorly sanitized filename allows attackers to execute commands on the server, potentially taking complete control of healthcare systems.