CVE-2021-33496 in transfer.sh
Summary
by MITRE • 05/24/2021
Dutchcoders transfer.sh before 1.2.4 allows XSS via an inline view.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/27/2021
The vulnerability identified as CVE-2021-33496 affects transfer.sh versions prior to 1.2.4, representing a cross-site scripting vulnerability that exploits the application's handling of inline view functionality. This issue specifically manifests when users interact with files that are displayed inline within the browser environment, creating a potential attack vector for malicious actors to execute arbitrary scripts in the context of authenticated users' browsers. The vulnerability resides in the application's insufficient input validation and output encoding mechanisms, which fail to properly sanitize user-supplied data before rendering it in web responses.
The technical flaw stems from the application's inadequate sanitization of file names and content when generating inline views, allowing maliciously crafted input to be interpreted as executable JavaScript code rather than plain text or markup. This occurs because the system does not properly escape or encode special characters in user-provided content before displaying it in HTML contexts, creating a classic XSS vulnerability. The vulnerability is classified under CWE-79 as a failure to sanitize user input, specifically manifesting as reflected cross-site scripting where the malicious payload is embedded in the application's response to a user request. Attackers can leverage this weakness by uploading files with malicious names or content that, when viewed inline, executes unauthorized scripts against users who access the affected transfer.sh instance.
The operational impact of this vulnerability extends beyond simple script execution, as it can enable attackers to steal session cookies, perform unauthorized actions on behalf of users, or redirect victims to malicious websites. Since transfer.sh is commonly used for file sharing and collaboration, the attack surface is broad, potentially affecting organizations that rely on the service for legitimate file transfers. The vulnerability is particularly concerning in enterprise environments where users might upload files containing malicious content, and the inline viewing feature is frequently used for previewing documents. This type of vulnerability can be exploited through various attack vectors including social engineering campaigns where users are tricked into clicking on malicious links, or through automated scanning tools that identify vulnerable endpoints. The threat is further amplified by the fact that the vulnerability affects the application's core functionality, making it difficult to mitigate without proper input validation and output encoding mechanisms.
Mitigation strategies for CVE-2021-33496 should focus on implementing proper input validation and output encoding mechanisms that prevent user-supplied data from being interpreted as executable code. Organizations should immediately upgrade to transfer.sh version 1.2.4 or later, which includes fixes for the XSS vulnerability through improved sanitization of inline view content. The recommended approach involves implementing comprehensive input validation that rejects or sanitizes potentially malicious characters, combined with proper output encoding when rendering user-provided content in HTML contexts. Security measures should also include content security policy headers that restrict script execution and prevent unauthorized code injection. Organizations using transfer.sh should also consider implementing additional security controls such as file type restrictions, mandatory virus scanning, and regular security assessments to prevent similar vulnerabilities from emerging in other components of their file sharing infrastructure. The vulnerability demonstrates the importance of implementing defense-in-depth strategies and proper secure coding practices as outlined in the OWASP Top Ten and MITRE ATT&CK framework, particularly focusing on the execution of malicious code through web-based attack vectors that leverage user trust and browser functionality.