CVE-2019-9842 in MiniBlog
Summary
by MITRE
madskristensen MiniBlog through 2018-05-18 allows remote attackers to execute arbitrary ASPX code via an IMG element with a data: URL, because SaveFilesToDisk in app_code/handlers/PostHandler.cs writes a decoded base64 string to a file without validating the extension.
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Analysis
by VulDB Data Team • 10/05/2023
The vulnerability CVE-2019-9842 represents a critical server-side code execution flaw in the madskristensen MiniBlog web application version 2018-05-18 and earlier. This vulnerability stems from inadequate input validation within the file handling mechanism of the application's PostHandler component, which processes user-submitted content. The flaw specifically affects how the application handles image uploads through data URLs, creating a pathway for remote attackers to inject and execute arbitrary ASPX code on the server.
The technical implementation of this vulnerability occurs in the SaveFilesToDisk method located within app_code/handlers/PostHandler.cs. When users submit content containing an IMG element with a data: URL, the application decodes the base64 encoded content and writes it directly to disk without performing proper extension validation or sanitization. This allows attackers to craft malicious image tags that contain executable ASPX code within the base64 payload, bypassing normal file upload restrictions and security controls.
The operational impact of this vulnerability is severe and multifaceted. Remote attackers can leverage this flaw to gain full control over the affected web server, potentially leading to data breaches, system compromise, and further lateral movement within network environments. The vulnerability enables persistent backdoor access and can be exploited to establish command and control channels, making it particularly dangerous for organizations relying on this blogging platform. The attack vector is relatively simple to implement, requiring only the injection of specially crafted HTML content that can be processed by the vulnerable application.
This vulnerability maps directly to CWE-434, which describes "Unrestricted Upload of File with Dangerous Type," and aligns with ATT&CK technique T1190, "Exploit Public-Facing Application," as it targets a publicly accessible web application component. The flaw demonstrates poor input validation practices and highlights the critical importance of implementing proper file type validation, content sanitization, and secure file handling procedures in web applications. Organizations should immediately implement mitigations including input validation, file extension restrictions, and content security policy enforcement to prevent exploitation of this vulnerability.
The remediation approach should involve updating the SaveFilesToDisk method to validate file extensions, sanitize base64 content, and implement proper file type checking before writing any content to disk. Additionally, organizations should consider implementing web application firewalls, input validation libraries, and regular security assessments to prevent similar vulnerabilities from being introduced in future development cycles. The vulnerability underscores the necessity of following secure coding practices and implementing defense-in-depth strategies to protect web applications from arbitrary code execution attacks.