Summary
Kunstmaan CMS contains an unrestricted file upload weakness in its media handling logic, allowing an authenticated backend user with media access to bypass extension checks using mixed-case executable filenames. If uploaded files are stored in a web-accessible directory and the web server executes their extension, an attacker may achieve remote code execution.
Kunstmaan CMS contains an unrestricted file upload weakness in its media handling logic, allowing an authenticated backend user with media access to bypass extension checks using mixed-case executable filenames. If uploaded files are stored in a web-accessible directory and the web server executes their extension, an attacker may achieve remote code execution.
Affected context
The issue occurs in Kunstmaan CMS media uploads when extension validation is performed case-sensitively but the filename extension is normalized to lowercase afterward. Risk is increased when authenticated backend users can upload media, uploaded files are reachable through the web server, and the server is configured to execute one or more permitted extension types.
The issue occurs in Kunstmaan CMS media uploads when extension validation is performed case-sensitively but the filename extension is normalized to lowercase afterward. Risk is increased when authenticated backend users can upload media, uploaded files are reachable through the web server, and the server is configured to execute one or more permitted extension types.
Exploit preconditions
- Attack surface: Backend media-upload functionality
- Required access: An authenticated user account with permission to manage media
- Trigger point: File extension validation and storage in the media directory
- Why exploitation is possible: Case-sensitive blacklist checks can be bypassed by mixed-case executable extensions, while subsequent normalization stores the extension in a form recognized by the web server
- Additional condition: The uploaded file must be placed in a web-accessible location served by a runtime that executes its extension
- Attack surface: Backend media-upload functionality
- Required access: An authenticated user account with permission to manage media
- Trigger point: File extension validation and storage in the media directory
- Why exploitation is possible: Case-sensitive blacklist checks can be bypassed by mixed-case executable extensions, while subsequent normalization stores the extension in a form recognized by the web server
- Additional condition: The uploaded file must be placed in a web-accessible location served by a runtime that executes its extension
Why this happens
The vulnerability results from inconsistent normalization and validation of uploaded file extensions. Kunstmaan checks the extension against a blacklist before converting it to lowercase, so differently cased representations may evade the check. The default blacklist may also fail to cover all server-executable extension types. This behavior maps to CWE-434 because the application permits potentially executable files to be uploaded without reliably restricting their type or execution context.
The vulnerability results from inconsistent normalization and validation of uploaded file extensions. Kunstmaan checks the extension against a blacklist before converting it to lowercase, so differently cased representations may evade the check. The default blacklist may also fail to cover all server-executable extension types. This behavior maps to CWE-434 because the application permits potentially executable files to be uploaded without reliably restricting their type or execution context.
Potential impact
- An attacker may execute arbitrary server-side code
- The web application could be compromised
- Sensitive files or application data may be accessed
- The attacker could alter content or abuse the CMS account's privileges
- The server may be used for further attacks against connected systems
- An attacker may execute arbitrary server-side code
- The web application could be compromised
- Sensitive files or application data may be accessed
- The attacker could alter content or abuse the CMS account's privileges
- The server may be used for further attacks against connected systems
General mitigation guidance
Mitigation generally focuses on allowlisting file types rather than relying on a blacklist, normalizing filenames before validation, and separating file type verification from filename extension checks. Uploaded content can be stored outside the web root or served through a non-executable handler, with execution disabled in upload directories by default. Additional controls include validating file signatures where appropriate, generating server-side filenames, restricting media permissions, applying least privilege to the application process, and limiting the capabilities of authenticated backend users.
Mitigation generally focuses on allowlisting file types rather than relying on a blacklist, normalizing filenames before validation, and separating file type verification from filename extension checks. Uploaded content can be stored outside the web root or served through a non-executable handler, with execution disabled in upload directories by default. Additional controls include validating file signatures where appropriate, generating server-side filenames, restricting media permissions, applying least privilege to the application process, and limiting the capabilities of authenticated backend users.
How teams detect issues like this
Detection can include static analysis for case-sensitive extension checks, lowercasing performed after validation, blacklist-only upload filters, and media paths exposed directly to executable web handlers. Code reviews should examine upload authorization, storage location, content validation, and web server execution policy. Runtime monitoring can alert on executable extensions, unexpected script requests in media directories, and anomalous backend upload activity. CI policy checks and automated security review tools may help identify these patterns before deployment.
Detection can include static analysis for case-sensitive extension checks, lowercasing performed after validation, blacklist-only upload filters, and media paths exposed directly to executable web handlers. Code reviews should examine upload authorization, storage location, content validation, and web server execution policy. Runtime monitoring can alert on executable extensions, unexpected script requests in media directories, and anomalous backend upload activity. CI policy checks and automated security review tools may help identify these patterns before deployment.
