CVE-2026-24687: 
C# vulnerability analysis and mitigation

CVE-2026-24687 is a path traversal and file enumeration vulnerability in Umbraco Forms, a form builder plugin for the Umbraco CMS. It allows authenticated backoffice users to traverse the server filesystem and read arbitrary file contents on Mac/Linux installations. Affected versions are Umbraco Forms 16.0.0–16.4.0 and 17.0.0–17.1.0; Windows-based and Umbraco Cloud deployments are not affected. The vulnerability was published on January 29, 2026, and patched versions (16.4.1 and 17.1.1) were released the same day. It carries a CVSS v3.1 score of 6.5 (Medium) and a CVSS v4.0 score of 6.0 (Medium) (GitHub Advisory, Umbraco Advisory).

The vulnerability is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory — Path Traversal). The root cause is insufficient validation of the fileName parameter in the export API endpoint (/umbraco/forms/api/v1/export), which allows an attacker to inject path traversal sequences (../ or ..\) to navigate outside the intended directory and access arbitrary files on the underlying filesystem. Exploitation requires low-level authenticated access (a valid backoffice user account) and is network-accessible with low attack complexity, though attack requirements are noted as "present" under CVSS v4.0, reflecting the need for a Mac/Linux deployment environment. No public proof-of-concept code has been identified (GitHub Advisory, Umbraco Advisory).

Successful exploitation allows an authenticated attacker to enumerate directory structures and read arbitrary files from the server filesystem, including sensitive configuration files, credentials, source code, and other confidential data. The impact is limited to confidentiality — there is no integrity or availability impact. The vulnerability is scoped to Mac/Linux Umbraco installations; Windows and Umbraco Cloud environments are unaffected. Exposure of credentials or configuration data could facilitate further lateral movement or privilege escalation beyond the Umbraco application (GitHub Advisory, Umbraco Advisory).

1. Obtain backoffice credentials: Acquire valid Umbraco backoffice user credentials through phishing, credential stuffing, or other means. Low-privilege backoffice accounts are sufficient.
2. Authenticate to the backoffice: Log in to the Umbraco backoffice on a target Mac/Linux installation running Umbraco Forms 16.0.0–16.4.0 or 17.0.0–17.1.0.
3. Identify the export endpoint: Locate the Forms export API at /umbraco/forms/api/v1/export.
4. Craft a path traversal request: Send an authenticated HTTP request to the export endpoint with a manipulated fileName parameter containing path traversal sequences, for example:

   GET /umbraco/forms/api/v1/export?fileName=../../../../../../etc/passwd

5. Enumerate the filesystem: Iteratively adjust the traversal depth and target path to enumerate directory structures and identify sensitive files (e.g., application configuration files, database connection strings, SSH keys).
6. Exfiltrate file contents: Read the returned file contents from the API response to extract credentials, secrets, or other sensitive data (GitHub Advisory, Umbraco Advisory).

• Network: Unusual or repeated HTTP GET/POST requests to /umbraco/forms/api/v1/export containing ../, ..%2F, ..\, or ..%5C sequences in the fileName parameter; requests originating from unexpected IP addresses or outside business hours.
• Logs: Web server or application access logs showing requests to the export endpoint with path traversal patterns in query parameters; HTTP 200 responses to such requests indicating successful file reads.
• File System: No direct file system artifacts are expected from read-only exploitation, but review of recently accessed sensitive files (e.g., /etc/passwd, application appsettings.json, .env files) may reveal unauthorized access.
• Application Logs: Umbraco application logs showing authenticated API calls to the export endpoint with anomalous fileName values outside expected form export filenames.

Upgrade Umbraco Forms to version 16.4.1 (for the 16.x branch) or 17.1.1 (for the 17.x branch), which contain the official patch. If immediate upgrading is not possible, apply the following interim mitigations:

• Configure a WAF or reverse proxy to block requests containing path traversal sequences (../, ..\) in the fileName parameter of the export endpoint.
• Restrict network access to the Umbraco backoffice to trusted IP ranges only.
• Block the /umbraco/forms/api/v1/export endpoint entirely if the export feature is not required.

Upgrading to the patched version is strongly recommended as the primary remediation (GitHub Advisory, Umbraco Advisory).

The vulnerability was credited to Kevin Joensen from Baldur Security, who responsibly disclosed the issue to the Umbraco team. The advisory was published by Umbraco maintainer rickbutterfield on January 29, 2026. Community reaction has been limited given the moderate severity and the requirement for authenticated access; no significant media coverage or notable researcher commentary beyond the initial advisory has been identified (Umbraco Advisory).