CVE-2026-25890: File Browser has a Path-Based Access Control Bypass via Multiple Leading Slashes in URL

Summary An authenticated user can bypass the application's "Disallow" file path rules by modifying the request URL. By adding multiple slashes (e.g., //private/) to the path, the authorization check fails to match the rule, while the underlying filesystem resolves the path correctly, granting unauthorized access to restricted files.

Details The vulnerability allows users to bypass "Disallow" rules defined by administrators.

The issue stems from how the application handles URL path normalization and rule matching:

1. Router Configuration: The router in http/http.go is configured with r.SkipClean(true). This prevents the automatic collapse of multiple slashes (e.g., // becoming /) before the request reaches the handler. 2. Insecure Rule Matching: The rule enforcement logic in rules/rules.go relies on a simple string prefix match: strings.HasPrefix(path, r.Path). If a rule disallows /private, a request for //private fails this check because //private does not strictly start with /private. 3. Filesystem Resolution: After bypassing the rule check, the non-normalized path is passed to the filesystem. The filesystem treats the multiple slashes as a single separator, successfully resolving //private/secret.txt and serving the file.

PoC Python minimal PoC

The following steps demonstrate the vulnerability: 1. Setup: - Admin user creates a folder /private and adds a file /private/secret.txt. <img width="971" height="719" alt="Screenshot20260123151608" src="https://github.com/user-attachments/assets/2071c92e-2bbe-46f8-a338-05b0f53d381a" /> <img width="890" height="386" alt="Screenshot20260123151551" src="https://github.com/user-attachments/assets/1def540a-de26-4666-a6ab-058d5927bfbe" /> - Admin adds a Disallow rule for user bob on the path /private. <img width="1005" height="1126" alt="Screenshot20260123151502" src="https://github.com/user-attachments/assets/e9b57d59-f4ab-41d8-b056-8ffdaa219963" />

2. Verification: - User bob requests GET /api/resources/private/secret.txt. - Server responds: 403 Forbidden. <img width="1193" height="721" alt="Screenshot20260123154446" src="https://github.com/user-attachments/assets/dd092a10-2f8c-4a3c-b48f-d540c483bb5a" /> 3. Exploit: - User bob requests GET /api/resources//private/secret.txt. - Server responds: 200 OK (Bypass successful). <img width="1193" height="721" alt="Screenshot20260123154544" src="https://github.com/user-attachments/assets/27ebb82c-f7c2-467d-ae82-f495ae3aa2d4" /> <img width="1196" height="818" alt="Screenshot20260123154618" src="https://github.com/user-attachments/assets/82035884-9a24-490d-b928-7bdd2dbe3193" />

Impact This vulnerability impacts the confidentiality and integrity of data stored in filebrowser. - Confidentiality: Users can read files they are explicitly forbidden from accessing. - Integrity: If the user has general write permissions but is restricted from specific directories via rules, they can bypass these restrictions to rename, delete, or modify files.