GitHub Security Advisory: GHSA-4mh3-h929-w968
Package: github.com/filebrowser/filebrowser/v2 (GO)
Severity: HIGH (CVSS 8.1)
CWE: CWE-706: Use of Incorrectly-Resolved Name or Reference, CWE-863: Incorrect Authorization
Vulnerable versions: <= 2.57.0
Patched in: 2.57.1

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

Details:
### 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](https://github.com/user-attachments/files/24823114/poc.py)

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="Screenshot_20260123_151608" src="https://github.com/user-attachments/assets/2071c92e-2bbe-46f8-a338-05b0f53d381a" />
<img width="890" height="386" alt="Screenshot_20260123_151551" 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="Screenshot_20260123_151502" 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="Screenshot_20260123_154446" 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="Screenshot_20260123_154544" src="https://github.com/user-attachments/assets/27ebb82c-f7c2-467d-ae82-f495ae3aa2d4" />
<img width="1196" height="818" alt="Screenshot_20260123_154618" 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 directori