CVE-2026-21857

### Summary Authenticated users with backup permissions can read arbitrary files within the webroot via path traversal in the Backup addon's file export functionality. <img width="664" height="899" alt="image" src="https://github.com/user-attachments/assets/fd1ca69e-b275-4daf-9a62-621cde6525f5" /> <img width="2358" height="445" alt="image" src="https://github.com/user-attachments/assets/fad81152-9e1b-413e-9823-09540a23e2fb" /> ### Details The Backup addon does not validate the `EXPDIR` POST parameter against the UI-generated allowlist of permitted directories. An attacker can supply relative paths containing `../` sequences (or even absolute paths inside the document root) to include any readable file in the generated `.tar.gz` archive. Vulnerable code: - `redaxo/src/addons/backup/pages/export.php` (lines 72-76) – directly uses `$_POST['EXPDIR']` - `redaxo/src/addons/backup/lib/backup.php` (lines ~413 & ~427) – concatenates unsanitized user input with base path This allows disclosure of sensitive files such as: - `redaxo/data/core/config.yml` → database credentials + password hashes of all backend users - `.env`, custom configuration files, logs, uploaded malicious files, etc. ### Affected versions ≤ 5.20.1 (confirmed working) ### Patched versions None (as of 2025-12-09) ### PoC – Extracting database credentials and password hashes 1. Log in as any user with Backup permission 2. Go to Backup → Export → Files <img width="1240" height="960" alt="image" src="https://github.com/user-attachments/assets/bc05ba18-9664-4be2-b637-4fec3a0f409a" /> 3. Intercept the request with Burp Suite <img width="2184" height="478" alt="image" src="https://github.com/user-attachments/assets/9fa754a1-2cd0-4d3d-a5cc-cfa34c8a1718" /> 4. Change one `EXPDIR[]` value to `../../../../var/www/html/redaxo/data/core` <img width="978" height="591" alt="image" src="https://github.com/user-attachments/assets/d15f5c7f-b72c-44cc-9be2-da8d3f26f124" /> 5. Send request → download archive <img width="423" height="131" alt="image" src="https://github.com/user-attachments/assets/db8a8bda-cdaf-4dea-812f-1e312da908e2" /> 6. Extract and open `data/core/config.yml` <img width="859" height="281" alt="image" src="https://github.com/user-attachments/assets/c8112ce1-5a1d-435f-953b-7eb4e711e042" /> Result: plaintext database password <img width="2534" height="1198" alt="image" src="https://github.com/user-attachments/assets/218ae917-868a-437e-98b0-6471b82c0b10" /> ### Impact Full compromise of the REDAXO installation: - Database takeover - Password hash extraction → offline cracking → admin access - When combined with other vulnerabilities → RCE CVSS 4.0 vector & score below. ### Credits Discovered by: Łukasz Rybak