1. Introduction
The Anonymous FTP Writable root Directory vulnerability allows unauthorized write access to the root directory of an FTP server. This enables attackers to upload malicious files, potentially compromising the server and its data. Systems running anonymous FTP services are typically affected. A successful exploit could lead to complete compromise of confidentiality, integrity, and availability of the FTP server.
2. Technical Explanation
The vulnerability stems from insecure default configurations that permit write access to the root directory without authentication. An attacker can then upload arbitrary files, potentially including malware or backdoors. This is often found in older FTP servers with weak security settings. CVE-1999-0527 details this issue. For example, an attacker could upload a malicious script and execute it on the server to gain control.
- Root cause: Insufficient access controls allowing write operations to the root directory of the FTP server without proper authentication.
- Exploit mechanism: An attacker connects anonymously to the FTP server and uses commands like ‘put’ or ‘upload’ to place files in the root directory.
- Scope: Affected platforms include servers running FTP services, particularly those with default configurations allowing anonymous access.
3. Detection and Assessment
To confirm vulnerability, check for write permissions on the root directory. A thorough assessment involves attempting to upload a test file.
- Quick checks: Use an FTP client to attempt to connect anonymously and list directory contents. Check if ‘put’ or ‘upload’ commands are permitted in the root directory.
- Scanning: Nessus plugin ID 26978 can identify this vulnerability, but results should be verified manually.
- Logs and evidence: Examine FTP server logs for successful write operations from anonymous users to the root directory. Look for events indicating file uploads or modifications in the root path.
ftp <target_ip>
User: anonymous
Password: <[email protected]>
put testfile.txt /
4. Solution / Remediation Steps
Restrict write access to the root directory of the FTP server. These steps should be performed during a scheduled maintenance window.
4.1 Preparation
- Ensure you have access to the FTP server’s configuration files and administrative credentials. A rollback plan involves restoring the backed-up configuration.
- A change window is recommended due to potential disruption of FTP services. Approval from the IT security team may be required.
4.2 Implementation
- Step 1: Edit the FTP server’s configuration file (e.g., ftp.conf, vsftpd.conf).
- Step 2: Locate the settings controlling anonymous access and write permissions.
- Step 3: Disable write access for anonymous users to the root directory. This may involve setting a ‘deny’ rule or restricting the anonymous user’s home directory.
- Step 4: Restart the FTP service to apply the changes.
4.3 Config or Code Example
Before
anonymous_enable=YES
local_enable=NO
write_enable=YES
After
anonymous_enable=YES
local_enable=NO
write_enable=NO
anon_upload_enable=NO
anon_mkdir_write_enable=NO
4.4 Security Practices Relevant to This Vulnerability
Implementing least privilege and secure defaults can prevent this issue. For example, limiting user permissions reduces the impact of a successful exploit. Input validation can block malicious file uploads.
- Practice 1: Least privilege – grant only necessary access rights to users and services.
- Practice 2: Secure Defaults – configure FTP servers with restrictive settings by default, disabling anonymous write access unless explicitly required.
4.5 Automation (Optional)
If using configuration management tools like Ansible, you can automate the changes. Only include if safe and directly relevant.
---
- hosts: ftp_servers
tasks:
- name: Disable anonymous write access in vsftpd.conf
lineinfile:
path: /etc/vsftpd.conf
regexp: '^write_enable=YES'
line: 'write_enable=NO'
notify: restart vsftpd
handlers:
- name: restart vsftpd
service:
name: vsftpd
state: restarted
5. Verification / Validation
- Post-fix check: Connect to the FTP server as an anonymous user and attempt to upload a file to the root directory. The operation should be denied with an error message like “550 Permission denied”.
- Re-test: Re-run the command from section 3 (attempting to ‘put’ a test file) – it should now fail.
- Smoke test: Verify that authorized users can still access and transfer files as expected.
- Monitoring: Monitor FTP server logs for any failed write attempts from anonymous users, indicating potential malicious activity.
ftp <target_ip>
User: anonymous
Password: <[email protected]>
put testfile.txt /
550 Permission denied.
6. Preventive Measures and Monitoring
Update security baselines to include restrictions on anonymous FTP access. Implement CI/CD pipelines with SAST tools to detect insecure configurations. A regular patch cycle ensures timely updates.
- Baselines: Update your organization’s security baseline or policy to explicitly prohibit anonymous write access in FTP server configurations.
- Pipelines: Incorporate Static Application Security Testing (SAST) into your CI/CD pipeline to identify insecure default settings during deployment.
- Asset and patch process: Implement a regular review cycle for FTP server configurations, ensuring they adhere to security best practices.
7. Risks, Side Effects, and Roll Back
Disabling anonymous write access may break existing applications that rely on it. A rollback involves restoring the original configuration file.
- Risk or side effect 1: Disabling anonymous write access could disrupt legitimate users if they depend on this functionality.
- Risk or side effect 2: Incorrectly configured settings may prevent all FTP access.
- Roll back: Restore the backed-up FTP server configuration file and restart the service.
8. References and Resources
- Vendor advisory or bulletin: CERT CA-1993-10
- NVD or CVE entry: CVE-1999-0527
- Product or platform documentation relevant to the fix: Refer to your FTP server vendor’s documentation for specific configuration instructions.