How to remediate – Apache <= 1.3.33 htpasswd Local Overflow

1. Introduction

The Apache <= 1.3.33 htpasswd Local Overflow vulnerability is a buffer overflow flaw in older versions of the Apache web server. This allows a local user to potentially gain elevated privileges, or a remote attacker to execute arbitrary commands if the 'htpasswd' command is accessible through CGI scripts. Successful exploitation could lead to complete system compromise. Confidentiality, integrity and availability may be impacted.

2. Technical Explanation

The vulnerability exists within the ‘htpasswd’ command in Apache versions 1.3.33 and earlier due to insufficient bounds checking when handling user input. An attacker can exploit this by providing a specially crafted input string that overflows a buffer, potentially overwriting adjacent memory regions. This could allow them to execute arbitrary code with elevated privileges if ‘htpasswd’ is run setuid or remotely through CGI scripts. Nessus identified this vulnerability based on the server version number; however, it may be a false positive.

• Root cause: Insufficient buffer bounds checking in the htpasswd command when handling user input.
• Exploit mechanism: An attacker provides an overly long input string to ‘htpasswd’, causing a buffer overflow and potential code execution.
• Scope: Apache web servers running versions 1.3.33 or older are affected.

3. Detection and Assessment

To confirm if a system is vulnerable, check the Apache version first. Then verify htpasswd configuration.

• Quick checks: Use the following command to display the Apache version: httpd -v or apache2 -v
• Scanning: Nessus plugin ID 30198 may identify this vulnerability, but results should be manually verified.
• Logs and evidence: Check system logs for unusual activity related to ‘htpasswd’ execution, particularly if it is run as a CGI script.

httpd -v

4. Solution / Remediation Steps

The following steps will help mitigate the Apache <= 1.3.33 htpasswd Local Overflow vulnerability. Ensure backups are taken before making changes.

4.1 Preparation

• Ensure you have access to the server’s configuration files and command line interface. A roll back plan is to restore from the backup taken in step 1.
• A change window may be required depending on your organisation’s policies, with approval from the system owner.

4.2 Implementation

1. Step 1: Ensure that ‘htpasswd’ does not run setuid. Check permissions using ls -l /usr/bin/htpasswd. If it has setuid bit enabled (e.g., `-rwsr-xr-x`), remove it with chmod u-s /usr/bin/htpasswd.
2. Step 2: Verify that ‘htpasswd’ is not accessible through any CGI scripts. Review Apache configuration files for references to ‘htpasswd’ in CGI directories.

4.3 Config or Code Example

Before

ls -l /usr/bin/htpasswd
-rwsr-xr-x 1 root root ... /usr/bin/htpasswd

After

ls -l /usr/bin/htpasswd
-rwxr-xr-x 1 root root ... /usr/bin/htpasswd

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of vulnerability. Least privilege is key, limiting the impact if exploited. Input validation prevents unsafe data from being processed. Safe defaults reduce the attack surface.

• Practice 1: Implement least privilege by running services with minimal necessary permissions to limit potential damage.

4.5 Automation (Optional)

Automation is not recommended for this specific vulnerability due to the need for careful configuration review.

5. Verification / Validation

Confirm that the fix has been applied correctly by verifying permissions and CGI access. Then re-run the initial detection method.

• Post-fix check: Run ls -l /usr/bin/htpasswd. The output should *not* show the setuid bit enabled (e.g., `-rwxr-xr-x`).
• Re-test: Re-run the initial detection method (httpd -v) and confirm that no longer flags this vulnerability, or that you have upgraded to a patched version of Apache.
• Smoke test: Verify basic web server functionality by accessing a simple webpage through the browser.
• Monitoring: Monitor system logs for any unusual activity related to ‘htpasswd’ execution.

ls -l /usr/bin/htpasswd

6. Preventive Measures and Monitoring

Regular security baselines help prevent this issue by enforcing secure configurations. CI pipelines can add checks for known vulnerabilities during deployment. A sensible patch or config review cycle is important.

• Baselines: Update your security baseline to include a requirement that ‘htpasswd’ does not run setuid and is not accessible through CGI scripts.
• Asset and patch process: Implement a regular patch management cycle for all systems, including Apache web servers.

7. Risks, Side Effects, and Roll Back

Removing the setuid bit from ‘htpasswd’ may affect certain applications that rely on it. Restoring from backup will return the system to its previous state.

• Roll back: Restore the system from the backup taken prior to making any changes.

8. References and Resources

Links only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: https://seclists.org/bugtraq/2004/Oct/356
• NVD or CVE entry: No direct NVD entry found for this specific version, but related vulnerabilities exist.
• Product or platform documentation relevant to the fix: https://httpd.apache.org/docs/