1. Introduction
The PHP mb_send_mail() Function Parameter Security Bypass affects web servers running vulnerable versions of PHP. This flaw allows attackers to potentially read and create arbitrary files on the server, leading to a compromise of confidentiality, integrity, and availability. Systems using PHP with the sendmail function and accepting remote input for the additional_parameters argument are at risk.
2. Technical Explanation
The vulnerability occurs because PHP does not properly sanitise user-supplied data when passed as additional parameters to the mb_send_mail() function, specifically when used in conjunction with sendmail. This allows an attacker to inject shell commands into the additional parameters, which are then executed by the server. The CVE associated with this issue is CVE-2006-1014.
- Root cause: Insufficient input validation on the additional_parameters argument of the mb_send_mail() function when used with sendmail.
- Exploit mechanism: An attacker can supply malicious commands within the additional_parameters string, which are then executed by the server’s shell. For example, an attacker could inject a command to read sensitive files like /etc/passwd.
- Scope: PHP versions prior to 5.2.1 are affected.
3. Detection and Assessment
You can confirm vulnerability by checking the PHP version installed on your server. A thorough assessment involves examining application code for use of mb_send_mail() with user-supplied input.
- Quick checks: Use the following command to display the PHP version:
php -v - Scanning: Nessus plugin ID 32689 may detect this vulnerability. This is an example only, and results should be verified manually.
- Logs and evidence: Examine web server logs for suspicious activity related to mb_send_mail() calls with unusual or lengthy additional parameters.
php -v4. Solution / Remediation Steps
Currently, there is no known patch available directly addressing this vulnerability. Mitigation focuses on avoiding the use of user-supplied data in the additional_parameters argument.
4.1 Preparation
- Stopping services is not required for this remediation, but it’s good practice to schedule downtime during peak hours. A roll back plan involves restoring from backup or snapshot.
- Changes should be approved by a senior administrator or security team member.
4.2 Implementation
- Step 1: Review all application code that uses the mb_send_mail() function.
- Step 2: Identify any instances where user-supplied data is passed as the additional_parameters argument.
- Step 3: Remove or sanitise the user-supplied input from the additional_parameters argument. Avoid using this parameter if possible.
4.3 Config or Code Example
Before
$additional_params = $_GET['params'];
mb_send_mail($to, $subject, $body, $headers, $additional_params);
After
// Do not use user-supplied data in additional parameters.
$additional_params = ""; // Or remove the parameter entirely if possible.
mb_send_mail($to, $subject, $body, $headers, $additional_params);
4.4 Security Practices Relevant to This Vulnerability
List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.
- Practice 2: Least privilege can limit the impact of a successful exploit. Ensure that the web server process has only the necessary permissions to perform its tasks.
4.5 Automation (Optional)
# Example Bash script to find vulnerable code (requires grep):
grep -r "mb_send_mail(" . | grep "$_GET"
# This will output lines containing mb_send_mail() and $_GET, indicating potential vulnerabilities. Review manually.
5. Verification / Validation
Confirm the fix by verifying that user-supplied data is no longer used in the additional_parameters argument of the mb_send_mail() function.
- Post-fix check: Re-run the code search from step 4.2 and ensure no instances are found where user input is passed to the parameter.
- Re-test: Attempt to exploit the vulnerability using a test payload with malicious commands in the additional_parameters argument. The command should not execute.
- Smoke test: Verify that email sending functionality still works as expected without the vulnerable code.
grep -r "mb_send_mail(" . | grep "$_GET" # Should return no results
6. Preventive Measures and Monitoring
Update security baselines to include restrictions on using user-supplied data in sensitive functions like mb_send_mail(). Implement code review processes to identify similar vulnerabilities during development.
- Baselines: Update your PHP security baseline to explicitly prohibit the use of user input in function parameters that could lead to command injection.
- Pipelines: Integrate Static Application Security Testing (SAST) tools into your CI/CD pipeline to automatically scan for potential vulnerabilities like this one.
- Asset and patch process: Regularly review PHP versions installed on your servers and apply security patches promptly.
7. Risks, Side Effects, and Roll Back
Removing or sanitising user input may break existing functionality if the application relies on it. Thorough testing is essential.
- Risk or side effect 1: Removing the additional_parameters argument could cause compatibility issues with some email clients.
- Risk or side effect 2: Incorrectly sanitised input might still allow limited command injection.
- Roll back: Restore the original code from your backup if functionality is broken. Revert any changes made to configuration files.
8. References and Resources
- Vendor advisory or bulletin: https://www.securityfocus.com/archive/1/426342/100/0/threaded
- NVD or CVE entry: CVE-2006-1014
- Product or platform documentation relevant to the fix: No specific documentation available. Refer to PHP’s official documentation on mb_send_mail() and input validation best practices.