CVE-2024-9026: PHP-FPM Information Disclosure Vulnerability

CVE-2024-9026 Overview

CVE-2024-9026 is a log manipulation vulnerability affecting PHP-FPM (FastCGI Process Manager) across multiple PHP versions. When PHP-FPM is configured with catch_workers_output = yes, attackers with local access can manipulate log message content to pollute final logs or remove up to 4 characters from log messages. This vulnerability is particularly concerning when PHP-FPM is configured to use syslog output, as it may allow further removal of log data.

Critical Impact

Attackers can tamper with PHP-FPM logs to hide malicious activity, evade forensic analysis, or corrupt audit trails essential for security monitoring and compliance.

Affected Products

• PHP 8.1.* versions before 8.1.30
• PHP 8.2.* versions before 8.2.24
• PHP 8.3.* versions before 8.3.12

Discovery Timeline

• 2024-10-08 - CVE-2024-9026 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-9026

Vulnerability Analysis

This vulnerability is classified under CWE-117 (Improper Output Neutralization for Logs), which describes a weakness where user-controllable data is written to application logs without proper sanitization. The flaw resides in PHP-FPM's worker output handling mechanism, specifically when the catch_workers_output configuration option is enabled.

The vulnerability allows local attackers with low privileges to manipulate log entries by crafting specific content that exploits the log processing logic. This can result in log pollution, where malicious or misleading data is injected into logs, or log truncation, where up to 4 characters can be removed from legitimate log messages. When syslog integration is enabled, the impact extends further, potentially allowing additional log data removal.

The integrity of logging systems is critical for security monitoring, incident response, and compliance auditing. By compromising log integrity, attackers can conceal malicious activities, create false trails, or impair forensic investigations.

Root Cause

The root cause lies in improper output neutralization within PHP-FPM's worker output capture mechanism. When catch_workers_output = yes is configured, PHP-FPM captures output from worker processes and writes it to configured log destinations. The vulnerability exists because the log processing logic does not adequately sanitize or validate the content before writing to logs, allowing specially crafted input to manipulate the resulting log entries.

Attack Vector

The attack requires local access to the system with low privileges. An attacker must be able to execute PHP code or otherwise influence the output generated by PHP-FPM worker processes. By carefully crafting the content that gets captured by the worker output mechanism, the attacker can:

1. Inject arbitrary content into log files, polluting legitimate log entries
2. Remove up to 4 characters from log messages, potentially obscuring critical information
3. When syslog is used, leverage protocol-specific characteristics to remove additional log data

The attack does not require user interaction and can be performed by any local user with the ability to influence PHP-FPM worker output.

Detection Methods for CVE-2024-9026

Indicators of Compromise

• Unusual patterns in PHP-FPM logs such as truncated entries or unexpected characters
• Inconsistencies between PHP-FPM logs and other system logs for the same events
• Log entries with missing or corrupted timestamps or severity indicators
• Evidence of log manipulation attempts in syslog facilities when PHP-FPM syslog output is enabled

Detection Strategies

• Implement log integrity monitoring to detect unauthorized modifications to PHP-FPM log files
• Deploy file integrity monitoring (FIM) solutions on log directories to alert on unexpected changes
• Cross-reference PHP-FPM logs with web server access logs and application logs for discrepancies
• Monitor for anomalous PHP process behavior that may indicate exploitation attempts

Monitoring Recommendations

• Enable comprehensive logging at multiple layers (web server, application, system) to provide redundant audit trails
• Configure centralized log aggregation to preserve log copies before potential manipulation
• Implement real-time log analysis rules to detect patterns consistent with log injection or truncation
• Review PHP-FPM configuration to identify instances where catch_workers_output = yes is enabled

How to Mitigate CVE-2024-9026

Immediate Actions Required

• Upgrade PHP to patched versions: 8.1.30 or later, 8.2.24 or later, or 8.3.12 or later
• Review PHP-FPM configurations across all environments to assess exposure
• Audit current log integrity and verify no tampering has occurred
• Consider disabling catch_workers_output if not required for operations

Patch Information

PHP has released security updates addressing this vulnerability. Administrators should upgrade to the following patched versions:

• PHP 8.1 branch: Upgrade to version 8.1.30 or later
• PHP 8.2 branch: Upgrade to version 8.2.24 or later
• PHP 8.3 branch: Upgrade to version 8.3.12 or later

Additional vendor advisories are available from PHP GitHub Security Advisory, Debian LTS, and NetApp.

Workarounds

• Disable catch_workers_output in PHP-FPM configuration if worker output capture is not essential
• Implement additional log validation and integrity checks at the log aggregation layer
• Use append-only log storage or write-once media to prevent log modification
• Deploy log forwarding to remote systems immediately upon generation to preserve original entries

# PHP-FPM configuration mitigation
# Edit your PHP-FPM pool configuration (e.g., /etc/php-fpm.d/www.conf)

# Disable worker output capture if not required
catch_workers_output = no

# If catch_workers_output must remain enabled, ensure logs are
# forwarded to a secure, centralized logging system immediately

# Restart PHP-FPM after configuration changes
systemctl restart php-fpm