CVE-2026-25237: Pear Pearweb RCE Vulnerability

CVE-2026-25237 Overview

CVE-2026-25237 is a remote code execution vulnerability affecting PEAR Pearweb, the framework and distribution system for reusable PHP components. Prior to version 1.33.0, the use of preg_replace() with the /e modifier in bug update email handling can enable arbitrary PHP code execution if attacker-controlled content reaches the evaluated replacement pattern.

Critical Impact

This vulnerability allows unauthenticated remote attackers to execute arbitrary PHP code on affected servers by crafting malicious input that gets processed through the vulnerable preg_replace() function with the /e modifier.

Affected Products

• PEAR Pearweb versions prior to 1.33.0

Discovery Timeline

• 2026-02-03 - CVE-2026-25237 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2026-25237

Vulnerability Analysis

This vulnerability stems from the use of PHP's deprecated preg_replace() function with the /e (PREG_REPLACE_EVAL) modifier. When this modifier is used, the replacement string is evaluated as PHP code after substitution. In PEAR Pearweb's bug update email handling functionality, attacker-controlled content can flow into this evaluated replacement, enabling arbitrary code execution.

The /e modifier has been deprecated since PHP 5.5.0 and removed in PHP 7.0.0 precisely because of security risks like this one. However, applications running on older PHP versions or those that haven't updated their codebases remain vulnerable.

Root Cause

The root cause is classified under CWE-624 (Executable Regular Expression Error). The vulnerability exists because:

1. The application uses preg_replace() with the /e modifier to process user-controlled input
2. User-supplied data in bug update emails is not properly sanitized before reaching the regex evaluation
3. The evaluated replacement allows injection of arbitrary PHP expressions that execute in the application context

This pattern represents a classic code injection vulnerability where regex replacement evaluation becomes an unintended code execution pathway.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Submitting specially crafted content through the bug update email handling system
2. Crafting input that, when processed by the vulnerable preg_replace() call, injects malicious PHP code into the replacement string
3. The injected code executes with the privileges of the web server process

For example, by including PHP function calls or shell commands within the crafted input, an attacker can achieve remote code execution, potentially leading to full server compromise, data exfiltration, or lateral movement within the network.

The vulnerability mechanism involves the deprecated /e modifier evaluating replacement strings as PHP code. When attacker-controlled data flows into this evaluation context without proper sanitization, arbitrary code execution becomes possible. For detailed technical information, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-25237

Indicators of Compromise

• Unusual PHP error logs referencing preg_replace() evaluation failures or unexpected code execution
• Web server access logs showing suspicious requests to bug tracking or email handling endpoints with encoded PHP payloads
• Unexpected outbound network connections from web server processes
• Creation of new files or modifications to existing PHP files in the web root

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block requests containing PHP code patterns in bug-related form submissions
• Implement application-level logging for all preg_replace() calls to monitor for exploitation attempts
• Use SentinelOne's Singularity Platform to detect anomalous process spawning from PHP/web server processes
• Monitor for regex patterns containing the /e modifier in code through static analysis tools

Monitoring Recommendations

• Enable verbose logging for the PEAR Pearweb application, particularly around bug tracking functionality
• Configure intrusion detection systems to alert on PHP code execution patterns in HTTP request bodies
• Implement file integrity monitoring on web application directories to detect unauthorized modifications
• Monitor process trees for unusual child processes spawned by web server workers

How to Mitigate CVE-2026-25237

Immediate Actions Required

• Upgrade PEAR Pearweb to version 1.33.0 or later immediately
• If immediate patching is not possible, disable or restrict access to bug update email handling functionality
• Implement strict input validation and sanitization for all user-controlled data
• Consider upgrading to PHP 7.0+ where the /e modifier is no longer supported

Patch Information

The vulnerability has been addressed in PEAR Pearweb version 1.33.0. The patch removes the dangerous use of preg_replace() with the /e modifier and replaces it with the safer preg_replace_callback() function, which does not evaluate replacement strings as PHP code.

For complete patch details and upgrade instructions, refer to the GitHub Security Advisory GHSA-vhw6-hqh9-8r23.

Workarounds

• Restrict network access to the PEAR Pearweb application to trusted IP ranges only
• Implement a reverse proxy with strict input filtering to sanitize potentially malicious payloads
• Disable the bug update email handling feature if it is not business-critical
• Deploy a WAF rule to block requests containing PHP code execution patterns targeting the vulnerable endpoint

# Example: Restrict access to bug handling endpoints via Apache
<Location "/bugs/">
    Require ip 10.0.0.0/8 192.168.0.0/16
</Location>

# Example: nginx configuration to limit access
location /bugs/ {
    allow 10.0.0.0/8;
    allow 192.168.0.0/16;
    deny all;
}