CVE-2021-21708: PHP Use-After-Free Vulnerability

CVE-2021-21708 Overview

CVE-2021-21708 is a critical Use-After-Free vulnerability affecting multiple versions of PHP. The vulnerability exists in PHP's filter functions when using FILTER_VALIDATE_FLOAT with min/max limits. When the filter validation fails under specific conditions, the PHP interpreter may access memory after it has been freed, potentially leading to application crashes, memory corruption, and remote code execution (RCE).

Critical Impact

This Use-After-Free vulnerability in PHP's filter validation functions can be exploited remotely without authentication, potentially allowing attackers to execute arbitrary code on vulnerable servers running affected PHP versions.

Affected Products

• PHP versions 7.4.x below 7.4.28
• PHP versions 8.0.x below 8.0.16
• PHP versions 8.1.x below 8.1.3

Discovery Timeline

• 2022-02-27 - CVE-2021-21708 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21708

Vulnerability Analysis

The vulnerability resides in PHP's input filtering mechanism, specifically within the FILTER_VALIDATE_FLOAT filter implementation. When this filter is used with min/max limit parameters and the validation fails, the code path triggers a Use-After-Free (UAF) condition. This occurs because the memory associated with the filter operation is deallocated prematurely, but subsequent code continues to reference and potentially modify this freed memory region.

The network-accessible nature of this vulnerability is particularly concerning, as PHP is predominantly used in web server environments where user input is processed through filter functions. An attacker can craft malicious input that triggers the filter validation failure, subsequently exploiting the UAF condition to potentially overwrite critical memory structures.

Root Cause

The root cause is a memory management flaw in PHP's filter extension (CWE-416: Use After Free). When FILTER_VALIDATE_FLOAT processes input with min/max constraints and the validation fails, the internal memory handling incorrectly frees a memory chunk while references to that memory still exist. The subsequent use of this freed memory can lead to heap corruption, where the attacker-controlled data may overwrite sensitive memory regions.

Attack Vector

The attack vector is network-based and does not require authentication or user interaction. An attacker can exploit this vulnerability by:

1. Sending crafted input to a PHP application that uses FILTER_VALIDATE_FLOAT with min/max limits
2. Triggering the filter validation to fail under specific conditions
3. Exploiting the resulting Use-After-Free condition to corrupt memory
4. Achieving code execution by controlling the contents of reallocated memory

The vulnerability specifically affects code patterns where filter_var() or filter_input() functions are called with FILTER_VALIDATE_FLOAT and the min_range or max_range options configured. For detailed technical information about the specific code paths involved, refer to the PHP Bug Report #81708.

Detection Methods for CVE-2021-21708

Indicators of Compromise

• Unexpected PHP process crashes or segmentation faults in web server logs
• Memory corruption errors or heap-related exceptions in PHP error logs
• Unusual patterns of failed float validation attempts in application logs
• Signs of exploitation attempts targeting filter functions with boundary values

Detection Strategies

• Monitor PHP error logs for memory-related errors and unexpected crashes when processing user input
• Implement web application firewall (WAF) rules to detect anomalous float values in input parameters
• Use runtime application self-protection (RASP) solutions to detect memory corruption attempts
• Audit code repositories for usage of FILTER_VALIDATE_FLOAT with min/max options

Monitoring Recommendations

• Configure centralized logging for all PHP application errors and warnings
• Set up alerting for patterns of repeated validation failures followed by crashes
• Monitor server health metrics for unexpected memory usage patterns
• Implement intrusion detection signatures for known exploitation patterns against PHP filter functions

How to Mitigate CVE-2021-21708

Immediate Actions Required

• Upgrade PHP to version 7.4.28 or later for 7.4.x installations
• Upgrade PHP to version 8.0.16 or later for 8.0.x installations
• Upgrade PHP to version 8.1.3 or later for 8.1.x installations
• Audit all PHP applications for usage of FILTER_VALIDATE_FLOAT with min/max limits

Patch Information

The PHP development team has addressed this vulnerability in PHP versions 7.4.28, 8.0.16, and 8.1.3. Organizations should upgrade to these versions or later to remediate the vulnerability. Detailed patch information is available through the PHP Bug Report #81708. Additional security advisories have been released by Gentoo Linux and NetApp.

Workarounds

• If immediate patching is not possible, consider implementing input validation at the application layer before using FILTER_VALIDATE_FLOAT
• Use alternative validation methods that do not rely on min/max range options until the patch is applied
• Deploy web application firewalls with rules to sanitize float input values
• Consider isolating PHP processes using containerization to limit the impact of potential exploitation

# Check current PHP version
php -v

# On Debian/Ubuntu systems, upgrade PHP
sudo apt update && sudo apt upgrade php

# On RHEL/CentOS systems with Remi repository
sudo dnf module reset php
sudo dnf module enable php:8.1
sudo dnf upgrade php

# Verify the upgrade
php -v