CVE-2020-7065: PHP Buffer Overflow Vulnerability

CVE-2020-7065 Overview

CVE-2020-7065 is a stack buffer overflow vulnerability in PHP's mb_strtolower() function when processing UTF-32LE encoded strings. In PHP versions 7.3.x below 7.3.16 and 7.4.x below 7.4.4, specially crafted invalid strings passed to the mb_strtolower() function with UTF-32LE encoding can cause PHP to overwrite a stack-allocated buffer. This memory corruption vulnerability can result in application crashes and potentially allow attackers to execute arbitrary code on affected systems.

Critical Impact

This stack buffer overflow vulnerability can lead to memory corruption, application crashes, and potentially remote code execution when processing malicious UTF-32LE encoded input through PHP's multibyte string functions.

Affected Products

• PHP versions 7.3.x below 7.3.16
• PHP versions 7.4.x below 7.4.4
• Debian Linux 10.0
• Canonical Ubuntu Linux (12.04 ESM, 14.04 ESM, 16.04 LTS, 18.04 LTS, 19.10, 20.04 LTS)
• Tenable Tenable.sc

Discovery Timeline

• 2020-04-01 - CVE-2020-7065 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-7065

Vulnerability Analysis

This vulnerability exists within PHP's multibyte string extension (mbstring), specifically in the mb_strtolower() function implementation. The root cause is a stack-based buffer overflow (CWE-121) that occurs during the processing of UTF-32LE encoded strings. When certain invalid or malformed UTF-32LE character sequences are passed to the function, PHP fails to properly validate the input boundaries before performing string operations, leading to out-of-bounds write (CWE-787) conditions.

The vulnerability is particularly dangerous because it can be triggered through network-accessible vectors in web applications that process user-supplied string data through PHP's multibyte string functions. An attacker can craft malicious input that, when processed, overwrites adjacent stack memory beyond the allocated buffer boundaries.

Root Cause

The vulnerability stems from improper bounds checking in the mb_strtolower() function when handling UTF-32LE encoded strings. The function allocates a fixed-size stack buffer for string conversion operations but fails to properly validate that the output does not exceed this buffer when processing certain malformed UTF-32LE input sequences. This improper input validation (CWE-787) allows an attacker to write data beyond the allocated buffer, corrupting adjacent stack memory.

Attack Vector

The attack requires network access and user interaction to exploit. An attacker can target web applications that process user-controlled input through the mb_strtolower() function with UTF-32LE encoding. The attack flow typically involves:

1. Identifying a PHP application that uses mb_strtolower() with user-controllable string input
2. Crafting a malicious string with specific invalid UTF-32LE byte sequences
3. Submitting the payload to the target application through web forms, API endpoints, or other input vectors
4. Triggering the buffer overflow when the application processes the input, potentially leading to code execution

The vulnerability exploits the string encoding conversion process. When PHP attempts to convert a malformed UTF-32LE string to lowercase, the improper handling of certain byte sequences causes a write beyond the stack buffer boundaries. For detailed technical information, refer to the PHP Bug Report #79371.

Detection Methods for CVE-2020-7065

Indicators of Compromise

• Unexpected PHP crashes or segmentation faults in applications using mbstring functions
• Anomalous web requests containing unusually large or malformed UTF-32LE encoded payloads
• Memory corruption errors in PHP error logs related to string processing operations
• Unusual process behavior or unexpected child processes spawned from PHP workers

Detection Strategies

• Monitor PHP error logs for segmentation faults and memory corruption errors, particularly those referencing mbstring or mb_strtolower()
• Implement web application firewall (WAF) rules to detect and block requests with abnormally large or malformed UTF-32LE encoded content
• Deploy intrusion detection signatures that identify exploitation patterns targeting PHP string encoding functions
• Audit application code for usage of mb_strtolower() with UTF-32LE encoding and user-controllable input

Monitoring Recommendations

• Enable PHP error logging at debug level to capture detailed stack traces during crashes
• Configure application performance monitoring to alert on sudden increases in PHP process crashes
• Implement file integrity monitoring on PHP binaries and critical application files
• Monitor network traffic for unusual patterns of requests targeting PHP endpoints with encoded payloads

How to Mitigate CVE-2020-7065

Immediate Actions Required

• Upgrade PHP to version 7.3.16 or later (for 7.3.x branch) or 7.4.4 or later (for 7.4.x branch)
• Apply security patches from your Linux distribution vendor (Debian, Ubuntu, etc.)
• Review application code for usage of mb_strtolower() with UTF-32LE encoding and consider input validation
• Implement web application firewall rules to filter potentially malicious encoded payloads

Patch Information

PHP has released patched versions that address this vulnerability. Organizations should upgrade to PHP 7.3.16 or 7.4.4 or later versions. Detailed changes can be found in the PHP ChangeLog Version 7.4.4.

For Linux distributions, security updates are available:

• Debian: DSA-4719 Security Announcement
• Ubuntu: USN-4330-1 Advisory and USN-4330-2 Advisory
• Tenable.sc: Tenable Security Notification TNS-2021-14

Workarounds

• Avoid using mb_strtolower() with UTF-32LE encoding until patches can be applied
• Implement strict input validation and sanitization for string data before passing to mbstring functions
• Consider using alternative encoding (such as UTF-8) where UTF-32LE is not strictly required
• Deploy web application firewalls with rules to detect and block malformed encoded payloads

# Check current PHP version
php -v

# Update PHP on Debian/Ubuntu systems
sudo apt-get update
sudo apt-get upgrade php

# Verify mbstring extension version
php -m | grep mbstring
php -r "echo phpinfo();" | grep -i mbstring