CVE-2020-7059: PHP fgetss() Buffer Overflow Vulnerability

CVE-2020-7059 Overview

CVE-2020-7059 is a critical out-of-bounds read vulnerability affecting the fgetss() function in PHP. When using fgetss() to read data with stripping tags, it is possible to supply specially crafted data that causes the function to read past the allocated buffer. This memory safety flaw can lead to information disclosure or application crash, potentially exposing sensitive data from server memory or causing denial of service conditions.

Critical Impact

This out-of-bounds read vulnerability in PHP's fgetss() function can expose sensitive memory contents to attackers or crash PHP applications, affecting web servers and applications running vulnerable PHP versions across multiple platforms.

Affected Products

• PHP versions 7.2.x below 7.2.27
• PHP versions 7.3.x below 7.3.14
• PHP versions 7.4.x below 7.4.2
• Tenable Tenable.sc
• Oracle Communications Diameter Signaling Router
• openSUSE Leap 15.1
• Debian Linux 8.0

Discovery Timeline

• February 10, 2020 - CVE-2020-7059 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-7059

Vulnerability Analysis

The vulnerability resides in PHP's fgetss() function, which is designed to read lines from a file while stripping HTML and PHP tags from the content. The function fails to properly validate buffer boundaries when processing input data containing malformed or specially crafted tag structures. This improper bounds checking (CWE-125: Out-of-bounds Read) allows the function to read memory beyond the allocated buffer limits.

When exploited, attackers can potentially extract sensitive information from adjacent memory regions, which may include configuration data, session tokens, encryption keys, or other application secrets stored in memory. Additionally, attempting to access unmapped memory regions will trigger segmentation faults, causing the PHP process to crash and resulting in denial of service.

Root Cause

The root cause of this vulnerability is insufficient input validation in the tag-stripping logic of the fgetss() function. When processing input streams containing malformed HTML or PHP tags, the function does not adequately check that read operations remain within allocated buffer boundaries. This allows memory reads to extend past the intended data region when encountering specially crafted tag sequences that confuse the parser's state machine.

Attack Vector

The vulnerability is exploitable remotely via network-based attacks against PHP applications that use the fgetss() function to process user-controlled or externally-sourced file content. An attacker can craft malicious input containing specially structured tag patterns that trigger the out-of-bounds read condition. Attack scenarios include:

• Uploading malicious files to web applications that parse uploaded content using fgetss()
• Submitting crafted data to PHP scripts that read from user-controlled streams
• Exploiting applications that process external data sources (feeds, APIs) through fgetss()

The vulnerability does not require authentication and can be exploited without user interaction, making it particularly dangerous for internet-facing PHP applications.

Detection Methods for CVE-2020-7059

Indicators of Compromise

• PHP application crashes or unexpected segmentation faults during file processing operations
• Unusual memory access patterns or core dumps related to PHP processes
• Error logs showing buffer-related issues in scripts utilizing fgetss() function
• Unexpected information disclosure in application responses containing memory fragments

Detection Strategies

• Monitor PHP error logs for segmentation faults and memory access violations during file operations
• Implement application-level logging around fgetss() function calls to detect anomalous input patterns
• Use static code analysis tools to identify applications using the deprecated fgetss() function
• Deploy runtime application self-protection (RASP) solutions to detect buffer overread attempts

Monitoring Recommendations

• Enable PHP error logging and monitor for unexpected crashes in web application processes
• Implement file integrity monitoring on PHP binary installations to verify patched versions
• Configure web application firewalls to detect and block suspicious file upload attempts
• Review application logs for unusual patterns in file processing operations

How to Mitigate CVE-2020-7059

Immediate Actions Required

• Upgrade PHP to patched versions: 7.2.27 or later, 7.3.14 or later, or 7.4.2 or later
• Audit applications for usage of the deprecated fgetss() function and replace with secure alternatives
• Apply vendor-specific patches for affected products including Tenable.sc and Oracle Communications Diameter Signaling Router
• Update Linux distributions with security patches from Debian, openSUSE, and Ubuntu

Patch Information

Security patches have been released by PHP and multiple vendors. The official PHP bug report is available at PHP Bug Report #79099. Distribution-specific patches are available through:

• Ubuntu Security Notice USN-4279-1
• Debian Security Advisory DSA-4626
• Debian Security Advisory DSA-4628
• Gentoo GLSA 202003-57
• Tenable Security Advisory TNS-2021-14
• Oracle Security Alert CPUJul2020

Workarounds

• Refactor application code to use fgets() combined with strip_tags() instead of the deprecated fgetss() function
• Implement input validation and sanitization before processing external file content
• Restrict file upload functionality and validate file contents before processing
• Deploy web application firewalls to filter potentially malicious input patterns

# Check current PHP version for vulnerability status
php -v

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

# Update PHP on RHEL/CentOS systems
sudo yum update php

# Verify patched version is installed (should be 7.2.27+, 7.3.14+, or 7.4.2+)
php -r "echo 'PHP Version: ' . phpversion() . PHP_EOL;"