CVE-2020-7061: PHP PHAR Information Disclosure Vulnerability

CVE-2020-7061 Overview

CVE-2020-7061 is an out-of-bounds read vulnerability affecting PHP versions 7.3.x below 7.3.15 and 7.4.x below 7.4.3 on Windows systems. The vulnerability exists in the PHAR (PHP Archive) extension, where extracting specially crafted PHAR files can trigger a one-byte read past the allocated buffer. This memory safety issue could potentially lead to information disclosure or application crashes.

Critical Impact

This vulnerability allows attackers to potentially access sensitive memory contents or cause denial of service by crashing PHP applications processing malicious PHAR files on Windows systems.

Affected Products

• PHP versions 7.3.x below 7.3.15
• PHP versions 7.4.x below 7.4.3
• Tenable Tenable.sc (versions using vulnerable PHP)

Discovery Timeline

• 2020-02-27 - CVE-2020-7061 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-7061

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), a memory safety issue that occurs when software reads data past the end or before the beginning of an intended buffer. In the context of CVE-2020-7061, the PHP PHAR extension on Windows fails to properly validate buffer boundaries when processing certain content within PHAR archive files.

The flaw is Windows-specific, indicating that the vulnerable code path involves Windows-specific file handling or path parsing routines within the PHAR extraction logic. When a malicious PHAR file is processed, the parser can be induced to read one byte beyond the allocated memory buffer, potentially exposing adjacent memory contents or triggering an access violation that crashes the application.

Root Cause

The root cause lies in insufficient bounds checking within the PHAR extension's file extraction routines on Windows. The code fails to account for edge cases in file content parsing, allowing a one-byte over-read condition. This type of vulnerability typically arises from off-by-one errors in loop conditions, incorrect buffer size calculations, or missing validation of input-controlled length values before memory access operations.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious PHAR archive file with specific content designed to trigger the boundary condition
2. Delivering the malicious PHAR file to a vulnerable PHP application (e.g., through file upload functionality, remote URL fetching, or other input mechanisms)
3. Causing the PHP application to process the PHAR file using the phar extension's extraction functionality

The one-byte over-read can expose sensitive information from adjacent memory locations, potentially revealing cryptographic keys, session tokens, or other security-critical data. Additionally, depending on memory layout and operating system protections, the out-of-bounds access could cause the PHP process to crash, resulting in denial of service.

For detailed technical information about this vulnerability, refer to the PHP Bug Report #79171.

Detection Methods for CVE-2020-7061

Indicators of Compromise

• Unexpected PHP process crashes during PHAR file processing operations
• Memory access violation errors in PHP error logs on Windows systems
• Suspicious PHAR files uploaded to web application directories
• Unusual patterns in file upload activity targeting PHP applications

Detection Strategies

• Monitor PHP error logs for segmentation faults or access violations related to PHAR operations
• Implement file upload validation to inspect PHAR archive contents before processing
• Deploy Web Application Firewalls (WAF) with rules to detect malicious PHAR file uploads
• Use application-level monitoring to detect anomalous PHAR extraction behavior

Monitoring Recommendations

• Enable verbose PHP error logging to capture memory-related exceptions
• Monitor system event logs on Windows servers running PHP for application crashes
• Implement network-level monitoring for unusual file upload patterns targeting .phar extensions
• Set up alerts for PHP process restarts or unexpected terminations

How to Mitigate CVE-2020-7061

Immediate Actions Required

• Upgrade PHP to version 7.3.15 or later for the 7.3.x branch
• Upgrade PHP to version 7.4.3 or later for the 7.4.x branch
• Review and restrict file upload functionality to prevent untrusted PHAR file processing
• Consider disabling the PHAR extension if not required by your applications

Patch Information

PHP has released patched versions addressing this vulnerability. Users should upgrade to PHP 7.3.15 or later, or PHP 7.4.3 or later. For detailed information, see the PHP Bug Report #79171. Linux distribution users should consult their vendor's security advisories, such as Gentoo GLSA 202003-57. Tenable.sc users should refer to the Tenable Security Notice TNS-2021-14 for specific guidance.

Workarounds

• Disable the PHAR extension in php.ini by setting phar.readonly = 1 and removing phar from enabled extensions
• Implement strict input validation to reject PHAR files from untrusted sources
• Use application-level filtering to block PHAR file uploads entirely if the functionality is not required
• Deploy network segmentation to limit exposure of vulnerable PHP applications

# Disable PHAR extension in php.ini
; Prevent PHAR file creation and modification
phar.readonly = 1

# Or comment out the PHAR extension entirely
; extension=phar