CVE-2020-7067: PHP Buffer Overflow Vulnerability

CVE-2020-7067 Overview

CVE-2020-7067 is an out-of-bounds read vulnerability affecting PHP's urldecode() function when PHP is compiled with EBCDIC support. The vulnerability allows attackers to read memory locations past the allocated buffer by exploiting erroneously used signed numbers as array indexes. While EBCDIC support is uncommon in typical PHP deployments, systems utilizing this character encoding—primarily IBM mainframe environments—are at risk of information disclosure through this flaw.

Critical Impact

Attackers can exploit this vulnerability to access sensitive memory contents beyond allocated buffers, potentially exposing confidential data processed by PHP applications.

Affected Products

• PHP versions 7.2.x below 7.2.30
• PHP versions 7.3.x below 7.3.17
• PHP versions 7.4.x below 7.4.5
• Tenable Tenable.sc (various versions)
• Oracle Communications Diameter Signaling Router
• Debian Linux 9.0 and 10.0

Discovery Timeline

• April 27, 2020 - CVE-2020-7067 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-7067

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read) and affects the urldecode() function in PHP when compiled with EBCDIC (Extended Binary Coded Decimal Interchange Code) support. EBCDIC is a character encoding system primarily used on IBM mainframe and midrange computer systems.

The core issue stems from improper handling of signed integer values when used as array indexes within the URL decoding logic. When the urldecode() function processes specially crafted input, the signed number handling can result in negative index values, causing the function to access memory locations outside the intended buffer boundaries.

While EBCDIC support is uncommon in standard PHP deployments, organizations running PHP on IBM z/OS, AS/400, or similar mainframe systems may have this support enabled. The network-accessible nature of this vulnerability means remote attackers can trigger the out-of-bounds read without authentication, potentially extracting sensitive information from process memory.

Root Cause

The root cause of CVE-2020-7067 lies in the incorrect use of signed integers as array indexes within the urldecode() function's EBCDIC handling code path. When processing percent-encoded characters in URL strings, the function uses integer values that can become negative due to sign extension or arithmetic operations. These negative values, when used as array indexes, cause the function to read from memory addresses before the start of the intended array buffer.

Attack Vector

The attack vector for this vulnerability is network-based and requires no user interaction or authentication. An attacker can exploit this vulnerability by sending maliciously crafted URL-encoded strings to a PHP application that processes them using the urldecode() function. On affected systems with EBCDIC support enabled, the crafted input triggers the out-of-bounds memory read.

The exploitation scenario involves:

1. Identifying a target PHP application running on an EBCDIC-enabled system
2. Crafting URL-encoded input designed to trigger negative array index calculations
3. Sending the malicious input to the application endpoint
4. Analyzing responses or application behavior to extract leaked memory contents

For technical details on the vulnerability mechanism, refer to the PHP Bug Report #79465.

Detection Methods for CVE-2020-7067

Indicators of Compromise

• Unusual URL-encoded requests with specific byte patterns targeting urldecode() processing
• PHP error logs showing memory access violations or segmentation faults during URL decoding operations
• Anomalous response sizes or content from PHP applications that may indicate memory leakage
• Increased memory-related warnings in PHP error logs from EBCDIC-enabled systems

Detection Strategies

• Implement web application firewall rules to detect and block anomalous URL-encoded request patterns
• Monitor PHP error logs for CWE-125-related memory access errors and segmentation faults
• Deploy intrusion detection signatures that identify exploitation attempts against urldecode() functionality
• Conduct regular vulnerability scanning to identify PHP installations running affected versions

Monitoring Recommendations

• Enable verbose PHP error logging on systems with EBCDIC support to capture early exploitation indicators
• Implement application-level monitoring for unusual memory consumption patterns during URL decoding operations
• Configure SIEM alerts for patterns indicative of information disclosure attempts targeting PHP applications
• Establish baseline behavior for URL processing endpoints to detect anomalous activity

How to Mitigate CVE-2020-7067

Immediate Actions Required

• Upgrade PHP to version 7.2.30 or later for the 7.2.x branch
• Upgrade PHP to version 7.3.17 or later for the 7.3.x branch
• Upgrade PHP to version 7.4.5 or later for the 7.4.x branch
• Apply vendor-specific patches for Tenable.sc, Oracle Communications Diameter Signaling Router, and Debian Linux

Patch Information

Patches addressing CVE-2020-7067 are available from multiple vendors. The PHP project has released fixed versions that correct the signed integer handling in the urldecode() function. Organizations should apply the following updates based on their environment:

• PHP: Upgrade to versions 7.2.30+, 7.3.17+, or 7.4.5+ as documented in PHP Bug Report #79465
• Debian Linux: Apply patches from DSA-4717 and DSA-4719
• Oracle: Refer to Oracle CPU October 2020 and Oracle CPU April 2021
• Tenable: Apply updates per Tenable Security Notification TNS-2021-14

Workarounds

• Disable EBCDIC support in PHP compilation if not required for your environment
• Implement input validation and sanitization for URL-encoded data before passing to urldecode()
• Deploy web application firewall rules to filter potentially malicious URL-encoded inputs
• Consider isolating EBCDIC-enabled PHP applications in network segments with restricted access

# Verify PHP version and check for EBCDIC support
php -v
php -i | grep -i ebcdic

# If vulnerable and EBCDIC not required, recompile PHP without EBCDIC support
./configure --disable-ebcdic
make && make install