CVE-2020-4739: IBM DB2 RCE Vulnerability via DLL Hijacking

CVE-2020-4739 Overview

IBM DB2 Accessories Suite for Linux, UNIX, and Windows, as well as DB2 for Linux, UNIX and Windows (including DB2 Connect Server) versions 9.7, 10.1, 10.5, 11.1, and 11.5, contains a DLL search order hijacking vulnerability. This flaw could allow a local authenticated attacker to execute arbitrary code on Microsoft Windows systems by placing a specially crafted DLL file in a compromised folder that is searched before the legitimate system directories.

Critical Impact

A local attacker can achieve arbitrary code execution on vulnerable IBM DB2 Windows installations, potentially leading to complete system compromise with full confidentiality, integrity, and availability impact.

Affected Products

• IBM DB2 version 9.7.0.0
• IBM DB2 version 10.1.0.0
• IBM DB2 version 10.5.0.0
• IBM DB2 version 11.1.0.0
• IBM DB2 version 11.5
• IBM DB2 Accessories Suite for Linux, UNIX, and Windows
• IBM DB2 Connect Server (all affected versions)

Discovery Timeline

• 2020-11-20 - CVE-2020-4739 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-4739

Vulnerability Analysis

This vulnerability stems from improper handling of DLL loading paths in IBM DB2 client components running on Microsoft Windows. When the DB2 application loads certain dynamic link libraries, it follows the Windows DLL search order, which can include the current working directory or other user-writable locations before trusted system directories.

An attacker who has local access to the system can exploit this behavior by placing a malicious DLL with a specific name in a directory that is searched early in the DLL loading sequence. When a legitimate DB2 process attempts to load the expected library, it inadvertently loads the attacker's malicious code instead, executing it with the privileges of the DB2 process.

The attack requires user interaction, as the victim must execute or interact with a DB2 component while the malicious DLL is present in a compromised folder. This is classified as CWE-426 (Untrusted Search Path).

Root Cause

The root cause of this vulnerability is the insecure DLL search path implementation in the IBM DB2 Windows client. The application does not restrict the directories from which DLLs can be loaded, nor does it validate the integrity or location of loaded libraries. This allows the Windows loader to find and execute malicious DLLs placed in directories that are searched before the legitimate system directories.

Attack Vector

The attack vector is local, requiring the attacker to have prior access to the target system. The exploitation process involves:

1. The attacker identifies which DLLs are loaded by IBM DB2 components without fully qualified paths
2. The attacker crafts a malicious DLL with the same name as a legitimately expected library
3. The malicious DLL is placed in a directory that appears earlier in the Windows DLL search order, such as the application's working directory or a user-writable path
4. When a user launches the vulnerable DB2 application or a process loads the targeted DLL, the malicious code executes with the privileges of that process

This technique is particularly effective when users run applications from directories containing downloaded or untrusted files, as the working directory is often searched before system directories.

Detection Methods for CVE-2020-4739

Indicators of Compromise

• Unexpected DLL files appearing in IBM DB2 installation directories or application working directories
• DLL files in user-writable folders that match names of legitimate Windows or DB2 system libraries
• Unusual process behavior or network connections originating from DB2 processes
• Process execution chains showing DB2 components spawning unexpected child processes

Detection Strategies

• Monitor file creation events in IBM DB2 installation directories and common working directories for suspicious DLL files
• Implement application whitelisting to prevent unauthorized DLLs from being loaded by DB2 processes
• Use endpoint detection and response (EDR) solutions to identify DLL side-loading or hijacking attempts
• Configure Windows Defender Attack Surface Reduction rules to block DLL loading from untrusted locations

Monitoring Recommendations

• Enable Windows Security Event logging for DLL load events (Event ID 7 in Sysmon)
• Monitor for DLL files with names matching common Windows system libraries appearing outside of %SystemRoot%\System32
• Track file hash changes for critical DLLs in the DB2 installation path
• Alert on any DB2 process loading DLLs from non-standard or user-writable directories

How to Mitigate CVE-2020-4739

Immediate Actions Required

• Apply the latest IBM DB2 security patches as referenced in IBM Support Document
• Review and restrict write permissions on directories in the DLL search path for DB2 applications
• Ensure users do not run DB2 client applications from directories containing untrusted files
• Implement application control policies to prevent unauthorized DLL execution

Patch Information

IBM has released security updates to address this vulnerability. Detailed patch information and download links are available in the IBM Support Document. Additional technical details about the vulnerability are documented in the IBM X-Force Vulnerability Report (X-Force ID: 188149).

Organizations should upgrade to the latest supported versions of IBM DB2 that include the security fix for this DLL search order hijacking vulnerability.

Workarounds

• Configure the SafeDllSearchMode registry setting to ensure safer DLL search order behavior on Windows systems
• Remove write permissions for standard users from directories that appear in the DLL search path
• Avoid running DB2 client applications from shared or untrusted directories
• Consider using Windows Defender Application Control (WDAC) or similar solutions to restrict DLL loading

# Registry configuration to enable SafeDllSearchMode (enabled by default on modern Windows)
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager" /v SafeDllSearchMode /t REG_DWORD /d 1 /f

# Verify the setting is applied
reg query "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager" /v SafeDllSearchMode