CVE-2026-3780: Installer Privilege Escalation Vulnerability

CVE-2026-3780 Overview

CVE-2026-3780 is an untrusted search path vulnerability affecting an application installer that runs with elevated privileges. The vulnerability exists because the installer resolves system executables and DLLs using search paths that can include user-writable directories. This flaw allows a local attacker to place malicious binaries with the same names as legitimate system files in these directories, causing the installer to load or execute the attacker-controlled code instead of the intended system files.

Critical Impact

Local privilege escalation allowing attackers to execute arbitrary code with elevated system privileges during the installation process.

Affected Products

• Foxit PDF software products (specific versions not disclosed)

Discovery Timeline

• 2026-04-01 - CVE-2026-3780 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-3780

Vulnerability Analysis

This vulnerability is classified as CWE-426 (Untrusted Search Path), which occurs when an application searches for critical resources using an externally-controlled search path. When the vulnerable installer executes with elevated privileges, it attempts to load system DLLs and executables. However, instead of using secure, absolute paths to system directories, the application relies on the default DLL search order or PATH environment variable, which may include directories writable by standard users.

The local attack vector requires the attacker to have local access to the target system. The attacker must prepare the malicious payload before the victim initiates the installation process. When the installer is executed by an administrator or through a privilege elevation prompt, it inadvertently loads the attacker's malicious binary with the elevated privileges of the installation process.

Root Cause

The root cause of this vulnerability is the application installer's reliance on the default Windows DLL search order without implementing proper security controls. When Windows searches for DLLs, it follows a specific order that can include the current working directory, user profile directories, and other locations that may be writable by non-privileged users. The installer fails to:

1. Specify absolute paths when loading system binaries
2. Implement DLL search order hardening
3. Validate the integrity of loaded modules
4. Use the SetDllDirectory("") API call to remove the current directory from the search path

Attack Vector

The attack requires local access to the target system and involves the following sequence:

1. The attacker identifies which DLLs or executables the installer attempts to load
2. The attacker creates a malicious DLL or executable with the same name as a target system file
3. The malicious binary is placed in a directory that appears earlier in the search path than the legitimate system directory
4. When a user or administrator runs the installer, the malicious binary is loaded instead of the legitimate system file
5. The attacker's code executes with the elevated privileges of the installer process

This type of attack is commonly known as DLL hijacking or DLL side-loading. The attacker can leverage this to achieve full system compromise, install persistent backdoors, or exfiltrate sensitive data with elevated access.

Detection Methods for CVE-2026-3780

Indicators of Compromise

• Unexpected DLL or executable files appearing in user-writable directories such as %TEMP%, %USERPROFILE%, or application-specific folders
• DLL files with system library names located outside of C:\Windows\System32 or other protected directories
• Anomalous process execution patterns showing the installer loading libraries from non-standard paths
• File system audit logs indicating creation of files with names matching common system DLLs in user directories

Detection Strategies

• Enable Windows DLL audit logging through Group Policy to track DLL loading events from non-standard locations
• Deploy endpoint detection rules that alert on installer processes loading unsigned or untrusted binaries
• Implement file integrity monitoring on common DLL hijacking target directories
• Monitor for process creation events where parent-child relationships involve known vulnerable installers and unexpected module loads

Monitoring Recommendations

• Configure Sysmon with appropriate rules to capture ImageLoad events, specifically filtering for DLLs loaded from user-writable paths by elevated processes
• Establish baseline behavior for legitimate installation processes to identify deviations
• Monitor Windows Event ID 7045 (Service Installation) and correlate with preceding DLL load events from suspicious locations
• Implement application whitelisting or code signing verification for all binaries loaded during installation processes

How to Mitigate CVE-2026-3780

Immediate Actions Required

• Review and apply the latest security updates from Foxit by consulting their Security Bulletins
• Ensure installations are performed from trusted, clean directories with restricted write permissions
• Clear user-writable directories (especially %TEMP% and Downloads folders) before running installers
• Run installers only from protected network shares or dedicated installation directories with proper access controls

Patch Information

The vendor has published security information through the Foxit Security Bulletins. Users should consult this resource for the latest patched versions and update instructions. Apply all available security updates to remediate this vulnerability.

Workarounds

• Temporarily restrict write access to directories commonly used for DLL hijacking attacks during installation processes
• Run installers from directories where only administrators have write access, such as C:\Admin\Installers\
• Clear the PATH environment variable of any user-controlled directories before executing the installer
• Consider using application control solutions to prevent execution of unsigned binaries in user-writable directories

# Example: Create a protected installation directory on Windows
mkdir C:\SecureInstall
icacls C:\SecureInstall /grant Administrators:F
icacls C:\SecureInstall /deny Users:W
# Copy installer to protected directory before execution
copy "%USERPROFILE%\Downloads\installer.exe" C:\SecureInstall\
# Run installer from protected location
C:\SecureInstall\installer.exe