CVE-2025-63685: Quark Cloud Drive RCE Vulnerability

CVE-2025-63685 Overview

CVE-2025-63685 is a DLL Hijacking vulnerability in Quark Cloud Drive v3.23.2. The application loads regsvr32.exe without validating its file path or digital signature. An attacker who can place a crafted malicious DLL in the application's startup directory achieves code execution when a user launches the program. The flaw is categorized under [CWE-491] and affects the desktop client distributed by Quark.

Critical Impact

Successful exploitation allows arbitrary code execution in the context of the user running Quark Cloud Drive, enabling persistence, credential theft, and lateral movement.

Affected Products

• Quark Cloud Drive 3.23.2
• Windows desktop client distributions of Quark Cloud Drive
• Installations where the application directory is writable by non-privileged users

Discovery Timeline

• 2025-11-20 - CVE-2025-63685 published to NVD
• 2025-12-16 - Last updated in NVD database

Technical Details for CVE-2025-63685

Vulnerability Analysis

Quark Cloud Drive v3.23.2 invokes regsvr32.exe during startup without specifying a fully qualified path or verifying the binary's digital signature. Windows resolves the executable using the standard search order, which begins with the application's own directory. Attackers exploit this behavior by dropping a malicious DLL or substitute binary into the startup directory. When the user launches Quark Cloud Drive, the planted payload loads inside the trusted application's process. The result is arbitrary code execution under the privileges of the logged-on user.

Root Cause

The root cause is insecure library and process loading. The application fails to enforce path validation and signature checks on dependencies it invokes at runtime. This matches the pattern described in [CWE-491], where insecure resource resolution allows substitution by an attacker-controlled artifact. Developers can prevent this by calling LoadLibraryEx with LOAD_LIBRARY_SEARCH_SYSTEM32 and by validating Authenticode signatures before execution.

Attack Vector

An attacker first obtains write access to the Quark Cloud Drive application directory. This can occur through a prior foothold, a phishing payload that drops a file, or supply-chain tampering of the installer. The attacker places a malicious DLL crafted to match the name expected by the application's loader resolution. The next time a user launches Quark Cloud Drive, Windows resolves the malicious DLL from the application directory and executes its code in-process. No further user interaction is required beyond the normal program launch. Refer to the GitHub CVE Discussion Thread for additional technical context.

Detection Methods for CVE-2025-63685

Indicators of Compromise

• Unsigned or unexpected DLL files present in the Quark Cloud Drive installation directory
• regsvr32.exe or related binaries loading from non-system paths instead of C:\Windows\System32
• Child processes spawned by Quark Cloud Drive that perform network connections to unfamiliar hosts
• Recently modified files in the application's startup directory not matching the vendor-installed manifest

Detection Strategies

• Monitor process creation events where the parent is the Quark Cloud Drive executable and the child is regsvr32.exe loaded from a non-system path
• Hunt for image-load events of unsigned DLLs originating in user-writable application directories
• Alert on file-write events that place .dll files into the Quark Cloud Drive install path outside of installer or update activity

Monitoring Recommendations

• Enable Sysmon Event IDs 1 (process creation), 7 (image loaded), and 11 (file create) with rules scoped to the Quark Cloud Drive directory
• Baseline the legitimate DLL set shipped with v3.23.2 and alert on any deviation
• Forward endpoint telemetry to a centralized analytics platform for cross-host correlation of DLL hijacking patterns

How to Mitigate CVE-2025-63685

Immediate Actions Required

• Restrict write permissions on the Quark Cloud Drive installation directory to administrators only
• Inventory all endpoints running Quark Cloud Drive v3.23.2 and prioritize remediation
• Remove any unsigned or unrecognized DLLs from the application directory and surrounding startup paths
• Block execution of unsigned binaries from user-writable directories using application control policies

Patch Information

No vendor advisory or patched release has been published in the NVD record at the time of writing. Monitor the GitHub CVE Discussion Thread and Quark's official channels for an updated build that addresses the insecure loading behavior. Until a patched version is available, treat all v3.23.2 installations as vulnerable.

Workarounds

• Deploy Windows Defender Application Control (WDAC) or AppLocker rules that block unsigned DLL loads from user-writable paths
• Move the Quark Cloud Drive installation under C:\Program Files\ where standard user write access is denied by default
• Audit and enforce ACLs on the application directory using icacls to remove Modify rights from non-administrative users
• Consider uninstalling Quark Cloud Drive on high-value systems until a fixed release is published

# Configuration example - restrict ACLs on the Quark Cloud Drive install directory
icacls "C:\Program Files\Quark Cloud Drive" /inheritance:r
icacls "C:\Program Files\Quark Cloud Drive" /grant:r "Administrators:(OI)(CI)F" "SYSTEM:(OI)(CI)F" "Users:(OI)(CI)RX"
icacls "C:\Program Files\Quark Cloud Drive" /remove "Authenticated Users"