CVE-2025-20128: ClamAV OLE2 Decryption DoS Vulnerability

CVE-2025-20128 Overview

A vulnerability in the Object Linking and Embedding 2 (OLE2) decryption routine of ClamAV could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to an integer underflow in a bounds check that allows for a heap buffer overflow read. An attacker could exploit this vulnerability by submitting a crafted file containing OLE2 content to be scanned by ClamAV on an affected device. A successful exploit could allow the attacker to terminate the ClamAV scanning process, resulting in a DoS condition on the affected software.

Critical Impact

Unauthenticated remote attackers can crash ClamAV scanning processes by submitting malicious OLE2 files, causing denial of service on systems relying on ClamAV for malware protection. This could leave endpoints unprotected during the service interruption.

Affected Products

• ClamAV (versions prior to 1.4.2 and 1.0.8)
• Cisco Secure Endpoint (Windows, macOS, Linux)
• Cisco Secure Endpoint Private Cloud

Discovery Timeline

• 2025-01-22 - CVE CVE-2025-20128 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-20128

Vulnerability Analysis

This vulnerability (CWE-122: Heap-based Buffer Overflow) resides in ClamAV's OLE2 decryption routine, which is responsible for processing Microsoft Compound Document File Format content commonly found in Office documents and other embedded objects. The root issue stems from an integer underflow vulnerability that occurs during bounds checking operations within the decryption process.

When ClamAV processes a file containing OLE2 content, it performs arithmetic operations to validate buffer boundaries before reading data. Due to improper handling of integer values during these bounds checks, an attacker can craft a malicious OLE2 file that triggers an integer underflow condition. This underflow corrupts the bounds check logic, allowing subsequent read operations to access memory beyond the allocated heap buffer boundaries.

The attack can be executed remotely without authentication by simply submitting a specially crafted file to any system using ClamAV for scanning. This includes email gateways, file servers, web applications, and endpoint protection systems that leverage ClamAV as their scanning engine. Upon processing the malicious file, the out-of-bounds heap read causes the ClamAV process to crash, disrupting scanning services and potentially leaving systems temporarily unprotected.

Root Cause

The vulnerability originates from an integer underflow condition in the OLE2 decryption bounds checking logic. When calculating buffer boundaries during OLE2 content processing, the code fails to properly validate that arithmetic operations do not produce underflow conditions. This allows attackers to manipulate OLE2 structures in a way that causes the bounds check calculation to wrap around, resulting in an incorrect (and excessively large) boundary value that permits out-of-bounds heap memory reads.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by delivering a maliciously crafted file containing OLE2 content to a target system where ClamAV will scan it. Common delivery mechanisms include:

The malicious file could be delivered via email attachments scanned by mail gateways, file uploads to web applications, shared network drives, or any other path where ClamAV automatically scans incoming content. When ClamAV's scanning engine processes the crafted OLE2 structure, the integer underflow triggers during the decryption routine, causing a heap buffer over-read that crashes the scanning process. This results in service disruption and may require manual intervention to restart the ClamAV daemon.

Detection Methods for CVE-2025-20128

Indicators of Compromise

• Unexpected ClamAV daemon (clamd) or clamscan process crashes or terminations
• Abnormal restart patterns for ClamAV services in system logs
• Files containing malformed OLE2 structures appearing in scan queues or quarantine areas
• Memory-related error messages in ClamAV logs indicating heap corruption or segmentation faults

Detection Strategies

• Monitor system logs for ClamAV process crashes, particularly looking for signals indicating memory access violations (SIGSEGV, SIGABRT)
• Implement alerting on ClamAV service availability to detect unexpected outages
• Deploy file integrity monitoring to identify suspicious OLE2-containing files being submitted for scanning
• Configure SIEM rules to correlate ClamAV crashes with recent file scan events to identify potential attack payloads

Monitoring Recommendations

• Enable verbose logging for ClamAV to capture detailed information about files being scanned before crashes occur
• Implement service health monitoring that alerts on ClamAV daemon unavailability or repeated restart cycles
• Monitor system memory usage patterns for anomalies that may indicate attempted exploitation
• Review email gateway and file upload logs to trace the source of potentially malicious files if ClamAV crashes are detected

How to Mitigate CVE-2025-20128

Immediate Actions Required

• Update ClamAV to version 1.4.2 or 1.0.8 or later immediately, as these versions contain the security fix
• Ensure automatic virus database updates are functioning to receive the latest protection
• For Cisco Secure Endpoint users, apply the latest software updates from Cisco
• Implement service monitoring to detect and automatically restart ClamAV if crashes occur as a temporary measure

Patch Information

Cisco has released software updates that address this vulnerability. ClamAV versions 1.4.2 and 1.0.8 contain the fix for the integer underflow in the OLE2 decryption routine. Users should upgrade to these versions or later as soon as possible. Detailed patch information is available in the ClamAV Security Patch Announcement and the Cisco Security Advisory. Debian users should refer to the Debian LTS Announcement for distribution-specific updates.

Workarounds

• There are no workarounds that address this vulnerability according to Cisco
• Implement process supervision to automatically restart ClamAV services if they crash, minimizing service disruption
• Consider implementing rate limiting on file submissions to scanning services to slow potential DoS attacks
• Deploy redundant scanning infrastructure where possible to maintain protection if one instance is impacted

# Example: Enable automatic restart for clamd service with systemd
sudo systemctl edit clamd.service
# Add the following lines to enable automatic restart:
# [Service]
# Restart=always
# RestartSec=5

# Verify ClamAV version after patching
clamscan --version
# Expected output: ClamAV 1.4.2 or 1.0.8 or higher