CVE-2025-20260: ClamAV Buffer Overflow Vulnerability

CVE-2025-20260 Overview

A vulnerability in the PDF scanning processes of ClamAV could allow an unauthenticated, remote attacker to cause a buffer overflow condition, cause a denial of service (DoS) condition, or execute arbitrary code on an affected device. This vulnerability exists because memory buffers are allocated incorrectly when PDF files are processed. An attacker could exploit this vulnerability by submitting a crafted PDF file to be scanned by ClamAV on an affected device.

Critical Impact

Successful exploitation could allow attackers to trigger a buffer overflow resulting in termination of the ClamAV scanning process (DoS), with potential for arbitrary code execution with the privileges of the ClamAV process.

Affected Products

• ClamAV (versions prior to 1.4.3)
• ClamAV (versions prior to 1.0.9)

Discovery Timeline

• 2025-06-18 - CVE CVE-2025-20260 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-20260

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow). The flaw resides in ClamAV's PDF scanning functionality, where improper memory buffer allocation during PDF file processing creates conditions for a heap-based buffer overflow. When ClamAV processes specially crafted PDF files, the incorrect buffer allocation can lead to memory corruption.

The vulnerability is exploitable remotely without authentication, meaning any system running ClamAV that scans untrusted PDF files is potentially at risk. This includes email gateways, file servers, web application firewalls, and any infrastructure where ClamAV is deployed for malware scanning purposes.

Root Cause

The root cause of this vulnerability is improper memory buffer allocation within ClamAV's PDF parsing engine. When processing PDF files, the application fails to correctly calculate and allocate the necessary buffer size, leading to a heap-based buffer overflow condition. This memory management error (CWE-122) allows data to be written beyond the allocated buffer boundaries, corrupting adjacent heap memory.

Attack Vector

The attack vector for this vulnerability is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious PDF file designed to trigger the buffer overflow condition
2. Submitting the crafted PDF file to any service that uses ClamAV for scanning
3. When ClamAV processes the malicious PDF, the buffer overflow is triggered

Common attack scenarios include sending malicious PDF attachments via email to organizations using ClamAV for email gateway scanning, uploading malicious PDFs to file sharing services protected by ClamAV, or targeting web applications that use ClamAV to scan user-uploaded files.

The vulnerability can be exploited to cause denial of service by crashing the ClamAV scanning process. While unproven in the wild, there is potential for attackers to leverage the buffer overflow for arbitrary code execution with the privileges of the ClamAV process.

Detection Methods for CVE-2025-20260

Indicators of Compromise

• Unexpected ClamAV process crashes or restarts, particularly when processing PDF files
• System logs showing segmentation faults or memory access violations from the clamd or clamscan processes
• Unusual PDF files with malformed structures in mail queues or upload directories
• Increased memory usage or anomalous behavior in ClamAV processes prior to crashes

Detection Strategies

• Monitor ClamAV process stability and implement alerting for unexpected process terminations
• Deploy network-based intrusion detection signatures for malformed PDF traffic targeting known ClamAV deployments
• Implement file integrity monitoring on ClamAV binaries and configuration files
• Use SentinelOne Singularity to detect memory corruption attempts and anomalous process behavior

Monitoring Recommendations

• Enable verbose logging in ClamAV to capture details about files being processed when crashes occur
• Implement centralized log aggregation to correlate ClamAV crashes across multiple systems
• Monitor for unusual patterns of PDF file submissions that may indicate exploitation attempts
• Track ClamAV version deployments across infrastructure to identify unpatched systems

How to Mitigate CVE-2025-20260

Immediate Actions Required

• Update ClamAV to version 1.4.3 or 1.0.9 (LTS) immediately
• Review all systems running ClamAV to ensure they are included in the update scope
• Consider temporarily disabling PDF scanning in ClamAV if immediate patching is not possible
• Implement additional security controls to pre-filter potentially malicious PDF files before ClamAV processing

Patch Information

ClamAV has released security patches addressing this vulnerability. Organizations should update to ClamAV version 1.4.3 or 1.0.9 (for the LTS branch) as detailed in the ClamAV Security Patch Announcement. Debian users should also review the Debian LTS Security Advisory for distribution-specific guidance.

Workarounds

• If patching is not immediately possible, consider disabling PDF scanning by configuring ScanPDF no in clamd.conf
• Implement network-level filtering to block or quarantine PDF files from untrusted sources before they reach ClamAV
• Deploy ClamAV in a sandboxed or containerized environment to limit the impact of potential exploitation
• Use SentinelOne Singularity for endpoint protection to detect and prevent exploitation attempts

# Temporary workaround: Disable PDF scanning in ClamAV
# Edit /etc/clamav/clamd.conf and add:
ScanPDF no

# Restart the ClamAV daemon
sudo systemctl restart clamav-daemon

# Verify the configuration is applied
clamscan --debug 2>&1 | grep -i pdf